|Posted on December 13, 2020 at 6:15 PM|
Background and objectives
A plant-based diet is an effective strategy in the treatment of obesity. In this 16-week randomized clinical trial, we tested the effect of a plant-based diet on body composition and insulin resistance. As a part of this trial, we investigated the role of plant protein on these outcomes.
Subjects and methods
Overweight participants (n = 75) were randomized to follow a plant-based (n = 38) or a control diet (n = 37). Dual X-ray Absorptiometry assessed body composition, Homeostasis Model Assessment (HOMA-IR) assessed insulin resistance, and a linear regression model was used to test the relationship between protein intake, body composition, and insulin resistance.
The plant-based vegan diet proved to be superior to the control diet in improving body weight, fat mass, and insulin resistance markers. Only the vegan group showed significant reductions in body weight (treatment effect −6.5 [95% CI −8.9 to −4.1] kg; Gxt, p < 0.001), fat mass (treatment effect −4.3 [95% CI −5.4 to −3.2] kg; Gxt, p < 0.001), and HOMA-IR (treatment effect −1.0 [95% CI −1.2 to −0.8]; Gxt, p = 0.004). The decrease in fat mass was associated with an increased intake of plant protein and decreased intake of animal protein (r = -0.30, p = 0.011; and r = +0.39, p = 0.001, respectively). In particular, decreased % leucine intake was associated with a decrease in fat mass (r = +0.40; p < 0.001), in both unadjusted and adjusted models for changes in BMI and energy intake. In addition, decreased % histidine intake was associated with a decrease in insulin resistance (r = +0.38; p = 0.003), also independent of changes in BMI and energy intake.
These findings provide evidence that plant protein, as a part of a plant-based diet, and the resulting limitation of leucine and histidine intake are associated with improvements in body composition and reductions in both body weight and insulin resistance.
Suboptimal nutrition is a major cause of obesity, chronic disease, and premature death across the nation and worldwide1,2. Certain dietary habits, such as high intakes of sodium and processed meat products and low intakes of fruits and vegetables, are associated with 45.5% of cardio-metabolic deaths in the United States3. Fortunately, research has shown a plant-based vegan diet to be beneficial in improving nutrient intake4, decreasing all-cause mortality, and decreasing risk of obesity, type 2 diabetes, and coronary heart disease5.
A plant-based vegan diet excludes all animal products and is centered around grains, legumes, vegetables, and fruits. While adequate in macro and micronutrients6, people sometimes question the ability to reach protein requirements on a plant-based vegan diet. A sufficient protein intake is necessary to supply nitrogen and amino acids to our cells to ensure the growth and maintenance of the protein pool in our bodies7. However, a diet based entirely on plants provides all essential amino acids and an adequate quantity of overall protein, even without the use of special food combinations6. Further, the consumption of exclusively plant proteins has been associated with reduction of the concentrations of blood lipids8,9,10,11, obesity12, and cardiovascular disease13,14,15.
The specific composition of dietary protein has been shown to influence the balance of glucagon and insulin activity14, which may play a role in body composition and insulin resistance12. A high intake of branched chain amino acids (leucine, isoleucine, and valine) can increase insulin resistance16. In addition, dietary restriction of sulfur containing amino acids (methionine and cysteine), is associated with a reduction in body weight, adiposity and metabolic changes in both adipose and liver tissues, which enhance insulin sensitivity and energy expenditure17. Plant protein low in sulfur also reduces blood lipids, homocysteine, and blood pressure18,19. Furthermore, low protein diets are also associated with increased life span, especially if the consumed protein is plant derived20.
In this secondary analysis of data from a 16-week randomized clinical trial21, we explore the effects of plant protein, as part of a plant-based diet, on weight control, body composition, and insulin resistance in overweight individuals.
This study demonstrated that the quality and quantity of dietary protein from a plant-based vegan diet are associated with improvements in body composition, body weight, and insulin resistance in overweight individuals. A decreased intake of animal protein and an increased intake of plant protein were associated with a decrease in fat mass, by 1.45 and 0.88 kg respectively. Exchanging plant protein for animal protein explains more than half of the reduction in fat mass in the vegan group (2.33 out of 4.3 kg). A large portion of fat mass reduction may be explained by the amino acid composition of plant protein, specifically by decreased leucine intake, which was associated with a decrease in fat mass by 0.82 kg, independent of changes in BMI and energy intake. Additionally, decreased histidine intake was associated with a decrease in insulin resistance, also independent of changes in BMI and energy intake. Finally, decreased intakes of threonine, leucine, lysine, methionine, and tyrosine were each associated with a decrease in insulin resistance. However, these associations were mainly driven by weight loss.
