The first issue of the Journal of Nutrition for 2015 has just published another article that adds to the body of evidence supporting the efficacy of a lower carbohydrate diets in reducing adiposity and improving blood sugar control and insulin function.
In the introduction to their article, the authors of this study argue that carbohydrate is the most obvious macronutrient implicated in the fight against type 2 diabetes and insulin resistance, as these conditions essentially reflect a degree of carbohydrate intolerance. Importantly they also highlight that an individual’s propensity to develop blood sugar dysregulation in response to their carbohydrate intake may be dictated largely by their genetic phenotype. This is to say that individuals who tend to display higher insulin responses to a given carbohydrate load are more likely to be sensitive to the stimulatory effects of dietary carbohydrate on fat deposition. Similarly it is theorised that the same people may respond more favourably to carbohydrate restriction.
Ethnicity is a variable that is known to affect diabetes risk, with African Americans exhibiting a higher risk of type 2 diabetes compared with their European counterparts. As such, there is intense interest whether a lower carbohydrate intake in such at risk populations may reduce the risk of developing diabetes.
For this reason, this latest study by researchers from University of Alabama sought to compare the effects of a ‘lower’-carb and ‘higher’-fat diet in three different groups of people; namely European Americans, African Americans and women with polycystic ovary syndrome (PCOS). Compared with their European counterparts, African Americans characteristically display a greater insulin response to glucose, which underlies their relatively high rates of type 2 diabetes. The most basic aim of the study was to determine if restriction of dietary carbohydrate would be beneficial for body composition and associated metabolic health in a group of overweight/obese man and women.
The three different groups of people were fed 1 of 2 diets. The lower-fat diet provided 55%, 18%, and 27% of energy from carbohydrate, protein and fat, respectively, while the lower-carbohydrate diet provided 43%, 18% and 39% respectively. Astute readers may recognise that these macronutrient ratios are not representative of a traditional very low carbohydrate diet, where carbohydrate and fat intake are lower and higher respectively. Importantly, both diets were matched for protein content so as to avoid potential confounding effects of altered protein intake on body composition and metabolic outcomes. While each of the European and African Americans did not have diabetes, their BMI (i.e. 25-45) meant that they were naturally at higher risk of developing type 2 diabetes. The European American and African American subjects received the diet for a total of 16 weeks, with the first 8 weeks consisting of a eucaloric intake and the last 8 weeks hypocaloric (i.e. 1000kcal less per day). The diets received by 30 women with PCOS were more or less identical, with the lower carb group receiving 41%, 19%, and 40%, of energy intake from carbohydrate, protein and fat respectively, while the lower fat group received 55%, 18%, and 27%, respectively. The diets both ran for 8 weeks only.
Interestingly and in accordance with the authors theory, African American subjects experienced a much greater reduction in fat mass after the lower-carbohydrate (i.e. 6.2 vs 2.9kg). In contrast the European Americans showed no significant difference in fat loss between the lower-carb and lower-fat diets.
Consistent with similar previous studies, women with PCOS receiving the lower-carbohydrate diet exhibited low fasting insulin and blood glucose scores together with increased insulin sensitivity. Concerning fat loss, PCOS women lost both intra-abdominal adipose tissue and intermuscular fat. However, in the lower-fat arm, women actually lost muscle mass.
The main take away from the study was that diets lower in carbohydrate intake can have beneficial effects on body composition, fat distribution and glucose metabolism in at risk populations such as obese/overweight African Americans and women with PCOS. As such, the consumption of large amounts of processed carbohydrate-containing foods in these population may be one of the major factors leading to both obesity and metabolic dysfunction.
Gower BA & Goss AM. A lower-carbohydrate, higher-fat diet reduces abdominal and intermuscular fat and increases insulin sensitivity in adults at risk of type 2 diabetes. The Journal of Nutrition. 2015;1S-7S.