Over the past several years, I’ve noticed a subtle shift in the way the media discusses dietary protein, with many experts promoting lower intakes. The push for lower intakes hasn’t only come from the mainstream crowing about red meat and colon cancer. Many voices from the alternative health communities are urging a reduction in protein. Even the ancestral health community counts among its ranks protein skeptics.
Today I’m addressing the standard arguments levied against high protein intakes. Hopefully, we can get to the bottom of the issue.
High protein diets stress the kidneys.
While it’s true that people with existing kidney damage or disease must limit protein intake, this isn’t the case in healthy people . Even type 2 diabetics with good kidney function can safely eat  a long-term high-protein diet. If anything, higher protein intakes protect against kidney disease by making it easier to avoid obesity .
High protein diets exceed your capacity for ammonia detox.
Protein metabolism begets ammonia, a toxin. The liver usually converts ammonia into urea, which is safely expelled through the urine. The average human can handle  about 230 grams of protein before ammonia-urea conversion tanks. After that, ammonia lingers. If your liver health is compromised, or your protein intake exceeds your ammonia detox capacity, ammonia toxicity can ensue.
Acute ammonia toxicity is neurotoxic, actually causing your astrocytes to swell . Rabbit starvation, which afflicted Arctic explorers living exclusively on lean protein (rabbits) and causes nausea, diarrhea, and eventually death , might derive from ammonia toxicity. A subtler, lower-grade ammonia toxicity likely exists in people with chronically-high intakes of protein. The dangers are mostly theoretical but based in physiology—ammonia is a toxin, and there is a limit to how much of it we can convert to urea.
High protein diets create toxins in the gut.
Sufficiently high intakes of protein can exceed the gut’s capacity to absorb it. The protein then passes on to the colon, where colonic bacteria ferment it and produce metabolic byproducts like ammonia, indoles, and phenols. As many of these compounds can have toxic effects, some have suggested that excess protein fermentation produces a toxic gut environment.
In one study , researchers gave subjects either a high protein diet, a low protein diet, or a normal protein diet. They analyzed the fecal water of each group for evidence of protein fermentation and ran a series of tests to determine the toxicity of each batch of fecal water. Surprisingly, while the high protein fecal water had elevated markers of fermentation, it was not toxic, and elevated protein fermentation metabolites actually correlated with lower cytotoxicity.
High protein diets give you cancer.
Vegans love citing the T. Colin Campbell research from the China Study, which appeared to show that high intakes of protein caused increased cancer deaths . While adequate intakes of protein (from casein) promoted the growth of existing tumors in those rodents, it also protected against the mutagens that cause the initial appearance of tumors. Protein was protective against cancer until they had it, at which point it accelerated the cancer’s progression. Rodents on the low protein diet were more susceptible to getting cancer after aflatoxin exposure. Once the rodents already had cancer, low protein was protective against further growth.
More accurate: adequate protein protects against the initiation of cancer (and probably other maladaptive ills), but patients with cancer should limit it. That makes sense. Context is everything.
Extra protein just converts to glucose.
Through the process of gluconeogenesis, we can convert protein into glucose. This has led some folks to believe that eating “extra protein” is like eating a piece of chocolate cake. So, does a high protein diet spike glucose levels? Is eating more protein just like cramming in extra sugar?
In one study , type 2 diabetics ate half a pound of steak (50 grams protein) and nothing else for breakfast. Compared to the control group who just had water, 50 grams of protein had almost no effect on glucose levels, adding just 2 grams to circulation. In another older study , eating up to 160 grams of protein in a single meal had no effect on blood glucose.
On the contrary, high-protein diets have been shown to improve glucose control in the population most at risk from high-carb intakes: type 2 diabetics .
High protein is unnecessary.
Protein is the most expensive macronutrient. In the wild, it takes the most energy to acquire. In the civilized world, it costs the most money to purchase. If we don’t need large amounts of protein, it makes sense to reduce our intake.
Many people lifting weights and cramming down protein shakes likely are eating more protein than they need. The more advanced you are as a lifter, the less protein you need. Beginners gain weight like crazy; experienced lifters do not.
Advanced lifters are closer to their genetic muscle ceiling. There’s less room to grow, so they grow more slowly, and muscle protein synthesis actually declines. Plus, their muscles have become more resistant to exercise-induced breakdown. It takes more to damage them, and there’s less to recover from. Overall, experienced lifters are more efficient with their protein and can maintain  nitrogen balance at 1.05 g/kg. If they want to gain muscle, 1.8 grams per kg (which is much lower than most people assume) seems to be the absolute ceiling for natural lifters . After that, the benefits level off, and you’re just wasting protein.
