The question of whether protein is harmful or not to health usually marks the beginning of the journey into fitness and bodybuilding. And for good reason.
Protein is an essential component of food necessary for the growth of the human body and the maintenance of its functions. Skin, nails, hair, blood cells, muscles, hormones, and enzymes require a regular intake of protein from food.
The recommended protein intake for physically inactive adults is 0.8 g per kilogram of body weight per day, for children 1.5 g/kg, for teenagers 1.0 g/kg, and for those who engage in sports 1.6-2.2 g/kg.
In sports and bodybuilding in particular, a diet with an excess of protein, significantly exceeding the recommended norms, is actively promoted by representatives of the sports nutrition industry and experts of various levels as the golden rule of muscle growth and as beneficial for weight loss.
We recommend: How much protein should you drink per day to gain muscle mass?
At the same time, many scientists warn of the significant risk of side effects from excess protein for the kidneys, liver, and bones, as well as the potential for cancer.
In this article, we will answer the question of whether protein is harmful or not, what the possible consequences and side effects of its consumption in large doses typical for athletes are, how protein is harmful to the kidneys, liver, and bones, and whether it increases the risk of cancer.
Main thoughts:
Protein doses up to 2.8 g/kg do not pose harm to kidney function in trained athletes
Large doses of protein are harmless for women with healthy kidneys
Protein is dangerous for the kidneys with a rapid increase in its amount in the diet
Training helps the kidneys to “process” large amounts of protein more easily
Insufficient protein in the diet may also reduce bone strength
Facts about the harm of protein
Common beliefs about the harm of protein include the following:
- protein is harmful to the kidneys;
- protein is harmful to the liver;
- protein causes cancer;
- consumption of protein increases bone fragility.
The premises for these claims are based on certain scientific facts.
For example.
It is known that an increase in protein in the diet is accompanied by a rise in calcium concentration in urine (we will discuss this in detail below). Based on this, it has been suggested that protein is harmful to bones, as it increases their fragility.
The basis for the assumption that protein is harmful to the kidneys and liver lies in the fact that these two organs are responsible for its disposal in the body. Therefore, a high-protein diet should increase the load on them, and the question is how well they are adapted for this.
Below, we will examine each of the mentioned facts in detail and provide scientific justification for why they are myths or, conversely, deserve attention.
We recommend: The harm of sports nutrition. Scientific facts
1 Is protein harmful to the kidneys?
The possible side effects of excessive protein intake on the kidneys stem from their functions in the body. In the context of the issue we are discussing, the following two are significant:
Filtering function
The kidneys are responsible for removing metabolic waste products from protein (and not only), foreign substances, and toxins from the body, as well as regulating the concentrations of essential minerals (calcium, potassium, sodium, etc.) either by excreting them in urine or by returning them back into the system.
The first question we should answer is: how does excess protein affect the filtering function of the kidneys?
Regulation of acid-base balance
The kidneys regulate the balance of acids and bases in the body, with regular shifts towards “acidosis” leading to catastrophic consequences for the body: it is a factor in the formation of kidney stones, the development of cancer, the leaching of calcium from bones, etc.
The second question we will seek to answer is: does acidity in the body increase with the consumption of large amounts of protein, along with the risk of forming kidney stones?
We recommend: The benefits and harms of milk: expert reviews
How does protein affect the filtering function of the kidneys?
With an increase in protein intake, the kidneys have to remove more metabolic waste products from the body, and in a healthy body, the kidneys can adapt to the increased load by increasing the filtration rate 24.
In certain kidney diseases, this ability is impaired, and strict control of protein intake is required to avoid exacerbation 25.
Let’s consider several indicative scientific studies.
Experiment: Study of the harm of protein to the kidneys in bodybuilders
To shed light on the question of the harm of protein to the kidneys, scientists studied bodybuilders and other professional athletes with a high average protein intake 5.
They analyzed dietary patterns over 7 days, blood and urine samples, to determine potential side effects on the kidneys from high doses of protein.
