What happens in our bodies after we eat animal and plant proteins? How do they affect our metabolism?

A review of controlled clinical trials comparing the effects of animal and plant proteins on metabolic health, analysing energy expenditure, glycaemia, insulinemia and lipemia after a meal, found very different characteristics. Animal proteins have a more complete amino acid profile, better digestibility, and higher bioavailability than plant proteins, and these differences are reflected in metabolism, with different short- and long-term effects on health.

This new review, published in Food Science and Nutrition, examined the effects of various proteins on thermogenesis, satiety, and insulin sensitivity in both the short and long term—the body’s ability to respond to insulin and utilise glucose effectively for energy. They compared the effects of animal and plant proteins on energy expenditure, lipemia (the presence of fats in the blood, such as cholesterol and triglycerides), glycaemia (the level of glucose in the blood), and insulinemia (the level of insulin in the blood).

Why animal proteins are superior

Animal proteins increase energy expenditure and diet-induced thermogenesis, the process by which the body generates heat after eating, and reduce blood glucose and lipemia compared to plant proteins due to their greater ability to stimulate insulin secretion and slow gastric emptying.

The different amino acid composition and bioavailability of animal and plant proteins could explain these differences. Plant proteins lack essential amino acids such as lysine, leucine and methionine and are less digestible (75-80%) than animal proteins (90-95%). In addition, plant proteins are less bioavailable due to the rigid walls of the plant cells in which they are encased, making it more difficult for digestive enzymes to access them and use them for the body’s functions. For these reasons, plant proteins are considered of lower quality than animal proteins, which are often referred to as “noble proteins” due to their superior biological value.

As a result of these differences, animal proteins can induce a higher energy expenditure than plant proteins, thanks to the greater thermogenic effect of some essential amino acids such as leucine, suggesting that animal-based protein foods, such as meat, may contribute to weight loss and the maintenance of the body’s lean muscle mass. Leucine, in particular, is a fundamental regulator of protein synthesis, with significant implications for muscle growth, recovery, ageing, and overall health. A diet rich in leucine-containing proteins, such as those of animal origin, can effectively support these processes, especially when combined with exercise.

The rate of protein oxidation, the process by which proteins are attacked by damaging free radicals, altering their structure and function, was also found to be lower in a meat meal and higher in a soy meal, demonstrating that animal proteins are not easily damaged and do not lose their functionality, protecting them from inflammation and its negative health consequences.

Metabolic benefits of animal protein

Animal and plant proteins also affect insulin secretion and glucose absorption after meals because they are digested and absorbed at different rates. Animal proteins are digested more quickly and release bioactive amino acids, such as valine, leucine, and isoleucine, into the blood. This improves insulin sensitivity, leading to short-term reductions in blood glucose levels, a condition associated with improved metabolic health that reduces the risk of developing type 2 diabetes, obesity, and cardiovascular disease. In contrast, plant proteins stay in the stomach longer and are digested more slowly, resulting in higher blood glucose levels. Maintaining blood glucose levels within a healthy range is crucial for protecting cardiovascular, metabolic, and overall health. Animal proteins have also been shown to be more effective than plant proteins in reducing blood levels of triglycerides and free fatty acids after a meal.

The hypothesised mechanisms of action are diverse, and further studies are needed to fully understand the more complex responses. Based on the results of this review, animal proteins appear to have metabolic advantages over plant proteins after ingestion. It is hoped that further research will continue to confirm and clarify the mechanisms that explain these differences. Future studies may be useful in planning optimal diets for weight control and preventing or managing chronic diseases such as diabetes and cardiovascular disease.