Challenges in Obesity Epidemiology D

Blackwell Publishing LtdOxford, UKOBRObesity Reviews1467-7881© 2007 Queen’s Printer and Controller of HMSO; published with permission; Journal compilation © 2007 The International Association for the Study of Obesity? 20078••111Review ArticleChallenges in obesity epidemiology D. Canoy & I. Buchan obesity reviews

Challenges in obesity epidemiology

D. Canoy and I. Buchan

NIBHI, Medical School, The University of Keywords: Adiposity, epidemiology, obesity, review. Manchester, Stopford Building, Manchester, UK

Accepted 24 November 2006

Address for correspondence: Dr I Buchan, NIBHI, Medical School, The University of Manchester, Stopford Building, Oxford Road, Manchester M13 9PT, UK. E-mail: [email protected]

OnlineOpen: This article is available free online at www.blackwell-synergy.com obesity reviews (2007) 8 (Suppl. 1), 1–11

Purpose and deﬁnitions Descriptive epidemiology

The purpose of this review is to summarize current knowl- Prevalence and secular trends in men and women edge about the epidemiology of obesity, and to identify the major gaps that a UK partnership of academia, industry Based on a sample representative of England in 2004, and government might address. 22.7% of men and 23.2% of women were obese and Obesity is an excess of body fat leading to ill health. To 43.9% of men and 33.9% of women were overweight assess fatness, the body mass index (BMI, kg m−2), origi- (13), suggesting that more than half of English adults were nating from Quetelet’s ‘average man’ (1), is widely used in either overweight or obese. In 2003, over half of Scottish clinical settings and population studies. BMI is an imprecise adults were either overweight or obese (64% of men and but useful measure of adiposity (2–4). For a given BMI, 57.3% of women) (14). There was little change in over- adiposity varies with age, sex and ethnicity (5); however, weight prevalence in England between 1993 and 2004, BMI correlates reasonably well with body fat mass (2,5,6) while obesity prevalence rose by 9.5% in men and 6.8% and the risks of obesity-related diseases (6–8). Adults with in women, at the same time the proportion considered a BMI >30 are classified as obese, whereas those with BMI lean (BMI 20–25) decreased by 10.6% in men and 8.5% 25–30 are classified as overweight (2). In children, defining in women (13). Similar trends were observed in Scotland overweight and obesity is complicated by the fact that (14). These repeated surveys suggest that the BMI of the weight varies with height as children grow. But methods to UK population shifted to the right. Although the preva- standardize childhood BMI for age and gender are avail- lence of obesity is usually higher in women, the prevalence able and give results that have been validated against other in both sexes in England has been comparable since 2000. measures of adiposity (9). Specious differences in pre- In addition, a threefold difference in prevalence of obesity valence can occur because of the adoption of different cut- has been observed at age 45 as compared with 16– off levels of child BMI standardized for age and gender 24 years (13). This difference between age groups has (10,11). The use of an international standard is a way to been consistent over time, while the mean BMI and the avoid this problem and enable comparisons of obesity prev- prevalence of obesity for each age group has been increas- alence between countries (12). Optimal cut-off levels for ing. The rapid rise in adiposity in the UK reflects a global measuring change over time within countries will, however, phenomenon – affecting both high- and low-income coun- vary across the world. tries (2,15).

This paper was commissioned by the Foresight programme of the Ofﬁce of Science and Innovation, Department of Trade and Industry © 2007 Queen’s Printer and Controller of HMSO; published with permission Journal compilation © 2007 The International Association for the Study of Obesity. obesity reviews 8 (Suppl. 1), 1–11 1

