Evidence Linking Fat To Obesity

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Cross-sectional evidence from studies linking intake data to degree of obesity clearly suggests that dietary fat is associated with overweight. In a recent review, Lissner and Heitmann (18) found that in more than 80% of the cross-sectional studies reviewed, dietary fat intake was associated with obesity. The diet of obese subjects has been found to contain 5-8% more fat than the diet of the normal weight control groups (19), and this is likely to be a conservative estimate if obesity-related underreporting of fat occurs. Furthermore, most experimental studies provide evidence that, compared to a covert low fat diet, spontaneous energy intake is

Table 10.1 Evidence linking fat intake to obesity

Normal Moderate overweight Obesity

^H Reported I I Measured

Figure 10.2 Reported and measured energy from protein among normal weight (BMI < 25), moderately overweight (BMI 25-30) and obese (BMI > 30) Danish men and women

Normal Moderate overweight Obesity

^H Reported I I Measured

Figure 10.2 Reported and measured energy from protein among normal weight (BMI < 25), moderately overweight (BMI 25-30) and obese (BMI > 30) Danish men and women increased following ingestion of a high fat diet (20-22). In addition, a number of studies have suggested that appetite control is dissociated from fat since energy intake of a subsequent meal was not suppressed by adding up to 60 g fat (2.3 MJ) to the previous meal (for review see Astrup and Raben (19)). Likewise, one study demonstrated that 2-year maintenance of weight loss was more successful on an ad libitum low fat diet than on calorie counting (23). Finally, randomized dietary fat intervention studies consistently show that weight is lost on a calorically unrestricted low fat diet, although rates of weight loss per day are modest (ranges between 17 g and 37 g per day) (18).

The specific mechanism for dietary fat in obesity development is generally believed to depend on passive over-consumption on the high fat diet, due mainly to its high energy density and also palatabil-ity related factors. However, the effect is likely to vary under different circumstances. For instance, fat intake has been reported to predict weight gain in sedentary, but not inactive women, suggesting that certain subgroups of the population may be particularly sensitive to over-consumption of a high fat diet (24). The hypothesis that genetic factors determine the identity of fat-sensitive individuals will be discussed in more detail later.

A possible energy independent effect of the fat related to a lower satiating power has also been suggested (25,26). Oscai et al. (27) demonstrated

Table 10.1 Evidence linking fat intake to obesity

The evidence for:

• The majority of cross-sectional surveys display a positive association between fat intake and obesity.

• These associations are likely to be underestimated due to social desirability biases

• Experimental studies suggest that spontaneous energy intake is increased following ingestion of a high fat meal compared to a low fat meal.

• Randomized short-term dietary intervention trials show weight loss, although modest, on calorically unrestricted low fat diets.

• Animal experiments show obesity development in rats fed equi-caloric high fat diets, compared to low fat diets

The evidence against:

• Secular trends for fat intake and obesity are in opposite directions in most countries.

• Inconsistent results from prospective observational studies linking fat intake to subsequent weight gain.

• No long-term clinical trials have supported promotion of weight gain from high fat hyperphagia.

• Experimental studies with equalization of energy density suggest the fat content per se is unrelated to subsequent energy intake.

that rats developed obesity when fed fat-rich, but not low fat equi-caloric diets. Other mechanisms include a particular storage preference (28) and/or a lower thermic effect of fat compared to carbohydrate (29) or protein. Proposed mechanisms linking fat to obesity development are given in Table 10.1 (29).

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