The results showed that despite higher concentrations of uric acid and calcium in the plasma, bodybuilders had creatinine (a breakdown product of creatine), urea, and albumin levels—markers of kidney health—within normal ranges.
For both groups of athletes, the nitrogen balance in the body was positive at a daily protein dose of more than 1.26 g/kg. This indicates a sufficient amount of protein in the diet for muscle mass gain.
The authors of the study conclude that protein doses up to 2.8 g/kg do not harm kidney function in trained athletes.
Protein doses up to 2.8 g/kg do not harm kidney function in trained athletes
But this is only a study of filtering function. It is still too early to relax, as an increase in uric acid and calcium concentration says something… More on this below.
We recommend: Whey protein in sports nutrition: types, properties, and numerous advantages
Experiment: Study of protein harm to kidneys in women
The influence of increased protein intake on kidney function in healthy women and those with mild kidney insufficiency was also studied 6.
It involved 1624 women aged 42-68 years, who provided blood samples over 11 years (from 1989 to 2000). The amount of protein in the diet was assessed by surveying women about the dishes they consumed. The filtering function of the kidneys was evaluated based on the level of creatinine in the urine.
The result was similar: large doses of protein are harmless for women with healthy kidneys.
Scientists also noted that non-dairy animal proteins in large amounts may contribute to impaired kidney function in women with moderate kidney insufficiency.
Large doses of protein are harmless for women with healthy kidneys
Experiment: Side effects of excess protein on kidneys with long-term intake
Most experiments aimed at studying the harm of protein to the kidneys are short-term (6-28 days), and little is known about the benefits or harms of large doses of protein over a long period.
In a study of long-term effects 7, 88 volunteers with healthy kidneys participated:
- 32 vegetarians;
- 12 bodybuilders not taking special sports proteins;
- 28 bodybuilders taking sports protein;
- 16 individuals not following any special diet.
The duration of the experiment was 4 months.
Kidney function was assessed by the creatinine content in urine, and the amount of protein consumed was measured by the nitrogen level in urine, which is proportional to the amount of protein in food.
The amount of protein in the diet varied among different participants from 0.29 g/kg (in vegetarians) to 2.6 g/kg of body weight (in professional bodybuilders on protein diets).
The result of the experiment: with prolonged consumption of large doses of protein, structural changes occur in the filtering elements of the kidneys, allowing them to cope with the increased load.
With an increase in the amount of protein in the diet, structural adaptive changes occur in the filtering elements of the kidneys
We recommend: Protein and alcohol: how does beer consumption affect muscle mass growth?
Experiment: The danger of a sudden increase in protein intake
Protein undoubtedly places a load on the kidneys 8,9. Experiments on mice show that a sudden increase in protein dosage in their diet (from 10-15% to 35-45%) causes kidney damage 10,11.
As for humans, a sudden increase in protein intake from 1.2 g/kg to 2.4 g/kg was associated with high levels of its breakdown products in the blood (which should not be present in normal levels), which researchers attributed to the body’s adaptation 12.
That is, a rapid increase in protein dosage can be dangerous for the kidneys. With a gradual increase, the filtering elements adapt.
Protein is dangerous for the kidneys with a rapid increase in its amount in the diet
Experiment: The consequences of excess protein for the kidneys during physical activity
Another interesting result from an experiment: when the amount of protein in the diet of rats was sharply increased, they experienced kidney function failure, the symptoms of which were significantly alleviated with physical activity 11.
Training helps the kidneys “process” a larger amount of protein more easily
For those with kidney damage, it is better to limit protein intake: this will help reduce the likelihood of further disease progression. Otherwise, protein consumption can be dangerous and lead to exacerbations 13.
We recommend: BCAA Amino Acids: harm and side effects
How does protein affect the risk of kidney stone formation?
So, everything discussed above only pertains to the filtering function of the kidneys, and it seems that they are quite capable of handling the increased load from excess protein.
Now let’s consider what scientific research says about the relationship between protein and the risk of kidney stone formation.
