5. Population nutrient intake goals for preventing diet-related chronic diseases:
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A high dietary intake of non-starch polysaccharides (NSP)/dietary fibre (protective). The nomenclature and definitions of NSP (dietary fibre) have changed with time, and many of the available studies used previous definitions, such as soluble and insoluble fibre. Nevertheless, two recent reviews of randomized trials have concluded that the majority of studies show that a high intake of NSP (dietary fibre) promotes weight loss.
Pereira & Ludwig (7) found that 12 out of 19 trials showed beneficial objective effects (including weight loss). In their review of 11 studies of more than 4 weeks duration, involving ad libitum eating Howarth Saltzman & Roberts (8) reported a mean weight loss of 1.9 kg over 3.8 months. There were no differences between fibre type or between fibre consumed in food or as supplements.
High intake of energy-dense micronutrient-poor foods (causative). There is convincing evidence that a high intake of energy-dense foods promotes weight gain. In high-income countries (and increasingly in low income countries) these energy-dense foods are not only highly processed (low NSP) but also micronutrient-poor, further diminishing their nutritional value. Energy-dense foods tend to be high in fat (e.g. butter, oils, fried foods), sugars or starch, while energy-dilute foods have a high water content (e.g. fruits and vegetables). Several trials have covertly manipulated the fat content and the energy density of diets, the results of which support the view that so-called “passive over consumption” of total energy occurs when the energy density of the diet is high and that this is almost always the case in high-fat diets. A meta-analysis of 16 trials of ad libitum high-fat versus low-fat diets of at least 2 months duration suggested that a reduction in fat content by 10% corresponds to about a 1 MJ reduction in energy intake and about 3 kg in body weight (9). At a population level, 3 kg equates to about one BMI unit or about a 5% difference in obesity prevalence. However, it is difficult to blind such studies and other non-physiological effects may influence these findings (10). While energy from fat is no more fattening than the same amount of energy from carbohydrate or protein, diets that are high in fat tend to be energy-dense. An important exception to this is diets based predominantly on energy-dilute foods (e.g. vegetables, legumes, fruits) but which have a reasonably high percentage of energy as fat from added oils.
The effectiveness over the long term of most dietary strategies for weight loss, including low-fat diets, remains uncertain unless accompanied by changes in behaviour affecting physical activity and food habits. These latter changes at a public health level require an environment supportive of healthy food choices and an active life. High quality trials to address these issues are urgently needed. A variety of popular weight-loss diets that restrict food choices may result in reduced energy intake and short term weight loss in individuals but most do not have trial evidence of long-term effectiveness and nutritional adequacy and therefore cannot be recommended for populations.
Probable etiological factors
Home and school environments that promote healthy food and activity choices for children (protective). Despite the obvious importance of the roles that parents and home environments play on children’s eating and physical activity behaviours, there is very little hard evidence available to support this view. It appears that access and exposure to a range of fruits and vegetables in the home is important for the development of preferences for these foods and that parental knowledge, attitudes and behaviours related to healthy diet and physical activity are important in creating role models (11). More data are available on the impact of the school environment on nutrition knowledge, on eating patterns and physical activity at school, and on sedentary behaviours at home. Some studies (12), but not all, have shown an effect of school-based interventions on obesity prevention. While more research is clearly needed to increase the evidence base in both these areas, supportive home and school environments were rated as a probable etiological influence on obesity.
Heavy marketing of fast-food outlets and energy-dense, micronutrient-poor foods and beverages (causative). Part of the consistent, strong relationships between television viewing and obesity in children may relate to the food advertising to which they are exposed (13-15). Fast-food restaurants, and foods and beverages that are usually classified under the “eat least” category in dietary guidelines are among the most heavily marketed products, especially on television. Young children are often the target group for the advertising of these products because they have a significant influence on the foods bought by parents (16). The huge expenditure on marketing fast-foods and other “eat least” choices (US$ 11 billion in the United States alone in 1997) was considered to be a key factor in the increased consumption of food prepared outside the home in general and of energy-dense, micronutrient-poor foods in particular. Young children are unable to distinguish programme content from the persuasive intent of advertisements. The evidence that the heavy marketing of these foods and beverages to young children causes obesity is not unequivocal. Nevertheless, the Consultation considered that there is sufficient indirect evidence to warrant this practice being placed in the “probable” category and thus becoming a potential target for interventions (15-18).
