Could Eating Too Much Protein Increase Your Risk of Stress Fractures?
What you eat provides not only the fuel to get you from point A to point B while running, but also the raw ingredients for all the stuff that makes up your body.
Eating properly is especially important to competitive runners as your body needs to be able to withstand and adapt to the rigors of heavy training in order to improve.
Practically everybody knows the basic role of these two nutrients: protein helps build and repair muscle tissue, and calcium is used to strengthen your bones. Both are important for recovery and improvement.
But protein and calcium interact in a complex fashion, and some studies have suggested that consuming too much protein can actually cause calcium loss, hence leading to weaker bones.
To figure out if this is something runners need to be worried about, we’ll have to consult the scientific literature.
The correlation between protein and calcium
It is well-established that increasing your protein intake will also increase your calcium excretion in your urine. A 1998 study of elderly subjects in Japan, for example, demonstrated a consistent positive correlation between protein intake and calcium levels in urine.
Protein intake and calcium loss
In a review study on the benefits and drawbacks of high dairy intake, Roland Weinsier and Carlos Kruimdieck at the University of Alabama describe how protein intake creates acids inside the body, which in turn bind to calcium and end up being excreted in urine. This is troublesome because low calcium intake can lead to low bone density.
Because of this, we would expect studies of dietary habits and bone health to find that high protein intake is associated with lower bone mass as a result of chronically high calcium loss.
Protein and muscular strength
Some studies of elderly people find this same result, but paradoxically, others find that high protein intake is associated with stronger bones in elderly people.
One simple explanation might be that, because inadequate protein intake leads to muscular weakness, this could explain the healthy bone strength in people with high protein diets.
Though muscular strength does play a role in bone health, the situation is more complicated—calcium and protein interact with one another in a manner dependent on their relative intake levels, as illuminated by Robert Heaney in a 2002 article.
Protein intake does indeed increase calcium loss via the urine. In response to this loss, the body releases a hormone to activate three mechanisms to increase the availability of calcium. One of these is to salvage calcium from skeletal bones, which can explain the loss in bone mass associated with high-protein diets in some studies.
But the two other effects, synthesizing an activated form of vitamin D in the kidneys, and directly increasing the absorption of calcium from the gut, both work to counteract the calcium lost when protein intake is high.
Heaney supports this by citing a study that found bone mass increased in people who took a calcium supplement and had high protein intake, but bone mass decreased when protein intake was low, even when the subjects were taking a calcium supplement.
Effects of protein and calcium in athletes
None of the studies we’ve looked at involved athletes. In fact, most of them involved elderly people at risk of bone fractures, which are a ways removed from competitive distance runners (no offense to any Masters athletes!).
However, calcium plays an important role in bone strength in younger, healthy distance runners too:
- Research reveals that calcium intake and physical exercise work together to build bone mass in young people, and low calcium intake was associated with stress fractures in distance runners in a 1990 study.
- Further, a 2008 paper reported that calcium supplementation decreased the risk of a stress fracture in female US Navy recruits.
The lesson to take from this is that your calcium and protein intake are both important, and should be proportional to one another.
For example, if you are actively trying to increase your calcium intake while recovering from a tibial stress fracture, you should make sure you increase your protein intake as well.
And if you are trying to eat more protein so you’re better recovered during a block of intense training, make sure you get enough calcium, too.
Dairy foods are a great source for both, as long as you aren’t lactose intolerant.
It can be overwhelming to try to cover all of the particulars when thinking about your diet, but one easy part is balancing out calcium and protein intake so both nutrients can work together to make you stronger.