Kidney stones are, in most cases, a “reward” for many years of “playing by the rules” of our body’s functioning. They form when urine becomes concentrated, which facilitates the formation of crystals from the minerals present in it.
According to scientific data, the risk factors for kidney stone formation are insufficient water intake and excess protein in the diet, which lead to increased concentrations of calcium, uric acid, and oxalates in the urine 28.
Animal protein is more harmful to the kidneys than plant protein in terms of the risk of kidney stone formation
There are several explanations for this statement.
Consumption of animal protein increases acidity in the body, which reduces the kidneys’ ability to return calcium from urine back into the system (this will be detailed below in the section Is protein harmful to bones?).
As a result, the concentration of calcium in urine increases, creating favorable conditions for the formation of kidney stones 29.
On the other hand, animal protein is a source of so-called purines – substances from which uric acid is formed. Uric acid is another known factor that increases the risk of kidney stone formation 30: the higher its concentration in urine, the higher the risk.
The solubility of uric acid significantly depends on the acid-base balance of urine. When pH decreases to 5.5-6.0 – which occurs with excessive protein intake – the solubility of uric acid decreases, and stone formation is further facilitated 30.
We recommend: The harm of glutamine and possible side effects
Experiment: The harm of a high-protein, low-carbohydrate diet to kidney health (risk of stone formation)
In this study, scientists aimed to determine how a popular low-carbohydrate diet for weight loss (low in carbohydrates, high in protein; there are many such diets, an example is the Dukan protein diet) affects kidney health.
In the experiment, subjects consumed a large amount of protein for 6 weeks 35.
As a result, pH dropped (acidity increased), the level of insoluble uric acid doubled (!), and the level of calcium in urine increased by 60% (!). The scientists’ conclusion: a diet with excess protein significantly increases the risk of kidney stone formation.
Experiment: The impact of excess animal protein on kidney health (risk of stone formation)
In this study, subjects with a history of kidney stones participated. Some of them were fed a high-protein diet for 2 weeks, while others were given a low-protein diet.
It was found that high consumption of purines from animal proteins increased the concentration of uric acid salts by 90%, uric acid itself by 200%, and ammonium ions, while lowering the pH level, which ultimately leads to the formation of uric acid and ammonium salt crystals 44.
In a study where subjects consumed food for three 12-day periods, first with a high content of plant protein, then a mix of plant and egg protein, and finally only animal protein, it was shown that the level of insoluble uric acid was highest on a diet rich in animal protein 39.
The scientists’ conclusion: urine crystallizes more easily on a diet high in animal protein, increasing the risk of kidney stone formation.
In addition to this, scientists showed that an excess of animal protein in the diet facilitates the formation of calcium oxalate crystals in urine 41, which is another physicochemical explanation for the side effect of excess protein in the form of kidney stone formation.
Based on all the factors mentioned above, the likelihood of kidney stone formation increases by approximately 250% with excessive consumption of animal protein 46.
A side effect of excessive protein consumption may be an increased risk of kidney stone formation by more than 200%. Moreover, animal protein is more dangerous in this regard.
We recommend: 13 scientific facts about the harm of energy drinks
2 Is protein harmful to the liver?
In a 1974 study, scientists found that feeding mice a diet with 35% protein content led to an increase in the concentration of certain liver enzymes in the blood, which scientists use to assess liver problems 20.
At least one animal study confirms the possibility of liver damage when alternating periods of high protein intake with a 5-day protein starvation 14.
Similar results were obtained in an experiment where, after 48 hours of fasting, mice were fed a mixture containing 40-50% casein 15.
Scientists concluded: consuming a large amount of protein (35-50%) after 48 hours of protein starvation may harm the liver. Shorter intervals of protein starvation were not studied.
Consuming large amounts of protein after periods of protein starvation can be dangerous for the liver.
A high-protein diet leads to an increase in the levels of certain liver enzymes (transaminases), which is an indicator used to diagnose various diseases, including liver damage 50.