2 Challenges in obesity epidemiology D. Canoy & I. Buchan obesity reviews

While English obesity prevalence rose three- to fourfold between different towns (27). London and Yorkshire had across the two decades from 1980 (16), it doubled in the slightly higher obesity prevalence in children compared USA over a similar period, with the USA starting from a with national average in England in 2001 and 2002, but higher baseline (17). From studies of US veterans, it has there was no clear gradient across different regions in the been proposed that the annual growth rate of median BMI country (28). Although regional differences in mean BMI was about 0.3% between 1900 and 1976 but almost dou- in England have been observed, there was no clear north– bled to 0.5% per year between 1988 and 2000 (18). south divide (29). A recent UK study has suggested a tem- Recently (1999–2004), obesity prevalence in US women poro-spatial/spread pattern to rising childhood adiposity seems to have stopped rising (19). (30). In developing countries, obesity is observed more in urban than in rural areas (31,32). Prevalence and secular trends in children Socioeconomic status and ethnicity The global epidemic of obesity has affected English children substantially. Using International Obesity Taskforce In England in 1996, obesity was commoner among those definitions for overweight and obesity (12), among 5–10- with less education, and in women of a lower occupational year-olds between 1974 and 2002/03 the prevalence of status (33). Birth cohort studies suggest that lower socio- overweight rose from 11.3% to 22.6% in boys and 9.6% economic status in childhood is associated with higher BMI to 23.7% in girls, and obesity rose from 1.8% to 6.0% in in adulthood (34,35). An increase over time (1995–2002) boys and 1.3% to 6.6% in girls (20). There was a two- to in mean BMI was observed among children in both manual fivefold increase in the prevalence of overweight and obe- and non-manual socioeconomic groups but the percentage sity in English children over the three decades up to the change was steeper for those in the manual group (36,37). early 2000s, with the most rapid rise occurring in the 1990s Although obesity varies with socioeconomic status in chil- (21,22). In Scotland from 1974 to 1994, the proportion of dren, the gap may have narrowed in the 1990s, at least in overweight 4–11-year-olds increased from 5.4% to 10.0% some areas (30). In developing countries, higher social sta- in boys and 8.8% to 15.8% in girls, and the proportion tus is associated with obesity but the burden of obesity obese increased from 1.7% to 2.1% in boys and 1.9% to shifts towards the lower socioeconomic groups as the coun- 3.2% in girls (23,24). try’s income rises, and affects women at the early stages of In the European Union (EU), there are around 22 million economic development (15). Education may be ‘protective’ overweight children (5 million of these are obese). Assum- against obesity in high-income countries but is a risk for ing a consistent rise in prevalence of overweight and obesity in low-income countries, depending on the stage of obesity, it is projected, that by 2010, 26 million will be economic development. overweight (rising by about 1.3 million per year) and, of The prevalence of obesity also varies with ethnicity. In these children, 6.4 million will be obese (rising by 350 000 England in 2004, the 22.7% of the general population per year) (25). was obese, the prevalence was higher in those of Black Rising BMI has been reported in children as young as Caribbean or Irish ethnicity (both 25.2%), and lower in 3 years old. In England between 1988 and 1998, the Bangladeshis (5.8%) or Chinese men (6.0%) (37). The proportion 3-year-olds above the 85th centile of the 1990 proportion of obese women across ethnic groups ranged British Growth Reference (26) rose from 14.7% to 23.6% from 17.2% (Bangladeshi) to 32.1% (Black Caribbean) and those above the 95th centile rose from 5.4% to 9.2% and 38.5% (Black Africans), whereas the general popula- (22). Rising BMI continued in Scottish pre-school children tion prevalence in women was 23.2%. Chinese women had between 1995 and 2001 (23). the lowest proportion of obese women (7.6%). However, Less is known about the trends in adolescents: the secular trends for these ethnic groups are less known. In sampling has varied too much over time to make valid the USA, the prevalence of obesity was generally higher comparisons. among non-Hispanic Blacks and Mexican-Americans when Three different definitions of childhood overweight and compared with non-Hispanic Whites in both men and obesity have been used in contemporary studies of UK women (38). Between the survey periods of 1976–1980 children – the prevalence figures cannot be compared and 1999–2002, the highest increase in prevalence was directly. observed among Black females (from 31.0% to 49.6%) and among Hispanic females (from 26.6% to 38.9%) when compared with other ethnic and gender groups (39). BMI Geographical variation increased from 1983 to 1999 among English children (5– Geographical variation in the prevalence of obesity among 11 years) of all ethnic groups (40). Among children (2– middle-aged men has been observed, with the difference in 20 years) in 1999, Afro-Caribbean girls and Indian and the prevalence of those with higher BMI by up to twofold Pakistani boys were more likely to be overweight than girls

