High protein diets are bad for you, some claim. Eating too much protein will damage your kidneys, cause bone loss, and *gasp* even give you cancer––but how much truth is there to this claim? Let’s find out.
High protein diets have long been associated with causing several health issues. Most notably, kidney damage, bone loss, and even cancer.
But do they, though? The short answer: no. The long answer: this article.
1. High Protein Diets and Kidney Damage
One of the most pervasive nutrition myths is that high protein diets increase the kidneys’ workload, which, over time, causes damage and eventually leads to kidney failure.
But this is only a problem for people with existing renal disease.
For instance, the Nurses’ Health Study published in 2003 looked at the change in GFR over an eleven-year period in individuals with pre-existing renal disease.
Their analysis showed an association between an increase in protein and a decline in renal function, suggesting that high protein intakes could accelerate kidney damage.
Wait, hold up––what the hell is GFR?
The glomerular filtration rate (GFR) is a test used to assess how well your kidneys are working. Glomeruli are small filters in the kidneys that filter waste from the blood.
The whole “protein is bad for your kidneys” thing stems from the idea that chronic intake of high protein damages these filters, which can lead to kidney failure.
But the same paper also looked at high protein intakes in 1,135 women with healthy kidneys and found no association between protein intake and changes in GFR, leading the authors to conclude that a high protein diet was not associated with kidney damage in women with normal renal function.
Skov AR et al. conducted a 6-month randomised control trial in 65 overweight but healthy participants placed on either a high-protein diet (~108g/d) or a low-protein diet (~70g/d).
At the end of the study, there was an increase in GFR rates in the high-protein group, but no adverse side effects were reported.
And a recent systematic review and meta-analysis published in the Journal of Nutrition with over 1,300 participants looked at high (~20% of total calories) and low (~5% of total calories) protein intakes. The researchers summarised their findings as follows:
The results of the current meta-analysis suggest a nonexistent or trivial effect of HP consumption on GFR in individuals with normal kidney function. These findings are in line with statements from the WHO and Institute of Medicine on protein intake and kidney function.
Furthermore, there is no evidential link that shows that HP intake somehow leads to declines in renal function in otherwise healthy persons and, as our analysis indicates, even in populations with greater risk for declines in renal function such as those with type 2 diabetes.
In line with these findings, Poortmans and Dellalieux investigated high and medium protein intakes in bodybuilders and well-trained athletes. They found that protein intakes as high as 2.8g/kg (1.2g/lb) didn’t impair kidney function.
More recently, Antonio et al. published two studies that looked at really high protein intakes––beyond what anyone reading this is likely to consume––in resistance-trained subjects.
In the first study, 48 participants (11 females and 37 males) consumed either 2.3g/kg (1g/lb) or 3.4g/kg (1.5g/lb) of protein per day for eight weeks. At the end of the study, there were no harmful effects on renal function despite protein intakes three and four times that of the RDA.
The second study had 14 resistance-trained males consume ~2.5g/kg–3.32g/kg (~1.1g/lb–1.5g/lb) of protein per day for one year. The researchers didn’t find any harmful effects of on measures of blood lipids, liver and kidney function.
And lastly, Antonio and Ellerbroek published a case report on well-trained bodybuilders consuming a high-protein diet over two years (3.2g/kg or 1.45g/lb during year one and 3.5g/kg or 1.58g/lb during year two). The researchers concluding:
The findings indicate that 2 yrs of a high protein diet in healthy resistance trained men had no effect on measures of body composition as well as liver or kidney function. Thus, there is no evidence to suggest that consuming a high protein diet over a 2-yr period causes any harmful side effects.
Also, a fun fact––GFR increases by as much as 65% in healthy women during pregnancy but drops back to baseline by three months postpartum. Yet, how many pregnant women are turning up with failed kidneys all of a sudden?
As long as you have don’t have pre-existing kidney issues, you don’t need to worry about high-protein intakes killing your kidneys, and it’s time to put this myth to bed.
2. Do High Protein Diets Cause Bone Loss?
Way back in the day, metabolic balance studies found that when people ate more protein, they had increased calcium in their urine. The theory was high protein intakes increased blood acidity causing the body to take calcium from bone to balance pH levels. Over time, this “leaching” of calcium would weaken the bones, and…well, shit, grandma just broke her hip.
But newer studies have poured themselves a nice Big Cup of Nope and contested these previous findings due to poor testing methods.
Fenton et al. conducted a meta-analysis in studies that used “superior methodological quality for the study of calcium metabolism” and concluded that:
There is no evidence from superior quality balance studies that increasing the diet acid load promotes skeletal bone mineral loss or osteoporosis. Changes of urine calcium do not accurately represent calcium balance.
Additionally, if it were true that high protein diets cause bone loss, then we’d expect to see an increase in bone fractures in people eating a high-protein diet. But we don’t. In fact, quite the opposite.