Two such cases are described in the scientific literature involving two fitness enthusiasts who consumed large doses of sports protein 49.
Both were hospitalized with severe abdominal pain, elevated transaminase levels in the blood, and hyperalbuminemia. After stopping protein consumption, their levels returned to normal.
Some types of nuts and seeds contain the toxin aflatoxin. Scientists have found that its carcinogenic (cancer-causing) effect is enhanced when the diet is high in protein 16 and, conversely, decreases when protein intake is reduced 17-19.
This is explained by the fact that this toxin is activated by a specific liver enzyme (scientists refer to it as ‘P450’), which is released in response to increased protein consumption.
Some types of nuts contain toxins whose effects are enhanced in the presence of high protein in the diet
The above is not a side effect of high protein intake for the liver: the harmful effects of aflatoxin can be avoided by limiting nut consumption in combination with protein foods. Nevertheless, it is important to keep this in mind.
There is no clear evidence of harm to the liver from high doses of protein: it is likely that protein is safe for a healthy liver, but there are reasons to believe that a sudden increase in protein intake may be dangerous
3 Is protein harmful to bones?
This is a very controversial and widely discussed question in science, to which researchers have not yet found a definitive answer.
Yes and no.
On one hand, numerous scientific studies indicate that a side effect of excessive protein consumption is “acidosis” of the body due to increased acid production, which should negatively affect bone strength, as the neutralization of acidity occurs partially by drawing calcium from the bones 27.
The situation is further complicated by the fact that the kidneys, one of whose functions is to regulate the balance of minerals in the internal fluids (calcium, potassium, sodium, etc.), cannot return calcium back into the system under conditions of increased acidity, leading to its “leaching” from the body through urine. Hypercalciuria develops 28,29.
Other studies suggest that this condition is not dangerous and does not lead to increased bone fragility 21. Moreover, protein is necessary for more effective absorption of calcium from food.
The authors of the mentioned scientific review claim that “none of the existing studies unequivocally confirm the negative impact of a high-protein diet on the mineral strength of bones. The only exception is cases where calcium intake from food is insufficient.“
However, there is a considerable amount of scientific evidence supporting the opposite.
Let’s look at some of them.
Experiment: The consequence of excess protein in the diet is the “leaching” of calcium from the body
One study showed that increasing the amount of protein in the diet from 47 g to 112 g per day resulted in an increase in the amount of calcium in urine, while the proportion retained in the body decreased. The reason was attributed to an increase in kidney filtration rate, decreased ability of the kidneys to reabsorb calcium from urine due to increased acidity 33.
When changing the amount of protein in the diet from 46 g to 123 g per day, the concentration of calcium in urine doubled 38,51.
A gradual increase in the amount of protein in the diet (42, 95, 142 g per day) leads to a proportional change in the calcium content in urine (168, 240, 301 mg) 37.
But does this lead to a deterioration in bone strength?
We recommend: The harm of fat burners and side effects: what makes natural supplements dangerous?
Experiment: A side effect of excess protein in the diet may be reduced bone strength
Analysis of signs of bone destruction in subjects who were on a well-balanced diet for 2 weeks and then consumed low, medium, or high amounts of protein for 4 days showed a significant increase in urinary calcium and a marker of bone destruction (N-telopeptide) on a high-protein diet 34. A similar result was obtained here 35.
However, what is curious is that in the case of a low-protein diet, the level of parathyroid hormone in the blood was elevated (hyperparathyroidism), which also indicated a weakening of bone strength.
The parathyroid hormone is responsible for regulating the balance of calcium and phosphorus in the body: when its level rises, bone tissue begins to break down, and the concentration of calcium in the blood increases to meet the body’s current needs for it 53.
The conclusion of the scientists: excess protein in the diet reduces bone strength, with the processes of bone destruction prevailing over the processes of formation.
