• Researchers report that taking in too much protein on a daily basis can produce heart health issues.
  • In particular, they note, consuming more than 22% of daily calories from protein can increase the risk of atherosclerosis.
  • Experts say some daily protein is necessary to maintain good overall health, but they advise that some of that protein should come from plant-based foods.

Consuming more than 22% of daily calories through proteins may increase the risk of atherosclerosis and even worsen the condition, according to a study completed at the University of Pittsburgh and published today in the journal Nature Metabolism.

To reach their conclusions, the researchers used human trials along with experiments involving mice and cells in Petri dishes. The human studies had a total of 23 participants.

Researchers used two different study set-ups for the human studies. Both involved asking participants to have two liquid meals – one with standard protein and one with high protein – about one to two weeks apart.

The first set-up involved 14 participants. The standard meal had 10% of total energy as protein, 17% as fat, and 73% as carbohydrates. The high protein meal consisted of 50% of energy from protein, 17% as fat, and 33% carbohydrates.

The second setup involved nine participants and was designed to mimic a “real-world” scenario. In this setup, the standard meal was representative of a person’s average protein intake and had 15% of total energy as protein, 35% as fat, and 50% as carbohydrates. The high protein meal represented the upper quartile of protein intake and consisted of 22% of energy from protein, 30% as fat, and 48% carbohydrates.

The scientists noted that leucine is an amino acid that contributes to the development and worsening of atherosclerosis. They found that higher dietary protein intake, specifically intake of more than 25 grams of protein per meal or 22% of daily energy requirements, led to higher leucine levels that activated a specific pathway in immune cells that is associated with atherosclerosis.

The second part of the study involved mice.

The researchers first created diets for the mice with graded protein contents that mimicked the average (15%) and high (22%) protein intake for a typical adult living in the United States.

They reported similar results to the human studies.

They added that the same pathway was activated in mice receiving more than 25 grams of protein per meal, or 22% of total energy intake, and that these mice were also more likely to promote atherosclerosis and cardiovascular disease.

The researchers conclude that high protein intake can contribute to atherogenesis.

The authors suggest that people approach high protein diets cautiously and that dietary guidelines are revised accordingly.

The authors note the recommendation from several organizations is that protein intake be about 11% of daily energy requirements to maintain nitrogen balance.

“High protein diets are very popular with the public, used for weight loss, bodybuilding, and an overall healthy lifestyle. For many years, reports in the research world using experiments in animal models showed that high protein diets can increase cardiovascular disease (also called atherosclerosis or hardening of the heart arteries), but no one knew why,” said Dr. Babak Razani, a professor of medicine at the University of Pittsburgh and one of the authors of the study. “In our previous 2020 paper, we used mouse models to define the mechanism for this by showing that protein ingestion activates macrophages, an immune cell that is a key driver of atherosclerosis, and we identified an important protein in the macrophages called mTOR that mediates this process.”

“Now, in the current study, we pin down the reasons why high protein diets are risky for our heart arteries,” Razani told Medical News Today. “First, we did several studies in humans to demonstrate that when people eat higher amounts of protein, our macrophages and the mTOR pathway get activated just like we found in animal models. Then, we identified, for the first time ever, the role of leucine – an amino acid highly enriched in animal-based proteins – as the driving force of atherosclerosis risk. Amino acids are the building blocks of proteins and there are 20 of them that make up any protein we ingest. But it is only leucine that is the bad actor in worsening the heart artery disease. We determined that eating meals containing about 22 percent kilocalories of protein is the threshold at which the protein and its leucine elevate risk.”

Two studies, one published in 2020 and one in 2023 came to different conclusions.

The first study, published in 2020, is the previous study by Razani and his colleagues that reported that high protein diets increase risk of cardiovascular disease.

The study published in 2023 found no statistical difference in cardiovascular outcomes between standard and high protein diets.

Even so, some cardiologists say they are moving away from recommending high protein diets and suggesting more plant-based and vegetable-based diets.

“This [new] study is too small to change anything,” said Dr. Stephen Tang, a cardiac electrophysiologist at Providence Saint John’s Health Center in California who was not involved in the research.

“I would not do anything different,” Tang told Medical News Today. “However, it does provide more evidence that high protein is not the way to go. Cardiologists are traditionally focused on cholesterol and high blood pressure – not protein. This study confirms that a plant-based diet is good for heart health.”

So, how much protein should included in a daily diet? How much is too much?

“While the results of the study are interesting, they don’t exactly reflect real world examples of over-consuming protein,” said Caroline Thomason, RD, CDCES, a dietitian based in Washington, D.C., who was not involved in the latest research.

“While it’s true that most western societies over-eat protein and under-eat nutrient dense plant-based foods like fruits, vegetables, and whole grains, protein as a food group is inherently satiating and quite hard to over-eat, especially in the context of a balanced diet,” Thomason told Medical News Today.

Another expert said there are ways to understand how this type of information can translate into everyday life.

“Whenever you eat excessive amounts of one nutrient, you end up limiting others, which can potentially cause health problems,” said Anne Danahy, RDN, a registered dietician nutritionist practicing in Arizona who was not involved in the latest research. “There’s good reason for the saying ‘everything in moderation.’ A healthy diet should provide balanced amounts of protein, fat, carbs, vitamins, and minerals.”

“Everyone’s protein needs are slightly different, so it’s best to talk to a dietitian who can assess your needs,” Danahy told Medical News Today. “However, I find for most healthy adults, a good goal is 20 to 30 grams of protein per meal, spread out evenly throughout the day.”

“Protein intake that exceeds this may not be used for muscle synthesis. It often ends up providing excess calories, and as this study suggests, it may cause more harm than good,” she added.

Danahy recommends eating more plant protein (from foods instead of supplements) for anyone looking to add extra protein to their diet.

“Several studies have associated high animal protein diets with an increased risk of cardiovascular disease,” she said. “Protein from animal sources may also contain saturated fats or other compounds that can boost inflammation and promote heart disease.”

“On the other hand, protein supplied by plant sources contains antioxidants, fiber, and other nutrients that can reduce the risk of heart disease,” Danahy concluded. “Nuts, seeds, legumes, and vegetables all contain protein and other health-promoting compounds.”

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