obesity reviews Challenges in obesity epidemiology D. Canoy & I. Buchan 3

or boys in the general population (41). In the USA, the are a greater risk for obesity than high-sugar, low-fat combined prevalence of overweight and obesity among 6– diets (51). In the UK, the proportion of energy consumed 11-year-olds in 1976–1980 was 6.5% and in 1999–2002 as fat has been increasing while that of carbohydrates was 15.8% (similar prevalence between boys and girls) has been decreasing since the 1970s (44). In the USA, (38). During this period, a high prevalence was noted in however, the proportion of energy consumed as fat has Black females (from 11.2% to 22.8%) and Hispanic males decreased (17). (from 13.3% to 26.5%) as compared with White males Although the contribution of sugar intake to the obe- (from 6.1% to 14.0%) and White females (from 5.5% to sity epidemic is debatable (50), intake of high-fructose 13.1%). The largest increases in prevalence were among corn syrup, the caloric sweetener used in soft drinks in Black males (from 6.8% to 17%) and Black females (from the USA in the 1970s through to the 1990s, has been 11.2% to 22.8%) (39). linked with obesity (52). Fructose as a percentage of energy intake is high in early infancy but its main source is milk products (53), whereas non-alcoholic beverages Factors influencing the obesity epidemic including soft drinks become a major source of added Adiposity is approximately the net result of the balance fructose later on. Increased intake of carbonated drinks in between energy intake and energy expenditure. The schools has been related to higher percentages of over- approximation is weak in children, where excess energy weight and obese children (54). In the USA, increased can be used for growth – it is also weak where adiposity consumption of soft drinks and sugared beverages has is estimated by BMI in populations with increasing physi- been observed since the 1970s (55). The consumption of cal activity, particularly in men, when a rising BMI can high-fructose corn syrup increased by >1000% over the reflect rising lean not fat mass. Generally, however, excess three decades up to the early 2000s, while cane sugar intake or reduced expenditure of energy would result in a intake decreased (52). In 1977, sweetened beverages positive energy balance, which, in the long term, could formed only 3.9% of total energy intake – by 1996 this cause weight gain. The rapid rise in excess weight in the had risen to 9%. population is therefore being driven by increased total Patterns of food consumption may also contribute to the energy intake, sedentary lifestyle or both. The rapid and obesity epidemic. Fast-food consumption has been associ- widespread nature of the obesity epidemic across geo- ated with more energy-dense food, higher-fat intake and graphical, ethnic, age and gender groups suggests that more consumption of sugar-containing drinks (56–58). In pervasive environmental and/or behavioural changes a survey in the USA, one in four ate fast food on a normal underlie the epidemic. Multiple genetic factors are likely day (56) and those who ate fast food during the survey day to explain the variation of the adiposity phenotype in had a higher BMI. The frequency of eating fast food has populations (42). been prospectively associated with increased weight gain (59). Fast-food expenditure increased from 20% in 1970 to 40% in 1995 in the USA – as a percentage of total energy Roles of dietary factors intake, it increased from 2% in 1970 to 10% in 1995 (60). Evolution has favoured a preference for energy-dense and There was a shift between 1977 and 1996 towards food fatty foods – the body defends against weight loss but eaten away from the home (45). Considering that food tolerates weight gain (43). Preference for energy-dense portion sizes increased in the USA between the survey foods, weak satiety and strong hunger traits leaves humans periods 1977–1978 and 1994–1996 particularly for soft susceptible to obesogenic environments. Evidence about drinks (62%), fruit drinks (48%), French fries (57%) and the roles of dietary factors in obesity is, however, difficult salty snacks (58%) (61), this trend implies patterns of food to produce as diet is difficult to measure – the following consumption could play an important role in the obesity findings should be interpreted with caution: epidemic. In the UK from 1970 to the mid-1980s, total energy In developing countries, obesity has been associated with intake decreased (44). In the USA during this period, higher socioeconomic status but the epidemic spread to there was a slight increase in the total energy intake in all those in lower socioeconomic groups when high-fat diets age groups (45), although another study suggested that become more affordable (32). Nevertheless, despite the there was no change in children’s energy intake (46). The globalization of the obesity epidemic, there remains a wide composition of diet might play a more important role difference in obesity prevalence between developed and than its total energy content in the obesity epidemic. developing countries that may be accounted for by differ- Excess fat intake can lead to excess weight (47,48). Even ences in dietary habit. For example, the dietary contribu- when the total energy intake is similar or reduced, a tion of fast food and soft drinks remains high among higher proportion of the energy provided by fat is associ- children in the USA but relatively low in Russia, China and ated with weight gain (49–51). Low-sugar, high-fat diets the Philippines (62).

