Will Eating Protein Wreck Your Arteries?

A new paper in Nature Metabolism makes this claim, but how does it hold up?

A new paper in Nature Metabolism claims to have shown that the amino acid leucine causes atherosclerosis at the exact same threshold as needed to stimulate protein synthesis.

The mechanisms are, in fact, the same. Leucine stimulates mTOR in muscle and that causes muscles to grow. According to this new paper, leucine stimulates mTOR in immune cells and that causes atherosclerotic plaque to grow.

Whereas all of nutritional science up to this point has referred to leucine as an “essential amino acid,” these authors refer to leucine as a “pathogenic amino acid.”

Since the leucine threshold for muscle protein synthesis and the newly claimed leucine threshold for atherosclerosis are approximately the same, these authors question whether middle-age and older adults should follow the existing recommendations to eat more protein to help stave off age-related declines in body composition:

… our data now suggest that a leucine threshold for mTORC1 signalling in macrophages can have pathophysiological consequences, driving an atherogenic environment that consists of autophagy dysfunction, generation of ROS and activation of pro-apoptotic pathways. It is noteworthy to point out that the threshold of dietary leucine-activated mTORC1 signalling in macrophages that we identified seems to coincide with the maximal stimulatory effect of dietary protein on muscle protein synthesis at about 25–30 g per meal, suggesting a complex integrated metabolic network of multi-organ physiological functions that ensure optimal health.

… it has been proposed that middle-aged and older adults consume at least 25 g to 30 g protein, particularly leucine-rich proteins, with each meal and a total of at least 1.0–1.2 [grams per kilogram bodyweight per day] to prevent age-associated declines in muscle mass, even though the effectiveness of this approach has not been firmly established. High protein intake is also recommended to help people lose weight. Many Americans and adults in other western cultures already consume such high amounts (≥1.2 [grams per kilogram bodyweight per day]) of protein. Moreover, the majority of protein in the diet of people living in western societies is derived from leucine-rich animal proteins. Here, we find such high protein intake and the corresponding leucine intake, have potential detrimental effects on vascular health.

So let’s take a look at what they actually showed.

The Findings

First, they did a series of experiments showing that high-protein meals in humans stimulate mTOR signaling in monocytes, a type of immune cell, that circulate in the blood.

Some critics focus on the fact that the protein drinks were highly processed junk drinks. This is true in the first experiment.

Junk Food Versus Real Food

The first experiment compared 10% and 50% of energy as protein, using liquid meals made from varying amounts of Nestle Boost Plus, Unjury whey protein isolate, non-fat dry milk powder, Sol Carb maltodextrin, canola oil, and water. Boost Plus is made mostly from water, glucose syrup, canola oil, and sugar, with a small amount (less than 2%) of soy protein isolate, vitamin and mineral mix, soy protein isolate, soy lecithin, natural flavor, salt, stevia extract, and carageenan.

The second experiment used “real foods” that were liquified in a blender. The ingredients were potatoes, beans, onions, carrots, corn, bacon, fats, broth and spices with varying amounts of protein isolates derived from egg, chicken, beef, and whey. This experiment compared a 450 Cal meal that was either 15% and 22% of energy as protein. The high protein meal took carbs down from 50% to 48% and fat down from 35% to 30%.

Leucine Stimulates mTOR in Immune Cells and Decreases Autophagy

These experiments both showed that higher protein intakes led circulating monocytes to have 1) more mTOR signaling and 2) less autophagy.

This trend corresponded to the percentage protein across the two experiments. They found seven amino acids were elevated in the blood after protein intake (leucine, isoleucine, valine, methionine, threonine, serine and arginine), and then screened human monocytes in vitro to see which amino acid caused mTOR to go up and autophagy to go down. They found this trend could be wholly replicated with the leucine at concentrations found in human blood after high-protein meals.

While the Boost Plus drink is highly processed trash, and while none of the ingredients are of the best quality in any experiment, this is absolutely the wrong place to put focus for three reasons:

• Everyone already knows leucine is a powerful stimulator of mTOR. These authors just showed that this also occurs in monocytes. Why wouldn’t it?

• Using real food instead of Boost Plus didn’t change the results, and the results could be replicated using the individual amino acid leucine in isolated cells.

