It’s hard to believe that in certain circles, protein has gotten a reputation as being harmful for bone health.  After all, Paleolithic hunter-gatherer diets typically contained a large proportion of meat, yet anthropologists can sometimes distinguish the remains of hunter-gatherers from those of agriculturalists solely by examining the bones: the high-protein eating hunter-gatherers typically had bones that were larger, stronger and denser, and showed fewer signs of chronic disease. 

Scientists believe the differences in physical activity between the two civilizations played a bigger role than any dietary changes, and sure, hunting and gathering no doubt required a lot of time on one’s feet, but ask any farmer: farming isn’t exactly sedentary work!  Even if a heavy physical workload was responsible for Paleolithic peoples’ stronger bones, we can still conclude that a high intake of animal protein didn’t work against building bone mass.    

So how did some people come to think that protein—animal protein, in particular—is harmful for bones?

Yes, Protein is Acidic, But…

The assumption that a high protein intake results in reduced bone mineral density comes primarily from the idea that protein presents an acid load upon digestion, and the body leaches calcium from the bones in order to buffer this acidity in the blood.  This is a great hypothesis on paper.  The problem is, experimental evidence in humans shows that it isn’t actually true.  Or, even if it is true, and protein causes calcium to be released from bone tissue, protein also increases calcium absorption, so the net effect on bone density is still positive—stronger bones. 

It’s true that animal proteins have an acid residue, but so do grains and sugars, yet you rarely hear anyone cautioning people against consuming pasta, rice, bread, and sugary desserts for the sake of their bone health.  (Vegetables and fruits are net alkaline foods, while pure fats and oils are neutral.)  People might cut these carbs out to lose weight or manage their blood sugar, but to protect their bones?  Crickets. 

Let’s take a closer look at the effect of protein consumption on bones. 

Protein is a Net Bone Builder

A systematic review and meta-analysis from no less than the National Osteoporosis Foundation determined that regarding bone health, “Current evidence shows no adverse effects of higher protein intakes.”  Of course, no adverse effects doesn’t necessarily mean higher protein intakes are beneficial; they might be neutral and have no effect one way or the other.  However, numerous studies have shown that higher protein intake is, in fact, beneficial for bone health.    

As long as calcium intake is adequate, higher animal protein intakes are protective against hip fracture among middle-aged men and women.  There is a beneficial effect of animal and dairy protein intake on bone strength and structure.  In healthy post-menopausal women, animal proteins (including dairy) were shown to have a beneficial effect on bone strength and microstructure, with a positive association between protein intake, bone failure load, and stiffness of the peripheral skeleton.

Compared to a diet calling for a daily protein intake of 0.8 grams/kg of body weight—which is typical of most guidelines for protein intake—a diet calling for substantially more—1.4g/kg/day—attenuated bone loss during weight loss in overweight middle-aged adults.  This is in line with mounting evidence that aging individuals should consume more protein, not less. 

This has been observed over and over again.  According to one review, “consuming protein (including that from meat) higher than current Recommended Dietary Allowance for protein is beneficial to calcium utilization and bone health, especially in the elderly.”

Low protein diets may induce secondary hyperparathyroidism due to reduced intestinal calcium absorption, while a high protein diet was shown to have no effect on serum parathyroid hormone levels.  Another study showed that, among post-menopausal women, a high protein intake did reduce parathyroid hormone levels, coupled with an increase in intestinal calcium absorption, along with higher IGF-1 levels, with no change to biomarkers of bone homeostasis.  The authors concluded, “The increased IGF-I and decreased PTH concentrations in serum, with no change in biomarkers of bone resorption or formation, indicate a high-protein diet has no adverse effects on bone health.”

The bottom line here is, higher protein intakes do cause increased urinary calcium excretion, which is where the acid load idea has its basis.  However—and this is a big however—they also cause increased calcium absorption, and the net effect is increased calcium retention.  Think of it this way: if person A makes $100,000 a year and spends $50,000, they’ll have more money in the bank than person B, who makes $60,000 and spends $30,000.  Even though person A spent more money than person B, they took in more initially, so they still retained more.  Getting back to protein, even if the body excretes more calcium on a higher protein diet, if it also took in more to begin with, the net effect is actually higher calcium retention.   

Moreover, the calcium excreted does not necessarily come from bone stores: “…recent findings do not support the assumption that bone is lost to provide the extra calcium found in urine.”  The excreted calcium may be some of the extra dietary calcium that the body does not require at that time.  (Anyone who absorbs 100% of the nutrients they ingest should offer themselves up as a research subject, because that is some mighty rare digestive efficiency!)

Bones: More Than Just Calcium

Bones aren’t just a conglomeration of minerals wrapped around nothing.  They’re not just calcium, phosphorus, magnesium and other structural elements arranged haphazardly with no scaffolding keeping them in place.  If bones were nothing but calcium, they would shatter anytime someone took a fall, like a piece of chalk dropped on the ground.  To the contrary, according to Roland Kröger, lead author of a recent paper that uncovered new revelations about the fractal organization of bone nanostructure:

“Bone is an intriguing composite of essentially two materials, the flexible protein collagen and the hard mineral called apatite.” (Source)

Being that protein is a primary structural constituent of bone, it’s hard to fathom that consuming protein might lead to weaker bones.  It would be like claiming higher protein diets are counterproductive for building muscle. 

Protein has Multiple Beneficial Effects on Bones

Aside from contributing to the structural integrity of bone, protein exerts an influence on various hormones that affect bone building and resorption.  Here’s the short list:

• Increased intestinal calcium absorption
• Increased IGF-1 (promotes bone building via increased osteoblast activity and higher renal phosphate resorption)
• Suppression of parathyroid hormone (reduced osteoclast activity)
• Improved muscle strength and mass, which may benefit skeletal strength

Takeaway Message

None of this means that anyone needs to go out of their way to gorge on protein, but certainly a reasonable protein intake isn’t harmful for bones.  In fact, post-menopausal women and older individuals of both sexes may be better off increasing their protein intake, rather than steering clear because of unfounded fears about the negative effects on bone density, which have not been substantiated in studies.