For instance, Kerstetter JE et al. looked at data in 1882 women aged 50 years and older and found the higher the protein intake, the higher the total femur bone mineral density.
A 2017 systematic review and meta-analysis from the National Osteoporosis Foundation looked at 16 randomised controlled trials and 20 prospective cohort studies. They found “no adverse effects of higher protein intakes” on bone health.
And a 2018 expert consensus paper endorsed by the European Society for Clinical and Economical Aspects of Osteoporosis, Osteoarthritis, and Musculoskeletal Diseases and the International Osteoporosis Foundation found protein intakes above the current RDA (0.8g/kg; 0.3g/lb) may help reduce bone loss and hip fracture risk. The same review also notes that low protein intakes pose a higher risk to bone health than high protein intakes.
Speaking of fractures, there’s some evidence to suggest that higher protein intakes (alongside calcium and vitamin D) can help speed up recovery in patients recovering from a recent hip fracture. 1
So, in sum, high protein diets won’t turn your bones to dust. In fact, high protein diets can decrease the risk of bone loss and fractures. This becomes especially important as people age.
3. What About Protein and Cancer?
First, we need to clarify something. When people say “high protein diets cause cancer”, what they’re actually referring to is red meat.
This is an important distinction to avoid lumping all protein sources––many of which, like white meat, fish, dairy, and plant proteins are healthy and inversely associated with cancer and mortality 2 3 4 5––under the same umbrella.
But even the link between red meat and cancer is a bit murky due to several study limitations.
The first problem is that studies linking red meat to cancer are observational––researchers look at data in retrospect and try to find a connection––and observational studies can only point to a potential correlation, but can’t determine the cause.
While observational studies can help kickstart a conversation which can lead to better-controlled trials, this isn’t always possible in some cases due to logistics, costs, and ethical concerns. This means that other factors can confound observational research.
For example, a 2012 study found that “red meat consumption is associated with an increased risk of total, CVD, and cancer mortality.” But what you won’t find in the abstract is where the researchers noted that:
Men and women with higher intake of red meat were less likely to be physically active, and more likely to be current smokers, drink alcohol and have higher BMI.
And:
A higher red meat intake was associated with a higher intake of total energy, but lower intakes of whole grain, fruit and vegetables.
Likewise, a meta-analysis published in the European Journal of Cancer explored the association between red and processed meat consumption and breast cancer. The participants who developed breast cancer were less physically active, more likely to be former smokers, had higher BMI and low vegetable intake.
So is it just red meat or was red meat one potential factor in a sea of unhealthy lifestyle factors? As it turns out, people who live a healthy lifestyle, regardless if they’re vegetarian or not, live longer than those who don’t. 6 7 Who knew?
Moreover, some studies didn’t differentiate between unprocessed and processed meat. This should go without saying, but a hot dog is very different from an unprocessed lean cut of steak. And if this difference isn’t accounted for, it’s hard to say if it’s red meat that’s the problem or a particular type of red meat.
Dietary assessment is another issue. Most studies used a 24-hour recall questionnaire. Not only are people notoriously bad at accurately reporting how much they actually ate, but a day’s worth of food intake isn’t always representative of how someone usually eats.
Finally, an association using experimental studies––that is, animal models––doesn’t always translate to humans.
For one, experimental models use levels of meat or meat components well above those found in human diets, even in cultures with high intakes of meat. Consider heme iron––a substance found in meat that’s been linked to an increase in the risk of several cancers. The amount of heme used in experimental models is 9-22 times more than that found in a regular human diet. 8
For two, experimental models use a purified diet void of the protective compounds present in a whole food diet like fruits and vegetables. 9
Even the World Health Organisation that’s listed red meat as “probably carcinogenic” to humans admits this is based on limited evidence and other explanations such as chance, bias, or confounding can’t be ruled out.
More recently, a systematic review and meta-analysis of 118 articles with more than six million participants looked at red meat consumption and incidences of cancer and cancer mortality.
The study found an association between red and processed meat and an increased risk for cancer when comparing the highest to lowest meat intake. However, the researchers also noted that the overall certainty of the evidence was low, and adults could continue eating red meat based on the current guidelines.
It’s important to note the authors said adults could continue with the current recommended meat intakes, they didn’t say there was no risk at all, nor are they encouraging people to increase their meat intake.
Where does this leave us?
The evidence is still inconclusive, but based on the literature as a whole, eating red meat in moderation is probably fine in the context of an overall healthy diet.
Lastly, remember that your overall lifestyle will have a bigger impact on cancer risk than just one food in isolation.
If you’re physically active; eat a majority whole food diet with lots of fruits, vegetables, and fibre; ensure you’re getting the majority of your protein from lean meats, fish, and plant sources while limiting your intake of red and processed meat; don’t smoke; keep your alcohol intake low––well, you’ll still die eventually but you’ll (probably) live a longer, healthier life.
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