A new study found that short-term high protein diets may hinder muscle endurance and short-term high carbohydrate diets may increase endurance (mSystems, May 17, 2022;7(3)). This study found that what elite long-distance runners ate determined which types of bacteria lived in their colons. A diet high in fruits and vegetables (high-carbohydrates) was associated with improved time-trial runs in highly trained competitive runners by 6.5 percent, while a high-meat diet (high-protein) was associated with a performance reduction of 23.3 percent.

The runners who had the least changes in the bacteria in their colons had the greatest improvement in time trial performance. The study found that:
• The high-carbohydrate diet that improved performance was associated with less change in the types of colon bacteria and an increase in the healthful types.
• The high protein diet that slowed exercise performance changed the types of bacteria in the colon (microbiome), reduced the number of different bacteria (diversity), and raised the levels of some types of viruses and bacteria.

How Diet May Influence Athletic Performance
Your muscles use primarily carbohydrates and fats for energy. The limiting factor for how fast you can move is how long it takes for oxygen to go from your bloodstream into the exercising muscles (Int J Sport Nutr Exerc Metab, 2018;28:451–463). Carbohydrates (sugar) require less oxygen to make energy than fats do, so as you start to run out of your limited sugar supply, you have to slow down. You have an almost endless amount of fat in your body, but only a meager amount of sugar stored in your muscles and liver. Restricting carbohydrates for a short period can increase the ability of your muscles to store extra sugar when you eat carbohydrates days later (Med Sci Sport Exerc, 2010;42:2046–2055).

You need some extra protein to stimulate muscle growth for exercise (Sports Med, 2019;49:59–68). Therefore many competitive athletes manipulate their training (Sports Med, 2018;48:1031–1048) by restricting carbohydrates and increasing protein for a week or two, and then eating a high carbohydrate diet for a few days before they compete. During the high-carbohydrate period, you should not “carbohydrate load” where you force yourself to eat a tremendous amount of carbohydrates such as pasta or bread (Eur J Sport Sci, 2015;15:3–12). Your muscles can store only a limited amount of sugar, so you can increase sugar storage in muscles only a little bit. Excess carbohydrates are converted to fat, and you don’t need extra fat for exercise. In 1973, I reported the case of a marathon runner who suffered a heart attack after carbohydrate-loading in the days before a marathon (JAMA, Mar 26, 1973;223(13):1511-2).

Another reason not to load on meat: a very high-meat diet contains sulfated amino acids such as cysteine and methionine, and fermentation of these acids in your colon can produce ammonia, phenols, and hydrogen sulfide, which can damage the lining of your colon and change the types of bacteria in your colon (Nutrients, Mar 10, 2018;10(3):337).

My Recommendations
Recent papers are showing that exercise may improve health by increasing the number of healthful colon bacteria and decreasing the number of harmful bacteria. A healthful plant-based diet appears to build endurance by increasing the number and diversity of healthful bacteria in your colon (mSystems, May 17, 2022;7(3)), so eating lots of fruits, vegetables, whole grains, beans and nuts may make you a better athlete.