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Deep Dive: Are High Protein Diets Less Healthy Than We Thought?

Posted on 28 February 2024

It’s previously been suggested that the optimal diet for longevity should contain 10%-15% protein as a proportion of total calories, and that this protein should come mainly from plant sources. However, many people consume more protein than this, and the majority of this protein tends to come from animal sources, not plants. High protein diets are popular as a weight loss strategy and for building muscle. Research in animals suggests that a higher intake of animal proteins accelerates atherosclerosis – the formation in the arteries of fatty deposits that impede blood flow and lead to heart attacks and strokes. 

Does this hold true in humans? Some studies say yes, though these were observational in nature. This means that researchers looked at people who were already consuming high protein diets by choice and found they had higher cardiovascular mortality rates than those on lower protein diets. However, a meta-analysis published last year (in which researchers analyse data from many individual studies) looked at 14 prospective studies with 221,583 total participants, and did not find any significant relationship between high protein intake and cardiovascular risk.

So, whether high protein diets pose a risk to cardiovascular health is still an area of active research. Not much is understood about what protein actually does inside the body with respect to atherosclerosis, and whether it’s something we should be concerned about. Recently, researchers found a potential link between protein and atherosclerosis, at least in animals. They found that high protein diets could increase the activity of diseased white blood cells that form the bulk of the fatty plaque. In the present study, which was published in Nature Metabolism they set out to investigate this link further. There are quite a few steps to this study, so buckle up if you want the full story. Otherwise, here’s a summary of the key points:

  • When a small group of human participants ate a very high protein meal, white blood cells reacted in a way that would probably favour the development of atherosclerosis.
  • This was due to the activation of a molecule called mTOR, which controls many processes such as cell growth and division.
  • This also occurred in people eating a more standard western diet type meal when 22% of calories came from protein, but not when 15% of calories came from protein.
  • In cell culture experiments with human and mouse white cells, the amino acid leucine appeared to be the main contributor to the effects of protein, activating mTOR four times more than any other amino acid in human cells.
  • In mouse models of atherosclerosis, high protein diets worsened atherosclerosis, as did moderate protein diets with added leucine, but not moderate protein diets with other added amino acids.
  • Despite all of this, the limitations of this study makes it hard to draw solid conclusions about human health, but should encourage further studies that can provide more answers.
A figure summarising the key findings and propositions of the study – that high plasma leucine after consuming protein activates monocytes and macrophages through mTOR signalling, promoting atherosclerosis and increasing cardiovascular risk.
Identification of a leucine-mediated threshold effect governing macrophage mTOR signalling and cardiovascular risk

High protein meals activate human white cells involved in atherosclerosis

The first part of the study was done in humans, with the goal of studying how amino acid levels changed in the blood of participants after a high protein meal, as well as how this affected white blood cells involved in atherosclerosis. Researchers first gave participants a 500 Calorie liquid meal that was very high in protein (50% of the calories were from protein, a much higher level than anyone would ever consume unintentionally). They found that there was a measurable effect on their white blood cells when compared to those who consumed a 500 Calorie meal of which only 10% came from proteins. Specifically, high protein meals activated a signalling molecule called mTOR within a type of white blood cell called a monocyte/macrophage. Macrophages are more developed, functional versions of monocytes, but for the sake of simplicity we’ll just be referring to them all as monocytes from now on.

Why is this important for atherosclerosis? Let’s break it down. Monocytes are key drivers of atherosclerosis. When monocytes recognise cholesterol that has been trapped within the walls of the arteries, they ‘eat’ the cholesterol and degrade it. The problem is that some cholesterol becomes oxidised, which prevents it from being broken down by the monocytes. The monocytes gradually fill up with this ‘fatty junk’ and become something known as foam cells. Foam cells cause a lot of inflammation within the walls of the arteries, which unfortunately produces more oxidised cholesterol and attracts more monocytes.

OK, what about mTOR? If you’ve read this site for a while or follow longevity research in general, you’ve probably heard of it. mTOR, or Mammalian Target of Rapamycin, is a master regulator of cell growth, allowing cells to adjust their activity depending on how much energy in the form of nutrients is available to them. When there is a lot of energy available to a cell, mTOR activity increases, signalling that it is a good time for the cell to grow, divide and be more active in general. When energy is scarce, mTOR activity decreases, which signals that the cell should engage mechanisms that will help it survive until energy becomes available again. This includes recycling damaged cellular components, repairing DNA and suppressing inflammation. Because of this, higher mTOR activity is generally a bad thing when it comes to age-related diseases like atherosclerosis.

The implication, therefore, is that high protein intake could promote atherosclerosis by activating monocytes via mTOR signalling, accelerating the accumulation of oxidised cholesterol. However, since the high protein liquid given to participants was very unrepresentative of a standard diet, the researchers did another experiment in which participants were given a mix of real protein-containing foods that were liquified. These meals contained either 15% or 22% protein as a proportion of calories, and the protein came from both animal and plant sources in similar ratios to a typical western diet. While still not exactly representative of a standard meal, these were more representative than in the first experiment and contained an amount of protein within the range of what an average person might eat. In this experiment, mTOR was still activated in the monocytes of people eating the 22% protein meal, but not in those eating the 15% protein meal.

Leucine – the primary culprit?

