Should you eat protein before exercise, or after?  

Eating protein after exercising may help rev up the body's muscle-making machinery, in both young and older men alike, a small study suggests. 

The study of 48 men - half in their twenties and the other half in their seventies -- found that in both age groups, consuming a protein drink after exercise led to a greater increase in muscle protein, compared with downing the drink after a period of rest. 

What's more, muscle protein increased at nearly the same rate in young and elderly men, the researchers report in the American Journal of Clinical Nutrition. 

The study has a number of limitations. Besides its small size, it did not look at actual muscle mass changes over time -- but only short-term changes in participants' muscle-fiber proteins after the protein drink. So it is not clear what kinds of gains older or younger adults might see from having their protein post-workout. 

Still, the findings do suggest that exercising before consuming protein may help the body put those nutrients to greater muscle-building use. 

And for older adults, they write, exercise should "clearly" be considered as a way to boost muscle-protein buildup in response to food -- and, by extension, to support healthy aging. 

The study included 24 older men with an average age of 74 and 24 young men with an average age of 21, none of whom regularly exercised. 

The researchers randomly assigned the men to one of two groups; in one, the men rested for 90 minutes, followed by 30 minutes of exercise -- pedaling a stationary bike and performing light strengthening exercises. In the other group, the men spent those additional 30 minutes relaxing. 

Afterward, men in both groups downed a drink containing 20 grams of protein, then had their blood levels of various amino acids (the building blocks of proteins) repeatedly measured. The researchers also took a small sample of tissue from each man's thigh muscle, right before the protein drink and 6 hours afterward, to measure changes in the amounts of protein in the muscle. 

Overall, muscle protein increased to a greater extent in the exercise group versus the inactive group, and both older and younger men showed similar benefits. 

It's well known that muscle mass tends to wane as people age, and some researchers have proposed that one reason may be that in older people, the body's muscle-protein production responds less efficiently to protein from food, and also to exercise. 

However, the current findings suggest that this may not be the case. 

Based on these findings, they conclude, it's possible that having protein after exercise allows for greater use of food-derived protein for muscle building, in young and old alike.