• There is no reason whatsoever to think canola oil or glucose syrup is a more powerful activator of mTOR than leucine and regardless of how bad for you processed junk is, nothing “bad for you” has been shown yet.

You Don’t Want All Your Immune Cells to Kill Themselves

In fact, as I pointed out in How to Finally Get Rid Of Your Lingering Cough, you want to have very robust activation of mTOR during the initial adaptive immune response to an infection or injury, because this is what drives T cell expansion by powering up the ability of these cells to burn glucose and glutamine.

Your circulating monocytes are meant to constantly surveil your body for things that don’t belong, and when they find something, they may be exactly what alerts T cells that it’s time to wake up with their mTOR emergency siren and get to work.

Why would you want your circulating monocytes to be focused instead on eating themselves through autophagy?

There are, of course, circumstances where you want autophagy to take place. For example, if your circulating monocytes have dysfunctional mitochondria, you want them to get rid of these mitochondria and replace them with new ones.

Leucine Aggravates Rotenone Poisoning In Isolated Cells

The authors of this paper tried to create a model of this by showing that the complex I inhibitor rotenone caused mitochondrial dysfunction and oxidative stress, and leucine dose-dependently aggravated it:

The “dysfunctional mitochondria” on the left is thought to reflect loss of mitochondrial membrane potential, meaning loss of the ability of the respiratory chain to support ATP production. All of the cells in these two panels have been treated with the respiratory chain poison rotenone, so it is a bit odd that they don’t include an unpoisoned control. Nevertheless, taken at face value the panel on the left suggests that leucine aggravates the loss of respiratory chain function caused by rotenone. The panel on the right shows it does the same to the oxidative stress caused by rotenone.

The authors’ interpretation is that by stimulating mTOR, leucine has decreased mitophagy, and thereby decreased the ability of the monocytes to clear out their rotenone-poisoned mitochondria.

However, they do not block mTOR to show that it abolishes the effect of leucine, and they do not compare leucine to a positive “extra fuel” control.

So, I would say that it is not at all clear from this that leucine is inhibiting the clearance of defective mitochondria. An alternative hypothesis would be that leucine is being burned for energy and generating more NADH. The oxidation of the NADH is blocked by rotenone, and this leads to reactive oxygen species (shown in the right panel), and an uncoupling response that diffuses the mitochondrial membrane potential (shown in the left panel) in order to prevent the buildup of excessive NADH.

I consider their own interpretation plausible, but I don’t see their experimental evidence as adequate to form it as a conclusion.

mTOR Is Needed for Mitochondrial Biogenesis

mTOR actually plays critical roles in mitochondrial turnover (see here and here) for example, but the normal exposure of mTOR is supposed to be cycling up in the fed state and cycling down in the fasted state, so presumably “all mTOR all the time” is going to cause dysfunctional mitochondria to accumulate.

The Defective Autophagy Hypothesis of Atherosclerosis

Aside from rotenone poisoning, another case where you want your immune cells to undergo mitophagy is in the highly inflammatory death spiral found in the center of an advanced atherosclerotic plaque known as the necrotic core.

The initial development of the plaque is driven by oxidative damage to lipoproteins, and the immune cells infiltrate the plaque to sequester the damaged lipoproteins to prevent their toxic load from killing the endothelial cells that line the blood vessel lumen. The more toxic, inflammatory, damaged lipoproteins they gobble up, the more likely they are to suffer toxicity themselves, and develop, for example, dysfunctional mitochondria.

It has been hypothesized that defective autophagy and mitophagy inside an atherosclerotic plaque leads to increased apoptosis and necrosis. Apoptosis is highly ordered, programmed cell death, and necrosis is extremely messy cell death. The more apoptosis and especially necrosis occur in the plaque, the more the guts of these cells spill out and lead to the accumulation of extremely inflammatory debris. The more inflammation there is, the more the collagen cap of the plaque is degraded. This, in turn, makes plaque rupture more likely, and plaque rupture is the central cause of narrowing the blood vessel lumen and the central cause of thrombotic events that cause heart attacks.

Leucine Aggravates Atherosclerosis in Mice With Genetically Defective Mitochondrial Function and Lipid Metabolism

In line with this, the authors of this paper showed that high-protein diets increased plaque burden and the size of the necrotic core in the aortas of ApoE knockout mice.