Plant vs. animal protein in weight regulation, body composition, and insulin resistance
Multiple randomized controlled studies have established the effectiveness of plant-based diets for weight loss25,26. Plant-based diets have also been shown to decrease the risk of developing diabetes in additional prospective studies27. The specific role of plant protein in weight regulation and metabolic health is of particular interest. In a study focusing specifically on the association between protein sources and body weight regulation using data from the European Prospective Investigation into Cancer and Nutrition study, increases in body weight were positively correlated with an increased intake of animal protein, especially in women28. Similarly, in a 2011 observational study, increases in animal protein consumption were found to be positively correlated with increases in BMI, while increases in plant protein intake were negatively associated with changes in BMI29.
Dietary protein triggers release of both insulin and glucagon12. Specifically, a higher intake of essential amino acids can stimulate secretion of insulin and up-regulate insulin like growth factor 1 (IGF-1)12. Essential amino acids are found in greater abundance in animal protein, compared to plant protein. In contrast, a higher intake of non-essential amino acids is associated with down-regulation of insulin secretion and increased glucagon secretion, resulting in stimulation of gluconeogenesis, hepatic lipid oxidation, lipolysis and reduction in both IGF-1 and cholesterol synthesis. Hepatic lipid oxidation promotes appetite control and lowers the respiratory quotient, which may play a role in body weight reduction, and may further be supported by the thermogenic effect of glucagon. Human adipocyte express IGF-1 receptors, thus down-regulation of IGF-1 activity can also promote leanness12. Non-essential amino acids in plant protein promote higher net glucagon activity than an omnivorous diet, promoting weight loss and reduction of LDL-cholesterol12.
The role of specific amino acids in insulin resistance and weight regulation
A 2018 prospective study that included more than 1,200 adults, who were followed-up for a mean of 2.3 years, showed that higher intake of branched chain amino acids (BCAA), especially leucine, can increase insulin resistance. Participants in the highest tertile for leucine intake had a 75% higher risk of developing insulin resistance compared with people in the lowest tertile (OR 1.75; 95% CI 1.09–2.82)16.
Increased serum concentrations of BCAA have been associated with increased risk of type 2 diabetes and underlying metabolic abnormalities30,31. High serum BCAA levels activate the mammalian target of rapamycin complex 1 (mTORC1) signaling pathway, leading to inhibition of glucose transport in muscle and fat tissues16. Animal protein from meat and dairy products contains a high percent of leucine. Therefore, these foods may stimulate the mTORC1 pathway, thus contributing to insulin resistance, and obesity32.
Randomized controlled trials have shown that reduced dietary intake of BCAA promote weight loss, reduce adiposity, and improve glycemic control and metabolic health33,34. In our study, the vegan group consumed less than 75% of the control group’s daily grams per day of BCAA. Our data also show that reduced dietary intake of leucine, in particular, was associated with decreased fat mass and reduced insulin resistance.
Additionally, our results suggest that a decreased intake of histidine, leucine, threonine, lysine, methionine, and tyrosine were all associated with a decrease in HOMA, with histidine being the only one having a significant association independent on changes in BMI and energy intake. The vegan group reduced both its absolute and relative intake of all six of these amino acids. The significant decrease in the consumption of sulfur-containing amino acids, i.e. cysteine and methionine, in the vegan group, is of particular interest. Several studies have shown that diets restricting sulfur-containing amino acids have shown beneficial effects in the prevention of chronic diseases, including type 2 diabetes, cancer, and cardiovascular disease14,17. Dietary restriction of methionine and cysteine without caloric restriction has been associated with reductions in body weight, adiposity, blood levels of insulin, IGF-1, and glucose17, as well as reductions in cardiovascular risk factors including blood lipids, homocysteine, and blood pressure18,19. Our results suggest that reduced intake of methionine through a plant-based diet may correlate with a decrease in both body weight and insulin resistance.
Meeting and exceeding the recommended daily intake on a plant-based diet
Higher animal protein consumption has been associated with increased risk of metabolic disease and mortality. A 2015 study using data from NHANES II reported the link between protein intake and mortality in men and women. Subjects in the high-protein group (consuming 20% or more of daily calories as protein) had a 73-fold increase in risk of diabetes mortality and a 74% increase in relative risk of all-cause mortality20. Our data suggest that both the decreased intake of animal protein and the amino acid composition of the plant-based diet are associated with decreased body fat and reduced insulin resistance.
The United States Department of Agriculture recommends a minimum of 46 g of protein per day for women and 56 g per day for men35. In the current study, all participants in the vegan group exceeded the recommended daily intake of protein and of each individual amino acid. While animal protein is higher in essential amino acids, containing significant amounts of leucine, histidine, threonine, methionine and lysine, consumption of plant protein, which is higher in non-essential amino acids, offers clear metabolic benefits. People following a plant based diet still consume more than 100% of the recommended dietary intake of essential amino acids. The main plant sources of these amino acids are legumes, grains, and vegetables. For example, 2 servings of oatmeal made from 100 g of oats contain 102% of recommended daily intake of tyrosine36.