By Michael Ormsbee, Ph.D., Florida State University

Edited by Kate Findley, The Great Courses Daily

Running and other endurance exercises are great for your health, but you can lose muscle mass if you don’t eat enough protein. Dr. Ormsbee explains how to meet your protein needs—both in terms of quality and quantity.

Nitrogen Balance

Protein is essential for a balanced diet, especially for people who lead an active lifestyle. To determine how much protein you need to eat, consider your nitrogen balance. Nitrogen is the part of the protein molecular structure that distinguishes it from carbohydrates and fat on a chemical level.

“In the lab, we can measure nitrogen balance, which is defined as the balance between nitrogen intake and nitrogen output,” Dr. Ormsbee said. “Protein foods can be analyzed for their nitrogen content, and this will take care of the nitrogen in part of the nitrogen balance equation.” 

Doctors can then analyze nitrogen from body output including urine, feces, sweat, skin, and hair. When nitrogen intake is higher than nitrogen output, you are in a positive nitrogen balance. 

In this scenario, you would be likely to be anabolic and in muscle growth mode. If you are trying to maintain or build muscle mass or lose fat, you want to be in a positive nitrogen balance.

When nitrogen intake is less than nitrogen output, you are in a negative nitrogen balance. You would likely be sacrificing muscle to support your protein needs in this case. This occurs during very long and intense workouts, if you don’t eat for extended periods of time, or during the normal process of aging, if you don’t pay attention to how much protein you eat. 

Protein Quality

Positive and negative nitrogen balance states are both very important to your overall body composition. Factors that may influence nitrogen balance include protein quality—like complete or incomplete. Complete proteins are those containing all the essential amino acids, which we must include in our diet because our body doesn’t make them naturally.

Amino acid content and total calorie intake also play a role. High-quality animal proteins like eggs and dairy products are best because of their amino acid content and their high digestibility—that is, they are easy to digest and absorb.

Plant proteins have a lower digestibility, meaning that they are harder to digest and absorb, and are low in some of the essential amino acids. A common recommendation for vegetarians is to increase protein intake by about 10 percent to account for this discrepancy. 

Protein and Exercise

The amount of protein in your diet can make a difference in overall body composition and health. However, protein needs can change based on how much and what type of exercise you do.

How does exercise affect our protein needs? Endurance and resistance exercise can use or burn a lot of calories; this fact alone would require more protein in your diet. 

Exercise also puts a lot of strain on your body’s protein stores. These protein stores are known as the amino acid pool.

One study measured nitrogen balance in young and middle-aged distance runners. These runners were randomly assigned to low-, moderate-, or high-protein intake diets from egg and milk proteins for 10 days, and nitrogen balance was measured in the final five days of each of the diets. 

Whether the athletes were young or middle-aged, a low-protein diet made all the runners on that diet fall into a negative nitrogen balance. The moderate-protein intake, which was just above the current recommended dose of 0.36 grams per pound of body weight, made about half of the runners go into a positive nitrogen balance, but the other half remained in a negative nitrogen state. When a high-protein diet was given, the majority of runners were in positive nitrogen balance.

Preserving Muscle Mass

Thus, in runners, regardless of age, research shows that protein intake that is higher than current recommendations was best to promote an environment that stops muscle wasting. In this study, the high-protein diet was 0.6 grams per pound. 

If you weigh 145 pounds, this would be about 80 grams of protein per day. If you weigh 185 pounds, this would amount to approximately 100 grams of protein per day.

Protein is not only good for those engaging in endurance exercises such as running—similar results have been found in studies involving weight-lifters. One study which followed a group of men over 13 days found that in the men who lifted weights, only the moderate- and high-protein diets led to a positive nitrogen balance.

Protein breakdown increases when you exercise hard. Ultimately, you have the best chance at maintaining or improving muscle mass by choosing a diet with high rather than adequate protein levels.

This article was edited by Kate Findley, Writer for The Great Courses Daily.

Dr. Michael Ormsbee is an Associate Professor in the Department of Nutrition, Food, and Exercise Sciences and Interim Director of the Institute of Sports Sciences and Medicine in the College of Human Sciences at Florida State University. He received his M.S. in Exercise Physiology from South Dakota State University and his Ph.D. in Bioenergetics from East Carolina University.