Abstract
Macronutrient composition of the diet influences the development of obesity and insulin resistance. The aim of this study was to assess the role of dietary fat quantity and fatty acid composition in body composition, insulin resistance, and insulin secretion. An open parallel randomized trial design was used. Overweight participants (n = 75) were randomized to follow a low-fat vegan (n = 38) or control diet (n = 37) for 16 weeks. Dual X-ray absorptiometry was used to measure body composition. Insulin resistance was assessed with the Homeostasis Model Assessment (HOMA-IR) index. Insulin secretion was assessed after stimulation with a liquid breakfast (Boost Plus, Nestle, Vevey, Switzerland). Self-reported 3-day diet records were used to assess dietary intake. A linear regression model was used to test the relationship between fat intake and body composition, insulin resistance, and insulin secretion. Changes in fat intake expressed as percent of total energy consumed correlated positively with changes in fat mass (r = 0.52; p < 0.001; and 0.347; p = 0.006, respectively), even after adjustment for changes in body-mass index (BMI) and energy intake (0.33; p = 0.01). Decreased intakes of C18:0 (r = 0.37, p = 0.004) and CLA-trans-10-cis12 (r = 0.40, p = 0.002), but increased intake of C18:2 (r = -0.40, p = 0.002) and C18:3 (p = -0.36, p = 0.006), were associated with a decrease in HOMA-IR, independent on changes in BMI and energy intake. The main fatty acids associated with changes in fasting insulin secretion were C12:0 (r = -0.31, p = 0.03), and TRANS 16:1 (r = -0.33, p = 0.02), both independent on changes in BMI and energy intake. Our findings demonstrate that, in the context of a low-fat vegan diet, decreased intake of saturated and trans fats and increased relative content of polyunsaturated fatty acids, particularly linoleic and α-linolenic acids, are associated with decreased fat mass and insulin resistance, and enhanced insulin secretion.
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Key Points
Several observational studies have assessed fatty acid intake in people consuming plant-based diets. Studies have shown that participants on vegan diets had a lower mean intake of saturated fatty acids and a much higher intake of linoleic acid, compared with omnivores. Similarly, the European Prospective Investigation into Cancer and Nutrition-Oxford study showed that individuals on vegan diets had very low intake of saturated fat and high intake of polyunsaturated fatty acids, compared with omnivores, pesco-vegetarians, and lacto-ovo-vegetarians. It has been shown previously that a plant-based diet increases the proportion of linoleic acid (C18:2) in serum phospholipids, and that this increase was related to increased insulin sensitivity in patients with type 2 diabetes. The overall aim of this 16-week randomized clinical trial was to assess the effect of diet changes on body weight and metabolism. The role of specific dietary fats on body composition, insulin resistance, and insulin secretion in overweight individuals with no history of diabetes was assessed. The study demonstrated that, in overweight individuals, both fat quantity and fat quality were associated with changes in body weight, body composition, insulin resistance, and insulin secretion. Decreased intake of saturated, trans, or total fat was associated with decreased fat mass. Changes in fatty acid composition of the diet were associated with changes in insulin resistance and insulin secretion. Decreased intakes of C18:0 and CLA-trans-10-cis12, but increased intake of C18:2 and C18:3, were associated with a decrease in insulin resistance, independent on changes in BMI and independent on changes in energy intake. The study demonstrated an inverse relationship between relative intakes of linoleic (C18:2) and alpha-linolenic (C18:3 n3) acids and insulin resistance. The relative consumption of both of these fatty acids as a proportion of total fat significantly increased on the vegan diet, and insulin resistance fell proportionately. The results also show an inverse association between intake of EPA, DHA and DPA, and postprandial insulin secretion.
The vegan diet resulted in a significant decrease in fat and protein intake and an increase in carbohydrate consumption. The changes in macronutrient intake were associated with changes in body weight, body composition, and insulin resistance. Furthermore, changes in micronutrient ntake may have influenced lipoprotein composition, transport, and metabolism. Not only was a decrease in fat consumption associated with a decrease in percent of body fat, but limiting the intake of saturated and trans fats and increasing the relative content of polyunsaturated fatty acids, particularly linoleic and alpha-linolenic acids, was associated with increased insulin sensitivity and enhanced insulin secretion. Different types of dietary fats not only have different short-term metabolic effects, but also different associations with total mortality. In the frame of a 16-week randomized controlled study, decreased fat intake, particularly saturated and trans fat, was associated with decreased fat mass. Decreased intakes of C18:0 and CLA-trans-10-cis12, and increased intake of C18:2 and C18:3, were associated with a decrease in insulin resistance, independent on changes in BMI and independent on changes in energy intake. The main fatty acids associated with unfavorable changes in fasting insulin secretion were C12:0 and TRANS 16:1, both independent on changes in BMI and independent on changes in energy intake. Selecting foods so as to limit the intake of saturated and trans fats and increase the relative content of polyunsaturated fatty acids, particularly linoleic and alpha-linolenic acids, may be a useful strategy for metabolic health.