- A small study by the University of Worcester found high-protein diets may decrease men's testosterone levels.
- Researchers fed 309 subjects a diet heavy in meat, fish, eggs, and protein shakes for eight weeks.
- Testosterone levels among study participants decreased an average of 37% by the end of the study.
High-protein diets may lead to reduced testosterone levels in men, a new study suggests.
The study, published in The Journal of Nutrition and Health and led by nutritionist Joe Whittaker, tested the testosterone levels of 309 men over the course of eight weeks.
Subjects were fed a diet that included 35% meat, fish, and protein shakes.
By the end of the study, their testosterone levels had decreased by an average of 37%. Participants experienced symptoms of decreased testosterone, including erectile dysfunction, fatigue, depression, and muscle weakness, Whittaker, a researcher of nutritional therapy at the University of Worcester in the UK, told Insider.
Whittaker said the findings suggest a diet where 35% of a man's total caloric intake is made up of protein — double the average intake for men, according to registered dietitian Erin Coleman via SF Gate — can lead to lowered testosterone levels. However, he believes, based on his research, it is still possible to see reduced testosterone from eating less than 35% protein.
Registered dietitian Bonnie Taub-Dix told Insider that she agrees that too much protein can lead to reduced testosterone in men, but it wouldn't apply to most people and would only affect those who consume excessive protein in an attempt to build muscle.
One limitation of the study is that it only tracked its subjects over an eight week span and did not account for other factors of the subject's lifestyle that could impact their testosterone levels.
How much protein is too much?
Men are advised to eat 2,200-2,800 calories per day, according to WebMD. Those calories should be made up of 2.5 cups of fruit, 3.5 cups of vegetables, 6-10 ounces of grain, three cups of dairy, 170-400 calories of fat, and just 5-7 ounces of protein, according to WebMD.
Diets made up of 35% protein on a 2,200-2,8000 calorie diet equals about 185-215 grams of protein per day, according to Mayo Clinic. 185 grams of protein is equal to about 1.5-2 lbs of chicken, 2-2.5 lbs of salmon, or 29-35 eggs, according to the US Department of Agriculture.
Taub-Dix said that men who eat high-protein diets to fuel their gains could actually make muscle building more difficult for themselves because low testosterone levels can slow down a man's bulking progress. Testosterone is the main hormone responsible for muscle growth, according to Healthline. Taub-Dix said a more balanced diet and training routine would help you to bulk up faster.
"Balance is key when it comes to getting in shape whether you want to lose weight, gain weight or stay right where you are," she said.
Low testosterone levels can lead to chronic illnesses
Whittaker said the study's test subjects experienced many short-term side effects of low testosterone levels, but he pointed to studies showing longterm low testosterone can lead to risks and conditions, like infertility and an increased risk of chronic illnesses, like cardiovascular disease, diabetes, and Alzheimer's.
Healthy testosterone levels among men are often associated with strong bone density, ability to build muscle and strength, mood and overall cognitive function regulation, and sex drive, Kevin Codorniz, MD, chief of endocrinology for Loma Linda University Health previously told Insider.
A high-protein, low-carb diet can suppress the immune system, according to researchers
The study also found that a high-protein and low-carb diet can increase cortisol in the blood which is the body's main stress hormone. Whittaker said high cortisol levels can lower the body's immune defenses.
"Cortisol is well-known to suppress the immune system, so high levels of cortisol on low-carb diets may suppress the immune system, making people more susceptible to catching infections such as colds, the flu, and COVID-19," Whittaker told Insider. "In the long-term, you could get things like osteoporosis (thinning bones), but I think that is unlikely."
Following a balanced diet and supplementing protein with carbs, fiber, and vegetables can help prevent hormonal imbalances and immune system suppression for men, according to Healthline.