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1 The Importance of Low-Fat Diets in Preventing Weight Gain

The role of a high-fat diet in inducing weight gain was highlighted by the WHO Consultation on Obesity (9) and gained general acceptance as an appropriate strategy for the prevention of obesity until questioned by some authors who expressed doubts about the efficacy and safety of this approach (48-50). Their skepticism was based on the association of increasing levels of obesity with decreasing proportion of dietary energy from fat in the U.S. national diet, a belief that high-carbohydrate diets resulted in increased CHD risk and the variable and inconsistent outcome of trials of low-fat diets. However, a review of U.S. dietary patterns over the period 1970-1994 by the U.S. Department of | Agriculture (51) revealed that, while there was a de- -a crease in the percentage of energy from fat (from 42% to < 38%), the absolute amount of fat in grams available for « consumption actually increased by 3%. This apparent paradox is possible because over the same time period the energy available per capita increased by 15% (from | 3300 kcal to 3800 kcal). Astrup et al. (52) have undertaken a meta-analysis of trials involving low-fat diets and shown that the lower the fat intake, the greater the weight loss, this loss being more marked in the more overweight subjects. In addition, recent detailed assessments on the matter (53-55) have all concluded that the balance of evidence supports an important role for dietary fat in the genesis of obesity and support the potential for weight gain prevention of reducing fat in the diet. This does not mean, however, that the other dietary components do not have an important role to play in the prevention ofweight gain. There is some evidence that the source of dietary carbohydrate and the glycemic index may influence cardiovascular risk factors (56) and has the potential to amplify the risk of the metabolic syndrome in those who gain weight (57).

2 Food Eaten Away from Home

One of the most profound changes to affect the food supply in almost all developed and many developing countries has been the rapid increase in the proportion of food prepared away from home. The proportion of household budget spent on food eaten away from the home is as high as 40% in the United States (51) and ~25-30% in many other counties such as Australia and the United Kingdom (58,59). The spread of fast-food outlets across the world is responsible for the biggest proportion of food eaten away from home, with the number of outlets increasing from 30,000 to 140,000 in the United States between 1970 and 1980 and fast-food sales increasing 300% in the same period (60). Some analyses have linked the increase in fast food consumption to increasing rates of obesity (61). French et al. (62) have found that fast-food use was associated with increased energy and fat intake as well as higher body weight in females (but not males) in the subjects who participated in the ''Pound of Prevention" study. The portion size of fast-food items has also been increasing rapidly in recent times and has been identified as a key issue in the consumption of dietary energy in excess of need (60). Evidence suggests that as portion size increases, the ability of consumers to estimate accurately their intake deteriorates (63). The sheer volume of sales achieved by fast food outlets and the association with increased fat and energy intake make them a useful target for obesity prevention efforts.

3 Sweetened Drinks

Evidence is accumulating from a range of sources that energy consumed as sweetened drinks is less well compensated for than when consumed as solid food (64,65). A recent longitudinal study has shown that children who consumed more soft drink at baseline were more likely to become obese and that every 1-serving increase in soft drink consumption resulted in a 1.6-fold increase in the risk of obesity (66). Special trials in adult volunteers allowing plentiful soft drinks, with either sugars or sweeteners, for 10 weeks in double-blind trials also show progressive weight gain only in the sugar drink-consuming group (67). Market research data suggest that soft drink consumption is increasing worldwide at a rate faster than any other food group (68) and is replacing water and milk as the most popular drinks among children. The widespread availability of soft drinks from vending machines could be contributing to this trend (60).

B Increase Physical Activity

Although it is difficult to accurately measure changes in physical activity, there can be little doubt that energy expenditure from activity has decreased substantially over the past 50 years. James (69) compared food intake data with population weight gain to estimate that it is likely that the average, adult energy expenditure in the United Kingdom decreased by ~800 kcal between 1970 and 1990. That this reduction in energy expenditure occurred in a period where surveys suggested that participation in leisure-time physical activity was increasing in the United Kingdom (70) supports the contention that the greatest contribution to this reduction in physical activity comes from the enormous changes in occupational and incidental activity. Prentice and Jebb (71) demonstrated the close association between increasing rates of obesity in the United Kingdom and two key indicators of inactivity (hours per week of television viewing and numbers of cars per household). Although there are few data to support the nature of this association, the extent of mechanization, computerization, and control systems imposed in the workplace and the shifting employment patterns away from manual to more sedentary occupations has markedly reduced the need for energy expenditure at work. In addition, the rapid increase in use of mechanized transport and labor-saving devices such as elevators has reduced the need to expend energy going about our daily lives. |

A number of studies support the benefit of increasing | physical activity in the prevention of weight gain, although it is not clear from current studies whether increased physical activity actually prevents or reverses age-related weight gain at the population level (72). | Cohort studies in both Finland and the United States 5

u have shown that weight gain is less in those who are more active (73-75), and similar findings have emerged © from assessments of a register of people who have been successful in losing and maintaining weight (76,77). It is I

also proving difficult to predict how much physical activity is required to prevent weight gain, with Scho-eller et al. (78) estimating that an additional 80 min of moderate activity or 35 min of vigorous activity may need to be added to our usual sedentary lifestyle to prevent weight regain in subjects who have previously been obese. However, there are numerous health benefits to be gained from regular exercise regardless of the impact of physical activity on weight gain prevention (79,80).