4 Challenges in obesity epidemiology D. Canoy & I. Buchan obesity reviews

between 1997 and 2003 decreased while the prevalence of Role of physical activity obesity increased (71). However, it remains unclear how Reduced energy expenditure has been associated with sedentary lifestyle could be an explanation for increasing weight gain (63), and several cohort studies have shown obesity prevalence in early childhood, particularly in those lower weight gain with higher levels of physical activity below 4 years of age (22). (64). In children, a dose–response relationship between the number of hours of television viewing and the prevalence Roles of early life factors of overweight has been observed (65). Sedentary lifestyle and increased mechanization could also be culprits in the Factors such as parental BMI, socioeconomic status and obesity epidemic. In England, the number of households birth weight have been shown to predict adult obesity with no cars reduced from 48% to 26% between 1971 and (34). However, the findings are inconsistent between 2000 (66), whereas the proportion of those with two or studies, many of which do not take important confound- more cars increased steadily from 8% to over 29% in ing or mediating factors into account. Breastfeeding has 2004–2005 (66,67). Between 1961 and 2003, the total been suggested to protect against obesity but findings are distance travelled by people by car or van increased from inconsistent (72–74). A meta-analysis concluded a small 157 billion to 678 billion passenger kilometres, and protective effect of breastfeeding, but there was significant between 1994 and 2003, this distance travelled increased evidence of publication bias (75). Although there are few by 4% per year. By 2004, 76% of men and 68% of women reports on secular trends of breastfeeding, some studies were travelling to work by car or van. show an increasing proportion of mothers breastfeeding Between the survey periods 1992–1994 and 2003, walk- since the 1970s (76–78). It has also been proposed that ing to and from school decreased from 61% to 53% among maternal smoking during pregnancy might be a risk factor 5–10-year-olds and from 44% to 41% among 11–16-year- (79). But smoking during pregnancy has been decreasing olds (66). During the same period, travelling by car or van since the 1970s (77,80). There has been no parallel change increased from 30% to 39% among 5–10-year-olds and in the link between obesity prevalence and breastfeeding from 16% to 23% among 11–16-year-olds. Perhaps the practice or maternal smoking during pregnancy in recent average distance from home to school, which has increased years. from 1.9 km to 2.3 km, has influenced the use of the car or van for this purpose. Trends in obesity-related diseases In 2000–2001, adults in England spent over 2 h on watching television and videos/DVDs but only about half The obesity epidemic is an actual and potential public an hour was spent on hobbies, games, or sports as daily health crisis. Higher BMI has been associated with prema- leisure activities (66). Between the survey periods of 1987, ture mortality. In one study, male and female non-smokers 1990–1991, 1996–1997 and 2002–2003, adult participa- who were obese at age 40 died 6–7 years earlier than their tion in physical activities (sports, games, walking at least non-obese counterparts (81). Obesity is associated with 2 miles) showed no clear decreasing trend (13), although hypertension, dyslipidaemia and insulin resistance (6) – the the proportion of those who achieved the physical activity secular trends in these conditions should indicate the target (at least 5 days a week of 30 min or more of mod- emerging disease impact of the obesity epidemic. erate to intense activity) between 1997 and 2004 increased Obesity is an important risk factor for type 2 diabetes. from 32% to 37% of men and 21% to 25% of women The incidence of type 2 diabetes has been rising since the (13,68). This increasing trend does not hide the fact that 1970s (82), and more recently a small but increasing pro- the vast majority during this period remained below the portion of the diagnoses have been in children (83–85). In recommended level of physical activity (13,68). During the contrast, death rates have been decreasing during the past spread of the US obesity epidemic, the proportion of those three decades in the UK, with more people surviving to who reported involvement in leisure-time physical activity older ages (86). Further, coronary heart disease has been regularly (but not intense) reduced from 33.2% in 1991 to declining since the 1970s in many Western countries 29.6% in 1998 (69). However, the proportion of inactive (87,88). In the UK, age-standardized death rates for circu- adults slightly decreased from 29.7% in 1991 to 28.6% in latory diseases decreased by 37.8% in men and 34.5% in 1998. Large-scale objective measurement of physical activ- women between 1995 and 2005 (86). The picture of car- ity in the general population is lacking. diovascular disease risk factors has also been improving. In There are also limited data on secular trends of physical the USA, mean cholesterol and the proportion of those with activity involvement of children. In the USA, the propor- high cholesterol levels declined in the 1980s and 1990s tion of those physically active in daily physical education (88). Between 1993 and 2003, mean systolic blood pressure classes decreased between 1991 and 1997 (70). Cardio- reduced from 137 to 132 mmHg among English adults. respiratory fitness among 9–11-year-olds in England There has been a substantial reduction in smoking (13),