In a large statistical study of over 85,000 women, it was found that women who consume more than 95 g of protein per day have a higher risk of bone fractures than those who consume less than 68 g per day. The risk of fractures is also increased among those who consume more than 5 servings of red meat per week and whose diet is dominated by animal protein over plant protein 36.
– Does additional calcium intake help prevent its leaching from bones on a high-protein diet?
No 40.
Individual scientific studies suggest that an excess of protein in the diet may lead to increased acidity in the body, “leaching” of calcium from bones, and decreased bone strength. Additional calcium intake in the form of a supplement does not solve the problem.
We recommend: Vitamin and mineral deficiency: how to understand which vitamins are lacking?
Experiment: A consequence of protein deficiency in the diet may also be a decrease in bone strength.
On the other hand, some data indicate that a protein diet promotes better calcium absorption, while insufficient protein intake is associated with an increased risk of bone fractures 21,22.
A positive correlation between the amount of protein in the diet and bone mineral density has been shown in experiments on elderly individuals and adolescents 58,59.
One reason for this may be that dairy products are often the source of protein, particularly whey protein or casein, which are rich in calcium 4.
Moreover, the consumption of dairy products stimulates the level of insulin-like growth factor IGF-1, which positively affects bone strength 56. See Milk: benefits and harms for the human body. Scientific facts.
Researchers found a significant decrease in calcium absorption from food on a low-protein diet (0.8 g per kg of body weight) and an increased level of parathyroid hormone, which, as mentioned above, is a sign of bone tissue breakdown 59. These side effects were not observed at protein doses of 1.0 and 2.2 g/kg.
Insufficient protein intake in the diet may also reduce bone strength.
Animal protein may be more harmful to bone health than plant protein
Interestingly, it seems that only a few amino acids in protein – those containing sulfur – have such a negative side effect on bone health.
In one experiment, when the amount of protein was increased from 50 to 150 g, the concentration of calcium in urine doubled; a similar effect was observed when sulfur-containing amino acids were added to a low-protein diet in an amount equivalent to 150 g of protein 42.
Sulfur-containing amino acids are found in large quantities in animal proteins, but not in plant proteins.
Research indicates that the higher the ratio of animal protein to plant protein in the diet, the greater the rate of calcium “leaching” from bones and the risk of fractures in women 52.
Animal food is a major factor in increasing the acidity of the body.
This is also stated by Dr. Colin Campbell, Professor Emeritus of Cornell University, one of the modern experts in understanding the causes of cancer, who calls animal protein – one of the main causes of the onset and progression of cancer due to its property of “acidifying” the body.
Conclusion: it may be a good decision to reduce the share of animal protein in the diet and increase plant protein 43.
But even if you do not want to do this, increasing the amount of vegetables and fruits in the diet can mitigate the dangerous consequences of excess animal protein: their consumption reduces acidity in the body 27.
There is an opinion that consuming soy protein may help strengthen bones, thanks to the isoflavones (phytoestrogens) contained in them 23.
In a comprehensive study on the harm of soy, the extreme danger of soy for health, including bone health, was shown. The reason, paradoxically, is the isoflavones.
We recommend: Phytoestrogens: questionable benefits during menopause, content in products, harm. Scientific facts
The side effects of protein in terms of bone strength impairment are more characteristic of animal protein types than plant types. Increasing the share of vegetables and fruits in the diet can reduce acidity in the body.
Sports protein may be more dangerous for bone health than natural protein
Some studies suggest that purified forms of protein (such as sports casein) are more harmful to bone health than natural forms (meat, dairy products) 56.
The reason is that in natural sources of protein there is significantly more phosphorus, which hinders the process of “enriching urine with calcium” 54-56.
When consuming natural proteins, an increase in calcium concentration in urine is observed only in cases where the phosphorus content has been artificially reduced to a level comparable to that found in refined types of protein 56.
Refined forms of proteins (sports protein) may be more harmful to bone health than natural protein products
We recommend: Soy protein isolate: 7 facts about the HARM of the exclusively “healthy” revolutionary product
5 The harm of very high protein intake
Very large doses of protein (over 200 g per day or more than 40% of total caloric intake) can have a toxic effect, leading to vomiting, diarrhea, and even death.