The lesion area was highest on the high-protein diets, but was not far behind when medium-protein diets were supplemented with leucine:

The necrotic core was only measured on low, medium, and high-protein, but the trend is the same:

Nearly all genetically engineered mice, including these mice, are made using C57BL/6J mice who have a universal defect in NAD(P)H transhydrogenase, which makes them vulnerable to oxidative stress from low NADPH and to excessive NADH accumulation stressing the mitochondrial respiratory chain (reductive stress), achieving something somewhat similar to rotenone poisoning.

I have written about this many times.

On this background they deleted the gene for ApoE, which is needed for normal LDL receptor uptake of lipoproteins. Without it, the mice spontaneously develop atherosclerosis.

The Mouse Model Is Unrealistic

These lab mice, like all lab mice, were housed in “a specific pathogen-free barrier facility,” with a 24/7 on-call vet, which essentially means that they never have an opportunity to get sick.

By taking mice with genetic mitochondrial dysfunction, completely deleting their normal lipoprotein metabolism, preventing them from ever getting sick, and then measuring nothing in them except the size of their aortic plaque, the authors have created a massively distorted perspective of what leucine is doing.

In this situation, leucine is never able to show any of these important roles:

• Activating mTOR to protect from infection.

• Activating mTOR to maintain healthy body composition and consequently healthy glucose metabolism and levels of basal inflammation.

• Activating mTOR to activate thyroid hormone production and to prevent its deactivation, which leads to higher LDL receptor activity, and is thereby a central protection against atherosclerosis.

Thus we only get to see the atherosclerosis already there where too much leucine too often could prevent plaque macrophages from autophaging their dysfunctional mitochondria and we never get to see leucine promote robust metabolism to prevent the development of atherosclerosis in the first place.

The findings of this study are robust to criticisms about the low quality of some of the ingredients or the presence of carbs in the diets because the breadth of evidence pinpoints leucine as the primary actor and it is highly consistent with the known role of leucine stimulating mTOR. The problem is the generalizations made from this study as protein net aggravating atherosclerotic risk rather than having one extremely narrow negative role (inhibiting plaque macrophage mitophagy) amidst many positive roles (leading to a robust rate of metabolism that minimizes atherosclerosis, for example).

Taking the Background Out of Context

The authors start out by saying, “More than a century ago it was discovered that rabbits that were fed a protein-rich diet developed intimal lesions within the aorta.”

They then cite two papers (here and here) as showing that, observationally, people who eat more protein have more cardiovascular disease mortality, and that this might exclude plant protein.

Their first claim is very misleading. As I have covered here, in 1909 Ignatowski produced atherosclerosis in rabbits by feeding them meat, eggs, and milk while pursuing the hypothesis that protein accelerates aging. However, in 1913 Anitschkov showed that cholesterol was the component in those foods responsible for this effect.

The two observational papers they cite are included in the most recent meta-analysis of the topic. High protein intake in this meta-analysis has no association with cardiovascular mortality or non-fatal stroke.

Interestingly, for the composite endpoint of non-fatal heart attack plus non-fatal stroke plus total cardiovascular mortality, there is a non-significant (P=0.19) trend toward a difference, but it is driven entirely by two outliers that happen to be the two studies cited in the Nature Metabolism paper!

These are Chen 2019 and Song 2016.

So while there might be more to learn here, it is misleading to place the findings of this study in the context of being the first study to mechanistically explain the robust body of evidence from Ignatowski 1909 to Chen 2019 showing that animal protein causes atherosclerosis.

Rather, it has shown a very narrow mechanism where leucine could have a negative effect in the midst of abundant other effects, many of which are essential and many of which are highly beneficial.

What Can We Learn From This Paper?

The most interesting finding of this paper is that deficient autophagy probably accelerates late-stage atherosclerosis by enhancing formation of the necrotic core.

It would be interesting to see if there is some cutoff where you maximize all the benefits of leucine and any extra is diverted into harmful effects, but if that is true we do not have the precision to find that cutoff right now.

Rather, this finding is likely one of many downsides to being “all mTOR all the time,” and it adds to the evidence that we want to cycle through the feeding and fasting states.

I would hypothesize from this that the Fasting-Feeding Reset I developed may play a role in facilitating atherosclerotic plaque reversal, but making that claim must await further evidence.