Interestingly, endurance athletes require more protein  than bodybuilders to remain in nitrogen balance.
High protein diets increase IGF-1 signaling.
IGF-1 is an important compound that helps us build and maintain bone and muscle mass. We need it to thrive, yet excessively high levels of IGF-1 may increase growth of unwanted tissues like tumors and hasten the aging process.
That said, there’s no indication that IGF-1 increases the formation of tumors. As in Campbell’s rats, it’s likely that IGF-1 makes the organism more robust, but once cancer is present, hastens its growth. And the proposed link between elevated IGF-1 and mortality in humans hasn’t been confirmed. It looks like both high and low levels are bad .
High protein diets reduce longevity.
A study from 2014 had a somewhat paradoxical finding : higher protein intakes had a negative effect on mortality in 50-year-olds, a neutral effect in 65-year-olds, and a beneficial effect in those over 80. Let’s assume for a second that the links are causal—that higher protein intakes are the proximate causes of the shifting mortality risks.
When older people eat more protein, even (or especially) if it comes from evil red meat, they get stronger, build more muscle, improve their ability to take care of themselves, and even think better. The studies didn’t track mortality, but loss of muscle, strength, independence, and cognitive function typically precede death in senior citizens. If higher protein intakes can improve those parameters, they should also improve survival in the population.
Another wrinkle is that dietary protein—especially of animal origin—is the best source of cysteine, a crucial backbone we use to produce the endogenous antioxidant glutathione. A group of researchers recently proposed  that a reduction in glutathione synthesis “underlies” the increased mortality linked to low protein intakes in the elderly. Glutathione protects our liver, helps metabolize toxins, and regulates oxidative stress; glutathione deficiency in older populations has been linked to neurodegeneration , heart attacks , and “accelerated aging .”
What’s the verdict?
I’d advise against a low-protein diet unless you have an express reason to do so. You’ll likely grow lethargic, lose muscle mass, gain fat mass, and be less resilient in the face of stressors and illnesses. Maybe it’ll buy you an extra year or two, but at what cost? Low-protein diets have been shown to:
- Slow the metabolism, increase insulin resistance, and cause body fat gain .
- Impair the immune system and make infections more severe .
- Reduce muscle function, cellular mass (yes, the actual mass of the cell itself), and immune response in elderly women .
- Impair nitrogen balance in athletes .
- Increase the risk of osteoporosis .
- Increase the risk of sarcopenia  (muscle wasting).
Meanwhile, high-protein diets confer a few potential risks, which I’ve laid out above.
There are clear benefits to higher protein intakes (lean mass retention, muscle growth, fat loss, increased satiety) and legitimate concerns (reduced longevity, excess IGF-1, too much growth of unwanted tissues). How do we square all the evidence? How do we balance the effects?
Short term is fine: Brief bursts of very high protein diets while lifting heavy things can probably help you shed excess weight and retain lean mass. Recent studies  find evidence of improved body composition and no evidence of any deleterious effects up to 4.4 g/kg for several weeks at a time. I’m not sure I’d continue eating that much protein for the rest of your life though.
Go high and low: Eat high protein one day, lower protein the next. If you’re trying to gain muscle, you probably need more protein on workout days. If you’re just maintaining, you can get away with far less.
Fast: When someone constantly eats, never going more than 5-6 hours between meals, that person never gives his/her body time to recover and prune damaged cells. Intermittent fasting  imposes periods of zero protein and zero food, giving your body a dose of autophagy  and a respite from mTOR/IGF-1 activation , and likely making higher protein intakes on feeding days safer.
Overall, I’d say using 100 grams per day as a baseline and going as low as you can without suffering the negative symptoms is worth exploring. It’s likely enough for the majority of people reading this, and if you cycle your protein or fast, you give yourself a chance at some nice autophagy and healthy growth. Plus, you can always increase your intake if you notice negative symptoms.
Athletes aiming for muscle growth and endurance athletes trying to avoid muscle loss  may go up to 1.8 g/kg. Dieting athletes should probably go at least that high to preserve lean mass.
Don’t forget my previous protein intake recommendations  for different populations.
Now I’d love to hear from you.
How much protein do you eat each day? How do lower—or higher—intakes affect you?