The impact of high-protein diets on weight, energy, and blood sugar regulation

Over the past two decades, there has been a marked increase in interest surrounding the role of dietary protein in enhancing health outcomes. This heightened attention is largely driven by growing evidence that links protein intake with a variety of metabolic benefits. A recent comprehensive review published in Frontiers in Endocrinology delves into the effects of dietary protein on energy intake, appetite regulation, and postprandial blood sugar levels, offering valuable insights into both the short-term and long-term impacts of protein consumption.

Immediate Effects of Protein Consumption

Current research suggests that high-protein diets play a significant role in promoting weight loss, reducing weight gain, and improving glycaemic control. One of the key benefits of protein intake is its ability to lower postprandial (after eating) blood sugar levels and suppress energy intake. These effects are thought to be linked to protein’s capacity to stimulate gastrointestinal (GI) hormones and other post-absorptive mechanisms. Additionally, protein consumption has been shown to reduce hunger and increase feelings of fullness, with these effects being dose-dependent in both healthy and obese individuals.

A notable meta-analysis has highlighted the ability of protein-rich diets to reduce hunger, lower food intake, and diminish the desire to eat, while simultaneously increasing feelings of fullness. This reduction in postprandial blood sugar levels is one of the mechanisms through which protein aids in appetite control. However, it is important to note that the benefits of increased protein consumption may only last for a period of six to twelve months.

Comparative studies on various protein sources have produced interesting findings. For example, preloads containing milk proteins such as casein or whey, as well as proteins from turkey, egg, tuna, or soy, have been evaluated for their effects on satiety and blood sugar regulation. Among these, whey protein has been found to have the most profound impact on reducing blood sugar levels, although some studies suggest that it might be less satiating than other proteins. Nevertheless, milk proteins have consistently demonstrated a stronger ability to lower blood glucose compared to proteins derived from peas, eggs, or fish.

The digestion of proteins and their by-products stimulates the release of GI hormones such as incretins, cholecystokinin (CCK), glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide 1 (GLP-1), and peptide tyrosine-tyrosine. These hormones are crucial in the regulation of postprandial blood sugar levels as they are transported to peripheral organs where they increase pyloric pressure and insulin secretion. This, in turn, helps to reduce blood sugar levels after meals.

The effects of different proteins on GI functions also vary, with whey protein generally showing greater efficacy than other sources. For instance, a study involving intraduodenal (directly into the duodenum) infusion of whey protein revealed a dose-dependent stimulation of plasma CCK, GLP-1, and pyloric pressures in both lean and obese participants. The high content of branched-chain amino acids, particularly isoleucine and leucine, in whey protein is thought to contribute significantly to its ability to reduce glycaemia and energy intake.

Medium- and Long-Term Effects

The impact of high-protein diets on weight loss has been extensively studied, particularly through ad libitum (eating at one’s own discretion) and energy-restricted dietary interventions. Regardless of the method employed, diets rich in protein have consistently led to greater weight loss compared to standard diets, especially in interventions lasting up to six months. Interestingly, ad libitum high-protein diets appear to be particularly effective.

A meta-analysis of 24 clinical trials observed a modest but significantly greater reduction in body weight and fat mass with an isocaloric energy-restricted high-protein diet compared to a standard diet over an average period of 12 weeks. Another meta-analysis, encompassing 74 trials, reported reductions in body weight, waist circumference, and body mass index (BMI) following six months on a high-protein diet.

However, the long-term effects of high-protein diets remain less clear, as studies extending beyond one year are limited. For instance, a meta-analysis of 15 clinical trials with interventions lasting at least 12 months found no clear benefits or detriments associated with high-protein intake concerning sustained weight loss. One 12-month study did observe modest improvements in weight maintenance with a high-protein diet compared to diets high in fat or carbohydrates, but the overall evidence remains inconclusive.

Plant Versus Animal Protein

The source of dietary protein—whether plant-based or animal-based—also plays a crucial role in determining health outcomes. Numerous studies have indicated that consuming large amounts of animal protein may increase the risk of developing diabetes, while plant proteins tend to offer protective effects.

Epidemiological studies consistently show a correlation between high animal protein intake, weight gain, and an elevated risk of obesity. For example, one study found that higher daily consumption of total and animal protein, predominantly sourced from chicken and red meat, was associated with significant weight gain over a period of 6.5 years.

In contrast, plant protein consumption has not been linked to either protective or adverse outcomes. In a study investigating the relationship between different protein sources and long-term weight gain across three cohorts, animal proteins were independently associated with weight gain over four years, whereas plant proteins were linked to weight loss.

Moreover, long-term consumption of animal proteins has been frequently associated with an increased risk of diabetes, while plant proteins appear to have neutral or even protective effects. The mechanisms underlying these differences are not entirely understood, but they may involve factors such as the glycaemic load, amino acid composition, and the insulinotropic (insulin-stimulating) effects of animal proteins, which could contribute to the beneficial effects observed with plant protein consumption.

Conclusion

There is substantial evidence supporting the role of increased protein intake in improving glycaemic control and promoting weight loss, particularly in the context of high-protein diets. However, the long-term health benefits of such diets are not yet fully understood, and more comprehensive, longitudinal studies involving diverse populations are needed to draw definitive conclusions. As research continues to evolve, it is clear that both the quantity and source of dietary protein are critical factors in determining the overall impact on health.