The mechanisms that underlie obesity are not fully understood and controversy surrounds the biochemical changes that accompany long-term weight gain. Obesity is a known risk factor for cardiovascular disease and one of the possible effects of excessive weight gain is an alteration in the homeostatic mechanisms controlling lipid levels. Achieving a negative energy balance by restriction of energy intake produces at least short term weight loss in some individuals, and this weight loss also favourably affects the circulating lipoprotein levels. However, it is unclear whether energy restriction, weight loss or reductions in particular macronutrients are the causative factor in the attenuation of hyperlidaemia. While the current consensus dogmatically clings to the claim that saturated fat and cholesterol are to blame for elevations in lipid levels, growing evidence in the scientific research suggests that carbohydrates, processed fats and excess energy are the causative factors.
The effects of fat, carbohydrate and energy restriction on circulating lipid levels has been investigated by researchers1 in hypercholesterolaemic subjects fed tightly controlled diet types. In the small study, 11 subjects consumed one of 4 types of diet for 4 weeks, separated by a 6 week washout period. Each diet contained the same amount of protein. Diet 1 (typical fat) consisted of 35% fat, 50% carbohydrate and meal 2 (low fat) consisted of 15% fat and 70% carbohydrate both at 100% of the individuals energy requirement. Diet 3 (low energy) and 4 (low fat and low energy) contained the same absolute fat levels as diet 1 and 2 respectively, but had calorie restriction of the carbohydrate content. The body weight of the subject consuming the low energy and low energy low fat diets decreased (3.6 and 3.2 kg, respectively) however, no control was performed to assess the reason for the weight loss.
In terms of the effects on lipoproteins, the low fat and low fat low energy diets caused a significant reduction in LDL cholesterol levels (8.2% and 8.0%, respectively). All diets increased HDL cholesterol levels relative to the traditional fat diet. Interestingly, the low fat diet increased triglyceride levels compared to the traditional fat diet, supporting previous data (here) that dietary carbohydrate is a significant cause of raised triglyceride levels. This is further supported by the fact that the low fat low energy diet had an increase in triglycerides whereas the low energy diet had a decrease in triglycerides, compared to the control. These results may suggest that it is not fat reduction that causes favourable shifts in LDL lipoprotein levels, but energy restriction or weight loss. These results also suggest that triglyceride levels are significantly affected by carbohydrate levels in the diet.
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