A high intake of sugars-sweetened beverages (causative). Diets that are proportionally low in fat will be proportionally higher in carbohydrate (including a variable amount of sugars) and are associated with protection against unhealthy weight gain, although a high intake of free sugars in beverages probably promotes weight gain. The physiological effects of energy intake on satiation and satiety appear to be quite different for energy in solid foods as opposed to energy in fluids. Possibly because of reduced gastric distension and faster transit times, the energy contained in fluids is less well “detected” by the body and subsequent food intake is poorly adjusted to account for the energy taken in through beverages (19). This is supported by data from cross-sectional, longitudinal, and cross-over studies (20-22). The high and increasing consumption of sugars-sweetened drinks by children in many countries is of serious concern. It has been estimated that each additional can or glass of sugars-sweetened drink that they consume every day increases the risk of becoming obese by 60% (19). Most of the evidence relates to soda drinks but many fruit drinks and cordials are equally energy-dense and may promote weight gain if drunk in large quantities. Overall, the evidence implicating a high intake of sugars-sweetened drinks in promoting weight gain was considered moderately strong.
Adverse socioeconomic conditions, especially for women in high-income countries (causative). Classically the pattern of the progression of obesity through a population starts with middle-aged women in high-income groups but as the epidemic progresses, obesity becomes more common in people (especially women) in lower socioeconomic status groups. The relationship may even be bi-directional, setting up a vicious cycle (i.e. lower socioeconomic status promotes obesity, and obese people are more likely to end up in groups with low socioeconomic status). The mechanisms by which socioeconomic status influences food and activity patterns are probably multiple and need elucidation. However, people living in circumstances of low socioeconomic status may be more at the mercy of the obesogenic environment because their eating and activity behaviours are more likely to be the “default choices” on offer. The evidence for an effect of low socioeconomic status on predisposing people to obesity is consistent (in higher income countries) across a number of cross-sectional and longitudinal studies (23), and was thus rated as a “probable” cause of increased risk of obesity.
Breastfeeding (protective). Breastfeeding as a protective factor against weight gain has been examined in at least 20 studies involving nearly 40 000 subjects. Five studies (including the two largest) found a protective effect, two found that breastfeeding predicted obesity, and the remainder found no relationships. There are probably multiple effects of confounding in these studies; however, the reduction in the risk of developing obesity observed in the two largest studies was substantial (20-37%). Promoting breastfeeding has many benefits, the prevention of childhood obesity probably being one of them.
Possible etiological factors
Several other factors were defined as “possible” protective or causative in the etiology of unhealthy weight gain.
Low-glycaemic foods have been proposed as a potential protective factor against weight gain and there are some early studies that support this hypothesis. More clinical trials are, however, needed to establish the association with greater certainty.
Large portion sizes are a possible causative factor for unhealthy weight gain (24). The marketing of “supersize” portions, particularly in fast-food outlets, is now common practice in many countries. There is some evidence that people poorly estimate portion sizes and that subsequent energy compensation for a large meal is incomplete and therefore is likely to lead to overconsumption.
In many countries, there has been a steady increase in the proportion of food eaten that is prepared outside the home. In the United States, the energy, total fat, saturated fat, cholesterol and sodium content of foods prepared outside the home is significantly higher than that of home-prepared food. People in the United States who tend to eat in restaurants have a higher BMI than those who tend to eat at home (25).
Certain psychological parameters of eating patterns may influence the risk of obesity. The “flexible restraint” pattern is associated with lower risk of weight gain, whereas the “rigid restraint/periodic disinhibition” pattern is associated with a higher risk.
Several other factors were also considered but the evidence was not thought to be strong enough to warrant defining them as protective or causative. Studies have not shown consistent associations between alcohol intake and obesity despite the high energy density of the nutrient (7 kcal/g). There are probably many confounding factors that influence the association. While a high eating frequency has been shown in some studies to have a negative relationship with energy intake and weight gain, the types of foods readily available as snack foods are often high in fat and a high consumption of foods of this type might predispose people to weight gain. The evidence regarding the impact of early nutrition on subsequent obesity is also mixed, with some studies showing relationships for high and low birth weights.
5. Population nutrient intake goals for preventing diet-related chronic diseases:
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