obesity reviews Challenges in obesity epidemiology D. Canoy & I. Buchan 5

which could have a stronger effect on mortality and circu- age, gender and ethnicity, but so can the sensitivities of the latory disease than obesity (87). This might partly explain instruments used to assess the determinants – these biases the complex picture of rising obesity and falling coronary need to be better understood. heart disease – but the full explanation is missing: generally, The determinants of energy balance interact, but the coronary heart disease is the main cause of excess mortality interactions are seldom reported. This is unsurprising in obesity, but the contribution of coronary heart disease because the interactions are highly complex, requiring to overall mortality is dropping – this might speciously larger than usual studies to characterize them. For exam- suggest that obesity is becoming a weaker determinant of ple, unhealthy diets among adolescents correlate both with disease. Given that BMI is a relatively weak cardiovascular television viewing and adverse perception of healthy eating risk factor, most of the trends in cardiovascular diseases are (94). Urbanization in developing countries is associated likely to be due to improved diet and lifestyle and improved with higher proportions of obesity (32) and could relate to treatment and control of risk factors other than obesity access to cheap energy-dense foods as well as reduced (89). A further unknown in relating BMI to disease risk is physical activity. The built environment has been associ- the equity of improved risk factor control and treatments ated with excess weight or body size, probably via inﬂu- between obese and non-obese people. If obesity is assumed ences on both diet and physical activity in the community to be a stable determinant of coronary heart disease, then (95). Fast-food outlets are sometimes clustered in deprived further gains of about 8% to 10% reduction in coronary areas (96). The interrelationships of the major determi- heart disease might be achieved if mean BMI was reduced nants of obesity over long periods of time need to be (87,89,90). It remains unclear whether or not increasing understood as a whole, dynamic system. Larger and more obesity prevalence will cancel out further reductions in wide-ranging studies are required to reveal the ‘big picture’ coronary heart disease incidence (or reverse the decreasing of the obesity epidemic. trend). Despite the reduction in mean cholesterol in the The need for a ‘big picture’ applies not only to current 1980s and 1990s in the USA, smaller reductions were populations but also across the life course. For example, observed during the late 1990s even with an increase in the the combined effect of increasing breastfeeding, decreasing use of cholesterol-lowering drugs during this period (88). energy and fat intake, and increasing physical activity (68) The obesity epidemic may have halted the downward trend might have cumulative and multiplicative effects only in mean cholesterol. revealed by a set of longitudinal studies. The search for individual modiﬁable risk factors might be less fruitful than the search for combinations of factors. Research weaknesses

The high-level weaknesses in the current epidemiology of Unclear consequences of long-term excess weight obesity are first the fragmentation of the understanding of energy balance in populations. There is also a shortage Obese people are more likely to suffer ill health than those of contemporary evidence about the emergence of obesity- who are lean. The studies that established this, however, related diseases. In addition, a sparse understanding of the involved adult men and women who were children and early life determinants of metabolic health and the lack of young adults when obesity prevalence was lower than it is studies of public health interventions to influence energy now. As such, they were likely to have had a lower adipos- balance are areas of significant weakness which have been ity in early life than today’s young. Given that adiposity identified elsewhere in this review. More specific problems tracks from childhood into adulthood (that is, the relative are: position of a child in the BMI distribution would be main- tained later into adulthood) (97,98), a greater proportion of the population now has excess weight for a longer period Inaccurate and incomplete assessment of of time. The health impact of long-term excess fat is poorly energy balance understood. One study associated early death with obesity Capturing dietary habit is difficult and under-reporting of at 18 years of age, but noted that the association was only dietary intake is seen more in obese people (91,92). Incom- partly explained by baseline BMI (99). Relevant longitudi- parable methods for assessing diet are used across different nal prospective studies are limited. studies. Diet evolves more quickly than the instruments used to assess it, such that changes over time in nutritional Obesity-related burden of disease might composition might not be captured in long-running studies. be underestimated Similar problems are associated with assessment of physical activity. Reporting biases, measurement errors and a lack There is conflicting evidence about the nature and of validated methods beset physical activity epidemiology strength of the association between BMI and mortality, as (64,93). The determinants of energy balance can vary with well as cardiovascular disease event rates (100). In a meta-