This condition is known as “protein poisoning” or, more exclusively, “rabbit starvation” 1, as rabbit meat contains very little fat and poisoning occurs due to an imbalance of protein and fat in the diet.
This explains the rule of protein-fat-carbohydrate ratio: the amount of protein should be less than 40% of total caloric intake, ideally 15-25% 2,3.
The harm from a large amount of protein may be due to the body’s inability to produce enough urine to eliminate its breakdown products.
When the amount of protein increases in the diet, the rate of urine production reaches a maximum, and if the protein dose is further increased, the volume of urine does not increase, fewer breakdown products are eliminated, and the concentration of amino acids and ammonia in the blood rises, which has a toxic effect.
That is why it is important to drink more water with an increase in protein dosage.
Large doses of protein are harmful to health and can have a toxic effect, as they disrupt the balance of fats and proteins in the body: the ideal amount of protein in the diet is 15-25% of total caloric intake
We recommend: How to properly take protein for muscle growth?
6 Protein and cancer
An increasing number of well-known experts (doctors, scientists, nutritionists) are stating that animal protein is one of the main causes of cancer.
And the most dangerous types of protein are those with the highest biological value, i.e., those that are most popular in bodybuilding (casein, whey, and egg protein and corresponding animal products).
This is stated by one of the modern experts on understanding the causes of cancer, Dr. Colin Campbell, Professor Emeritus of Cornell University. An interview with him can be read in the article Protein and Cancer: Animal Protein – One of the Main Causes of Cancer. Interview with Leading Expert | Scientific Facts.
The cause of breast, intestinal, and prostate cancer in 80% of cases is attributed by scientists to dietary habits, specifically – high meat consumption 31.
Scientific studies unequivocally confirm the existence of a correlation between red meat consumption and colorectal cancer 32.
Several possible mechanisms for this are mentioned:
- the formation of heterocyclic amines in meat during cooking, which are carcinogens 60;
- saturated fats, which are themselves a factor that increases cancer risk 26;
- the formation of certain substances (NH3 and NOC) from bacterial residues in meat that enter the digestive tract, which stimulate the development of colorectal cancer and its characteristic chromosomal mutations 31.
The correlation between animal protein consumption and cancer risk is confirmed by statistical studies: men who consume beef, pork, or lamb five or more times a week have a higher risk of cancer than those who consume these types of meat less than once a month.
Moreover, the fattier the meat (the more calories come from saturated fats), the greater the cancer risk 47.
An analysis by Italian scientists of about 8000 cases of various forms of cancer showed that consuming red meat more than 7 times a week (compared to less than 3 times a week) increases the risk of stomach cancer by 60%, colorectal cancer by 90%, pancreatic cancer by 60%, bladder cancer by 60%, breast cancer by 20%, endometrial (uterine) cancer by 50%, and ovarian cancer by 30% 48.
Animal protein is a factor in the development and progression of cancer. The most dangerous are sports proteins with high biological value and red meat.
We recommend: The Harm of Casein. Sports Casein – The Worst Protein. Casein and Cancer
So is protein harmful?
Based on the facts presented above, the following conclusions can be drawn.
Excess protein in the diet does not seem to be dangerous for the filtering function of the kidneys, provided that the amount is gradually increased.
The consequence of consuming a large amount of protein may be the formation of kidney stones, especially if the protein is of animal origin and there is insufficient water in the diet.
Both excess and deficiency of protein can be dangerous for bone health: in both cases, bone tissue is destroyed, although for different reasons. Animal proteins and refined (sports proteins) are more harmful to bones than plant-based and natural proteins.
Increasing the proportion of plant foods (fruits, vegetables) while consuming animal protein reduces the negative side effect of increased acidity in the body and the associated leaching of calcium from bones and formation of kidney stones.
Animal protein is one of the main causes of the onset and development of cancer.