Next, researchers set out to investigate which components of protein were responsible for activating mTOR signalling. All proteins are made out of building blocks called amino acids. Researchers looked at which amino acids increased the most in the blood of participants after consuming the high-protein meals. They then conducted lab tests with cultured human immune cells and singled out one amino acid – leucine – as being particularly good at activating mTOR. Leucine is an essential amino acid, meaning that it cannot be produced by the body and must be ingested. The researchers found that leucine activated mTOR about four times as well as any other amino acid applied at the same concentration. When researchers instead applied the amino acids to cells at the concentrations observed in participants’ blood, they still found leucine to be the main mTOR activator, despite not being the most concentrated amino acid.

Measurements of the ability of different amino acids to activate mTOR in cultured human macrophages, with leucine (Leu) in red. Researchers used fluorescent tags to measure where mTOR was within the cell, and how much this overlapped with the presence of amino acids (colocalization). More overlap means that the amino acid was better at activating mTOR.
Identification of a leucine-mediated threshold effect governing macrophage mTOR signalling and cardiovascular risk

What does high leucine do to mice?

Next, researchers wanted to see if a high protein diet actually promoted atherosclerosis. Since testing this in humans would be very challenging, they opted for a mouse model of atherosclerosis – the ApoE -/- mouse. These mice lack the ability to remove cholesterol from their blood and so develop atherosclerosis very quickly. A total of 46 mice were fed a western type diet containing either 7%, 21% or 46% protein as a proportion of calories. Sure enough, the mice fed the highest protein diet suffered from increased atherosclerotic plaque formation after 8 weeks when compared to the other two groups.

Cross sections of aortas from mice fed a low protein (LP), moderate protein (MP) or high protein (HP) western type diet, with cholesterol stained in red.
Identification of a leucine-mediated threshold effect governing macrophage mTOR signalling and cardiovascular risk

Finally, the researchers wanted to confirm the role of leucine. 67 mice were fed one of six western diets at random. These diets contained either a moderate or high amount of protein, but some of the moderate protein diets had added leucine to match the leucine content of the high-protein diet, while others had all other amino acids added apart from leucine. They found that mice fed moderate protein diets high in leucine developed significantly larger atheroslcerotic plaques when compared to those without added leucine, while those fed a moderate protein diet with other added amino acids did not. They also found that when fed a typical western diet, 22% protein as a proportion of calories seemed to be the threshold at which atherosclerosis progression started to increase.

Should humans be worried?

So, the general thrust of this paper is that high protein diets may increase the risk of atherosclerosis and that leucine is an important player, with its ability to activate monocytes through mTOR signalling being a plausible mechanism. However, there are a few reasons why we should be cautious when drawing conclusions about human health.

Firstly, the human part of the study only showed that mTOR activity increased with a high protein meal, not that this led to increased atherosclerosis in humans – that effect was only shown in mice. The number of human participants was fairly small (14 and 9 in the first and second experiments respectively) and they had an average BMI of 28, which is in the middle of the overweight range. While this doesn’t make the findings any less relevant to human health (overweight and obese people get more atherosclerosis after all) it’s important to keep in mind since obesity is associated with increased mTOR activity. Finally, while the meals the participants ate may have contained realistic levels of protein, they still weren’t very representative of a standard meal, and we also don’t know what happened to leucine or mTOR activity throughout the day as measurements were taken a few hours after the meal.

As for the mouse experiments, some of the groundwork tests used very small sample sizes. For example, when establishing how high protein meals affected blood amino acid concentrations and mTOR activity, as few as 3-6 mice were used. For the more important experiments (those comparing the effects of different diets on atherosclerosis), there were around 10-15 mice per group, which would generally be considered adequate, though more is always better.

We already have to be careful about findings in mice translating to humans, but it’s also worth pointing out that mouse models of atherosclerosis are genetically engineered to develop atherosclerosis at a rapid rate. While they are widely used as a model for humans because they’re usually the best we can do, they represent an exaggerated version of a disease that takes decades to develop in humans. There are also subtle differences between the human and mouse metabolism that mean findings like these won’t always translate between our species.

The take-home message

All in all, this is an interesting study demonstrating a mechanism that could help explain why diets high in animal protein (which tends to have more leucine) have been linked to heart disease in animal studies and human observational studies. The authors propose 22% of calorie intake as being a threshold at which protein starts to increase cardiovascular risk. 

However, due to the way the findings are divided between humans and mice, more research will be needed before we draw any solid conclusions. Previous studies have suggested that higher protein intake may promote healthy ageing, largely by helping to combat age-related muscle loss. In this context, leucine may actually be beneficial because it promotes protein synthesis and muscle growth. Having more muscle is probably protective against atherosclerosis because it improves cholesterol levels and protects against diabetes, both major risk factors for this disease. The other common motivation for choosing a high protein diet is to achieve weight loss, which is also protective against atherosclerosis.

So, you can see why we need to be cautious about making extrapolations based on a molecular mechanism. What level of protein intake is healthy is going to depend on all kinds of factors like age, sex, the source of the protein and how much fat is present in the rest of the diet. Since high protein diets are quite popular, further investigation is definitely needed. Until more research is available, the safest approach is to get your protein from healthier sources like plants and fish while limiting red meat. Even if protein does turn out to play a role in atherosclerosis, high cholesterol in the blood will remain the most important risk factor that can be modified through diet.

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