6 Challenges in obesity epidemiology D. Canoy & I. Buchan obesity reviews

analysis involving 388 622 individuals and 60 374 deaths, Population-level preventive measures and obesity was associated with higher mortality but the interventions are understudied pooled risk for the overweight group was not significant (100). In patients with coronary heart disease, lower BMI Trials suggest that moderate weight loss is feasible and was even associated with higher mortality (101). Consid- improves metabolic health (113–115). However, some ering that the amount of fat may differ between ethnic observational studies suggest that intentional weight loss is groups even with the same BMI (102), it is plausible that associated with higher mortality in spite of improving met- the underlying differences in fatness might affect how abolic risk factors – these could be confounded by uninten- BMI predicts risks in different populations. One study tional loss of lean mass in the diseased who intend to loose suggested that fat distribution measures predict myocar- fat mass (116–118). Other studies show reduced mortality dial infarction more strongly than BMI does (103). In the with intentional weight loss (119). The epidemiology is not highest two-fifths of waist : hip ratio, the population clear but it is biologically plausible that intentional weight attributable risk was 24.3% while in the top two-fifths of loss with reduced metabolic risk factors could improve BMI, it was 7.7%. The impact of obesity could have been health. underestimated. Individual approaches to reducing weight might be effec- tive in the short term, and in extreme cases, efficient. How- ever, diet and physical activity are interdependent and are Childhood obesity: a problem poorly understood both affected by influences beyond the individual. Among Children are experiencing an obesity epidemic at the same children, prevention of excessive weight gain is paramount time as adults. But it is difficult to imagine how trends in but they are clearly not in control of the environment that adult energy balance relate closely to those of young chil- promotes excessive energy intake and/or insufficient energy dren. There is an association between childhood and paren- expenditure. tal obesity, but it is unclear whether this is an epigenetic There is a lack of research into population measures to phenomenon (metabolism imprinted on offspring) or sim- help people maintain a healthy weight (120). Although ply an indication of a family lifestyle that leads to a positive population-based intervention studies on reducing the bur- energy balance. den of obesity are needed, more research should be carried There is also inconsistency in defining obesity in child- out into preventing excessive weight gain and maintaining hood, as the adiposity associated with BMI varies in weight loss. strength with the degree of adiposity (104). The defini- Expecting the general population to adopt healthy life- tion of childhood overweight and obesity is generally styles is unrealistic if the social and built environment does based on statistical rather than biological grounds (10). not promote it. Not enough is known about the role of The classification derived from how BMI relates to mor- geographical factors and the built environment on obesity, tality in adults (105) could be more meaningful biologi- and the impact of food systems and distribution on food cally, although the generalizability of such classifications choices and dietary habits. The evidence of how to reverse across populations has been questioned (106). The rela- the obesogenic environment through public health action tionship of childhood BMI to adult health outcomes is is lacking. Population-based programmes to reduce obesity understudied. are part of many health policies but are grossly under- Some patterns of early growth have been linked with studied (121). The randomized controlled trials that later adiposity (107). Studies in this area are limited and attract most research kudos are largely infeasible in this inconsistent (108), and it is unclear how these patterns area – alternative designs, such as quasi-experiments are explain the increasing obesity prevalence over time. Cohort required (31). and period effects of growth patterns on later obesity need to be investigated. It is plausible that cohorts from different Policy analysis is too narrow periods may have different susceptibilities to the adverse effects of an obesogenic environment. For the health policymaker, obesity epidemiology presents The obesity epidemic might have different consequences many dimensions and uncertainties. Capturing all dimen- in children than in adults because children are developing sions, from biological to social, in policy scenarios with within the obesogenic environment. Child growth and obe- a limited evidence base requires extreme co-ordination. sity interact (109), but the long-term consequences are Dividing health policy, for example, into separate ‘task- unknown. Interventions to prevent excessive weight or to forces’ for obesity and physical activity, as in the UK (122), reduce weight in children are understudied (31,110,111). could lead to narrow analysis that is less informative than Animal studies suggest that there might be lasting, modifi- a more complete approach. The opportunity costs are very able risk factors in infancy, but these are unstudied as yet high because more than half of the population in many in humans (112). countries are classified as overweight or obese. Therefore,

