Current medical recommendations are to limit fat intake to around 30 % of calorie intake to reduce the risk of cardiovascular disease. However, the association between high fat diets and disease is controversial. That being said, the nutritional literature does contain examples of seemingly well designed studies demonstrating such an association. For example, in a recent study published in the journal of Nutrition1, researchers assessed the impact of a dietary fat on the risk of developing insulin resistance, obesity, β-cell dysfunction and metabolic syndrome. Food intakes were assessed using food frequency questionnaires and insulin function and body fat percentage were measured using an oral glucose tolerance test and dual energy absorptiometry, respectively. The results showed that increasing fat intake was associated with increasing adiposity and reduced insulin sensitivity. Compared to the low fat (<20 %) group, the high fat (>35 %) group was fatter, had more trunk fat and showed poorer insulin sensitivity.
Other studies have found similar associations between fat intake and body weight and insulin sensitivity. But is the association proof of a cause and effect or is there more to this than meets the eye? The Masaai and Eskimo tribes eat very high fat diet composed almost exclusively of animals fats, and yet do not suffer from obesity, cardiovascular disease or metabolic syndrome. In addition, it is known that low carbohydrate diets containing a high content of fat are able to cause weight loss, without the need for calorie restriction (here). It is therefore likely that confounding variables are the causative agents in the association between fat intake and metabolic syndrome. For example, those that eat very high fat diets on the whole tend to eat low quality foods, as modern Western foods are generally high in both fat, sugar and processed foods. In this respect, it is likely the combination of fat with sugar, and the low quality of the food they are contained within, that are the drivers of the association.
Another interesting aspect to this study was the fact that adjustment for physical activity did not alter the association between dietary fat and parameters of the metabolic syndrome. This would tend to suggest that exercise levels are not able to influence the causative factor in the association between high fat diets and disease. If we assume that the causative agent is indeed the high fat high sugar Western diet, then the observation that physical activity is not beneficial makes sense based on other research. This is because exercise is known to be ineffective at reversing the metabolic dysfunction associated with poor quality Western diets. The authors of this study also found that using total energy rather than total fat intake yielded similar associations. Further, the fructose intake of the individuals was associated with the total energy intake. As fructose is a known driver of metabolic syndrome, this points strongly to fructose being the causative agent in the associations shown in this study.
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