A 10-week muscle-building and dietary program involving 50 previously untrained, healthy middle-aged adults with overweight found no evidence that eating a high-protein diet increased strength or muscle mass more than consuming a moderate amount of protein while training. Published in the American Journal of Physiology: Endocrinology and Metabolism, the study is one of the most comprehensive investigations of the health effects of diet and resistance training in middle-aged adults, the researchers say. 

Participants in the study were between the ages of 40 and 64 years, had no previous weightlifting experience and were randomized into moderate- and high-protein diet groups. The team assessed participants’ strength, lean-body mass, blood pressure, glucose tolerance and several other health measures before and after the program. The intervention involved a standard strength-training protocol of three sessions per week. To standardize protein intake, participants ate a freshly cooked, minced beef steak and drank a carbohydrate beverage after every training session. They were also sent home with an isolated-protein powder to mix with water and consume every evening throughout the 10 weeks of the study.

“The moderate-protein group consumed about 1.2 grams of protein per kilogram of body weight per day, and the high-protein group consumed roughly 1.7 grams per kilogram per day,” explains Colleen McKenna, a graduate student in the Division of Nutritional Sciences and a registered dietician at the University of Illinois Urbana-Champaign. McKenna, who led the study with kinesiology and community health professor Nicholas Burd, says the team kept calories equivalent in the meals provided to the two groups with additions of beef tallow and dextrose.

The participants also kept food diaries and McKenna counseled them every other week about their eating habits and protein intake.

In an effort led by University of Illinois Urbana-Champaign food science and human nutrition professor Hannah Holscher, the team also analyzed gut microbes in fecal samples collected three times throughout the course of the study: (1) at the beginning of the intervention; (2) after the first week, during which participants adjusted to the new diet but did not engage in physical training; and (3) at the end of the 10 weeks. Previous studies have found that diet alone or endurance exercise alone can alter the composition of microbes in the digestive tract.

“The public health messaging has been that Americans need more protein in their diet, and this extra protein is supposed to help our muscles grow bigger and stronger,” Burd explains. “Middle age is a bit unique in that as we get older, we lose muscle and, by default, we lose strength. We want to learn how to maximize strength so that as we get older, we’re better protected and can ultimately remain active in family and community life.”

The Recommended Dietary Allowance (RDA) of protein for adults is 0.8 grams of protein per kilogram (2.2 pounds) of body weight per day to avoid developing a protein deficiency. The team tried to limit protein consumption in the moderate-protein group to the RDA, but their food diaries revealed that participants were consuming, on average, 1.1 to 1.2 grams of protein per kilogram of body weight per day. Those in the high-protein group ate about 1.6 to 1.7 grams of protein per kilogram per day, which is twice the recommended amount.

Burd and his colleagues hypothesized that getting one’s protein from a high-quality source such as beef and consuming significantly more protein than the RDA would aid in muscle growth and strength in middle-aged adults engaged in resistance training. At the end of the 10 weeks, however, the team saw no significant differences between the groups. Their gains in strength, as well as their body fat, lean body mass, glucose tolerance, kidney function, bone density and other “biomarkers” of health, were roughly the same.

The only potentially negative change researchers recorded between the groups involved alterations to the population of microbes that inhabit the gut. After one week on the diet, those in the high-protein group saw changes in the abundance of some gut microbes that previous studies have linked to negative health outcomes. Interestingly, Burd and his colleagues found that their strength-training intervention reversed some of these changes, increasing beneficial microbes and reducing the abundance of potentially harmful ones.

What the Research Means to Health and Exercise Professionals

Your dedicated clients want to know how to maximize their resistance-training efforts, both in and out of the gym. And that includes what foods they eat. Adding protein powder to a smoothie or eating animal-based protein at both lunch and dinner are not uncommon practices for those looking to increase muscle size and strength. That extra protein, however, may not be necessary.

“We found that high-protein intake does not further increase gains in strength or affect body composition,” Burd says. “It didn’t increase lean mass more than eating a moderate amount of protein. We didn’t see more fat loss, and body composition was the same between the groups. They got the gain in weight, but that weight gain was mainly from lean-body-mass gain.”

Burd says these findings make him question the push to increase protein intake beyond 0.8 to 1.1 grams per kilogram of body weight, at least in middle-aged weightlifters consuming high-quality, animal-based protein on a regular basis.

McKenna said the team’s multidisciplinary approach and in-depth tracking of participants’ dietary habits outside the laboratory makes it easier to understand the findings and apply them to daily life.

“We have recommendations for healthy eating and we have recommendations for how you should exercise, but very little research looks at how the two together impact our health,” McKenna suggests. To help bring these elements together, the study team included exercise physiologists, registered dietitians and experts on gut microbiology. “This allowed us to address every aspect of the intervention in the way it should be addressed,” she says. “We’re honoring the complexity of human health with the complexity of our research.”

Given this complexity, it is important to stay away from one-size-fits-all recommendations, and instead urge your clients to follow established guidelines and take the time to find out what works best for them. Just as some people feel better following a plant-based diet, others thrive by including moderate amounts of animal-based protein. Urging your clients to choose whole-food sources of all macronutrients, protein included, is well within your scope of practice and is good advice for both maximizing strength-training results and for general health and well-being. In fact, helping your clients make healthy choices and positive lifestyle changes should be considered an integral part of your job.