obesity reviews Challenges in obesity epidemiology D. Canoy & I. Buchan 7

obesity policy analysis needs to be a continuous process – Improved measures of adiposity linking public health surveillance with cumulative synthesis of the emerging science. Monitoring the performance of Body mass index is inexpensive and useful for crude assess- obesity policies on population weight/BMI alone is attrac- ment of population adiposity. However, indices of fat dis- tively simple but likely to be misleading: the problem is tribution have been shown to predict metabolic disease how to improve health when weights are increasing, not risks independently of BMI (103,124). Although measuring just how to change weights. fat distribution is advocated (2), the use of these indices is uncommon in clinical or public health practice. In addition, the significance of fat distribution in children is poorly Future research understood. Future studies should explore better metrics of The high-level tasks for future epidemiology in obesity are adiposity and examine their usefulness in predicting disease first to piece together extant findings and datasets into a risks. Simple but more predictive measures of adiposity more complete ‘meta-epidemiology’ of energy balance. Pro- phenotype are needed. spectively, there is a need to characterize the emergence of obesity-related diseases through continuous, longitudinal Life course studies of metabolic health studies via routine healthcare data. And, at the same time, to establish cohort studies capable of identifying the early The obesity epidemic affects all age groups. Heavier babies life determinants of metabolic health. In addition, the intro- and children with higher BMI are more likely to become duction of public health interventions and new technolo- obese in later life. Infants and pre-school children are get- gies to encourage a healthy energy balance need to be ting heavier but what drives this is not known. Some stud- subject to (quasi-)experimental study. ies suggest that increased post-natal growth may predispose Specific areas of research that need nurturing are children to become obese later in life, but what influences described in the following sections. this accelerated growth is not known. More research should be performed to explore the causes and consequences of obesity throughout the life course. Life course Improved characterization of the studies of metabolic health, starting at the time of antenatal obesogenic environment health care, should be established, recruiting parents con- The factors affecting energy balance should be further tinuously and following up their offspring both longitudi- explored and characterized at an individual level. These nally and serially across successive birth (sub-)cohorts. factors are interrelated and the nature of their associations Research should also be performed on the inter-genera- may vary with age or with time. Studies are needed to tional effects of obesity, aiming to disentangle hypotheses indicate which factors are relevant in specific contexts (age on metabolic ‘programming’ through genetic or epigenetic group, ethnicity, etc.). Future studies should also identify mechanisms, and transmission of dietary behaviours and factors that prevent excess weight gain and maintain a lifestyle from parents to children. healthy weight. Better and better-integrated assessments of diet, feeding Vulnerable groups behaviour and physical activity need to be developed. Large numbers of individuals then need to be assessed in this way. There is a need to understand the determinants and conse- In particular, the factors that maintain energy balance need quences of obesity specific to potentially vulnerable sub- to be identified (123). Information and biotechnologies populations. These include ethnic and migrant groups, should be harnessed to increase the feasibility and scope of socially or economically deprived groups, and the elderly. such studies. The percentage of fat increases while muscle mass decreases with age (125). The balance between the adverse effects of excess fat on metabolic health and the ‘protective’ Rapid translation between biology and effect of weight on bone health needs to be explored in the population studies elderly. The impact of the obesity epidemic among migrant Knowledge about the biological mechanisms of appetite families needs to be investigated (121). regulation, energy storage and metabolic health is emerging via a myriad studies into different pathways. The challenge Public health surveillance and for both biology and epidemiology is to assemble the infor- interventional research mation in ways that explain the effects measured across populations, and shape hypotheses. Bioinformatics is It has been estimated that in 2002, the direct and indirect central to this, but must be introduced into mainstream ‘obesity cost’ in the EU was €32.8 billion, whereas the cost epidemiology. of subsidizing ‘healthy foods’ was €10 billion, a third of

8 Challenges in obesity epidemiology D. Canoy & I. Buchan obesity reviews

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