[This article originally appeared in the
May 2000
issue of Northwest Runner
magazine.]
As far as I can tell, most runners understand that adequate carbohydrate intake is an important component of any training program. However, the importance of dietary protein is perhaps less obvious. Do runners need more of it than couch potatoes? If so, how much more?
As recently as a month ago, I myself didn't know the answers to these questions. Since nothing cures ignorance like a fast-approaching deadline, I decided to make protein intake the subject of this month's column. (A related topic -- iron intake -- will be covered in next month's column.)
Keeping your balance
How can scientists tell whether someone is getting enough protein in his/her diet? The traditional approach has been to assess the subject's "nitrogen balance," i.e., to determine whether his/her nitrogen intake is sufficient to replace the amount lost in the urine, feces, and sweat. Since the amino acids in protein are our main source of nitrogen, a negative nitrogen balance indicates that the subject probably isn't ingesting enough protein (Lemon, International Journal of Sport Nutrition 5: S39-61, 1995).
Based on the concept of nitrogen balance, the United States Recommended Daily Allowance (USRDA) for adults has been set at 0.8 grams of protein per kilogram of body mass. According to this guideline, someone who weighs 60 kilograms should consume 0.8 x 60 = 48 grams of protein per day in order to cover his/her daily nitrogen losses (Paul, Sports Medicine 8: 154-76, 1989).
With this issue in mind, a number of researchers have set out to define the protein needs of endurance athletes using the nitrogen-balance approach. The first published study on this topic was that of Tarnopolsky et al. (Journal of Applied Physiology 64: 187-93, 1988), who collected and analyzed the urine, feces, and sweat of six elite male endurance athletes (runners or cross-country skiers), each of whom was studied during a period of low protein consumption and during a period of high protein consumption. Tarnopolsky's calculations revealed that, on average, these athletes required a minimum of 1.37 g protein per kg per day to stay in nitrogen balance.
While these three studies came up with three different numbers, they collectively suggest that runners do indeed require more protein in their diet than the USRDA would indicate, even though the USRDA includes a considerable "safety margin" intended to reflect the varying needs of different individuals (Paul, 1989). And studies of male weightlifters (reviewed by Lemon, 1995) confirm that they too have increased protein needs -- probably about 1.4-1.8 g/kg/day. Thus, if you're a serious runner who pumps iron on the side, it's a safe bet that you need more protein than your couch-potato contemporaries.
What about women?
After reading the above paragraphs, one might wonder whether anyone has checked into the protein needs of female runners. The answer is a qualified yes. Nitrogen-balance studies of exercising women are unfortunately rare; however, a pair of reports from the Tarnopolsky lab (Tarnopolsky et al., Journal of Applied Physiology 68: 302-8, 1990; Phillips et al., Journal of Applied Physiology 75: 2134-41, 1993) have concluded that female athletes break down less protein during exercise than male athletes do and consequently may not need to eat quite as much protein per kilogram of body weight. More work is needed in this area.
A third question which may come to mind is: is it possible to overdose on protein such that it does your body more harm than good? Once again, the final word on this subject has yet to be written. However, there are signs that excessive protein consumption makes the kidneys "work harder" and thus contributes to the development of long-term renal problems (Brenner et al., New England Journal of Medicine 307: 652-9, 1982). In addition, high protein loads can interfere with the body's ability to absorb dietary calcium (Garlick et al., European Journal of Clinical Nutrition 53: S34-43, 1999).
A final point to be made is that this article has not distinguished between animal and plant sources of protein. As many of you know, individual plant products do not contain all of the amino acids we need, so vegetarians must eat a variety of foods to insure adequate intake. However, this point has been discussed at length by other columnists and textbooks and will not be considered further here.
The bottom line
So you're a runner. So your body needs extra protein. Does this mean you need to stock up on mammoth steaks and giant tubs of tofu? Probably not. As a runner, you likely eat more than your sedentary sidekicks -- and that means that you likely take in more protein than they do. As long as you consume enough calories to maintain your body weight, and as long as at least 15% of those calories come from protein, you're probably meeting your daily protein needs. (It's worth doing a quick calculation to make sure this is true. Assume your goal to be 1.5 g/kg/day just to be safe, and count each gram of protein as 4 calories' worth of nourishment.) In short, there's no need to stop eating spaghetti . . . as long as you remember to throw in an occasional meatball!