C Decrease Sedentary Behaviors

Technological advances have allowed a reduction in hours spent at work and in undertaking household chores leading to a substantial increase in leisure time while at the same time spawning the development of numerous entertainment options to fill this time. Almost all of these new entertainment options such as television, video games, and computers are sedentary activities requiring little energy expenditure. In recent times these activities, which initially were used to complement existing forms of leisure activity, are occupying more hours in the day and displacing more active pursuits and games. This has raised concerns for both adults and children, and a number of studies have found association between the number of hours spent watching television and increased levels of BMI in children (81-83). Although small, these associations, have been relatively consistent but have not been demonstrated for other sedentary behaviors.

It is important to make a distinction between lack of physical activity and sedentary behavior as the mechanism for their impact on body weight may be different and a high level of sedentary behavior can coexist with a high level of physical activity. In the study of Andersen et al. (83), most of the children reported relatively high frequency of activity and although there was a strong association between TV watching and weight status, the association between physical activity and fatness was weak. Robinson (84) indicates that the mechanisms by which sedentary behavior influences body fatness are still to be elucidated. He suggests that a reduction in energy expenditure from a displacement of physical activity seems logical (although not clearly found) but television viewing may also be associated with an increased dietary intake, potentially driven by food advertising. It is interesting to note that studies of the treatment of overweight children have found that reinforcing decreased sedentary behavior leads to a greater weight loss than promoting increased physical activity. (85).

D Improve Infant Feeding and Maternal Nutrition

Infant feeding practices may have an important influence in the etiology of childhood overweight and obesity, and breastfeeding may assist with postpartum weight management in mothers. In addition, eating practices and food preferences developed in early childhood may persist throughout childhood and adolescence and contribute to the genesis of weight problems.

WHO has recently reevaluated the importance of breastfeeding in terms of subsequent health, but did not pay particular attention to the issue of obesity in later childhood. Breastfeeding induces faster longitudinal growth in newborn babies and results in babies that are thinner than bottle-fed babies. The choice of recommendation for exclusive breastfeeding until ~6 months is based on a series of small studies that reveal that the rate of growth of the breastfed child decelerates from soon after birth and crosses that of the NCHS standard group of predominantly bottle-fed babies at the age of ~6 months (86).

There has nevertheless been a renewed emphasis on the potential value of breastfeeding recently with a series of studies (see Table 3) (87), suggesting that breastfeeding may protect against the development of obesity in older children. However, not all studies have shown an association, and it was apparent that breastfeeding

Table 3 Recent Studies Examining the Impact of Breastfeeding on Later Obesity

Study (Ref.)

Sample

Age at follow-up

Breastfeeding (>6 months) effect on overweight and obesity rates odds ratio (95% CI)

Von Kries et al., 1999 (90)

9357 children

5-6 years

0.75 (0.57-0.98)

Gillman et al., 2001 (88)

8186 girls and 7155 boys

9-14 years

0.78 (0.66-0.91)

Hediger et al., 2001 (146)

2685 children

3-5 years

0.84 (0.62-1.13)

needs to be exclusive and sustained for at least 4 months to show any impact on weight in later childhood (88,89). The longer the breastfeeding period, the less likely the children are to become obese later. Von Kries et al. (90) found a dose-dependent protective effect of breastfeeding on the development of obesity with 3-5 months of exclusive breastfeeding being associated with a 35% reduction in obesity at age 5-6 years. However, Bute, in her recent analysis, highlights the difficulty in separating out the breastfeeding per se from confounding factors such as the socioeconomic status of the family, the educational level of the mother, and their degree of concern in modifying the children's subsequent diet (91). As Dietz (89) indicates in a recent editorial on the issue, the percentage of obesity cases preventable by breastfeeding may be small, with a maximum population attributable risk of 15-20%, but the numerous additional benefits, lack of potential risks and cheapness of breastfeeding, make it worth promoting vigorously.

Whether the effects of breast feeding on subsequent obesity levels are influenced by the fatty acid composition of the breast milk is unknown, but the diet of the pregnant woman influences the n-6 and n-3 supply to the fetus through the mechanisms of essential fatty acid (EFA) transport in the placenta (92) with the mother's diet also affecting her own body membrane lipid levels and the composition of the breast milk. The EFAs stimulate lipogenesis and cellular proliferation (93), and vegetarian mothers have reduced intakes of EFAs with an increase in the n-6/n-3 ratio (94). Insulin resistance limits the delta-9 desaturase responsible for the essential chain elongation of the EFAs, so overweight and obese mothers may also be influencing their babies' adipocyte proliferation depending on their own diet and whether or not they breastfeed.

The value of breastfeeding in promoting a faster return to prepregnancy weight is less clear. Sebire et al. (95) found in a retrospective analysis of 287,213 pregnancies in England that women who were breastfeeding their children on discharge from hospital were significantly less likely to be obese. However, this may be a consequence of the reluctance of overweight women to initiate and sustain breastfeeding (96) rather than a direct impact of breastfeeding on maternal weight. Studies of women who have breastfed in Brazil (97) and the United States (98) have shown that the long-term association between breastfeeding and maternal weight and fat stores is limited but does increase in strength when only women who exclusively breastfed are included in the analysis.

There is now a reasonable amount of evidence to suggest that low-birth weight infants are at increased risk of a combination of hypertension, dyslipidemia, and glucose intolerance (a cluster known as syndrome X) in adulthood (99). This condition is exacerbated in those who having been born small subsequently develop obesity in adulthood. However, the connection between low birth weight and the development of obesity appears slight at best (100), and these abnormalities are more likely to be mediated by catchup growth or abdominal obesity. Poor perinatal nutrition is only one of a number of factors, including age of parity, smoking, and sexually transmitted disease that influence birth weight, but the benefits of improving maternal nutrition are likely to be numerous.

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