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Throwdown, round 2: plant vs. animal protein for type 2 diabetes

In round one of this fight, an earlier study showed that a diet with proteinrich plants didn't provide a metabolic advantage. Round two explores metabolic impacts from a different angle: amino acid composition of plant versus animal protein

Study under review: Comparison of the effects of diets high in animal or plant protein on metabolic and cardiovascular markers in type 2 diabetes: A randomized clinical trial

Introduction

The optimal diet for treating metabolic diseases like obesity and type 2 diabetes remains controversial, with evidence suggesting a variety of nutritional strategies that can be effective so long as people adhere to them. However, some interventions may be more effective than others. One strategy that’s gaining steam involves eating more protein.

Even though the term “high-protein diet” varies in definition from study to study, evidence to date[1] supports the idea that eating more protein than the recommended daily allowance of 0.8 grams per kilogram bodyweight results in less hunger and a reduced appetite, increased energy expenditure, and a preservation or increase of lean body mass. A recent meta-analysis[2] of 74 randomized controlled trials showed that eating a higher protein diet (27% vs. 18% of calories on average) significantly reduced several cardiometabolic risk factors, including body weight, BMI, waist circumference, blood pressure, triglycerides, and fasting insulin, while also significantly increasing HDL-cholesterol and satiety. A strong body of evidence supports the claim that a high-protein diet can facilitate dietary adherence, health improvement, and long-term fat loss.

But it’s possible that not all protein is the same when it comes to health. A meta-analysis[3] of observational research totaling over half a million people suggests that animal protein is associated with an increased risk of developing type 2 diabetes, while plant protein is not. However, plants are bundles of fiber and bioactive compounds that could explain the risk difference, and people who eat more veggies typically[4] have[5] a healthier lifestyle. These confounders make it impossible to relate differences in type 2 diabetes risk to differences in plant and animal protein, per se.

In general, animal proteins are higher in branched-chain amino acid content (BCAAs; leucine, isoleucine, and valine) and sulfurous amino acids (methionine and cysteine) than plant proteins. The role that these amino acids play in the pathology of type 2 diabetes is contested. Dietary methionine restriction has been shown in animals[6] to increase insulin sensitivity. Similarly, BCAAs, especially leucine[7], are potent stimulators of the mTOR pathway, and chronic stimulation of mTOR has been implicated[8] in insulin resistance. Some evidence suggests[9] that the metabolism of BCAAs is disrupted in people with type 2 diabetes, resulting in an exacerbation of insulin resistance (as shown in Figure 1), and dietary intake of BCAAs has been associated with the risk of developing type 2 diabetes.

Figure 1: Role of BCAAs and mTOR in type-2 diabetes

Adapted from: Lynch & Adams. Nat Rev Endocrinol. 2014 Dec.

On the other hand, animal studies suggest[10] that mTOR activation in the brain reduces food intake and bodyweight. These effects and brain mTOR signaling are brought about[11] through increased dietary leucine intake. Moreover, BCAA consumption, especially[12] isoleucine[13], has been shown to have beneficial effects[14] on glucose metabolism.

To investigate how the amino acid composition of a diet influences health outcomes in people with type 2 diabetes, the study under review examined the effects of a high-protein diet containing predominantly either animal or plant protein on insulin sensitivity in participants with type 2 diabetes.

High-protein diets are well established to promote fat loss and health improvement in people with obesity and type 2 diabetes. How differences in the amino acid composition of plant and animal proteins may affect health remain less well investigated. The study under review compared the effects of a high-protein diet containing mostly animal protein or plant protein in people with type 2 diabetes.

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