by Jeff S. Volek, Ph.D., R.D.
In order to enhance development of muscle strength and size, you need to be engaged in resistance training. This is mandatory. In order to get the most bang for your resistance training buck, you need to have adequate testosterone levels. Testosterone is a steroid hormone secreted from specialized cells in the testes in men and in smaller amounts from the ovaries in women. Testosterone has potent anabolic (muscle building) effects on muscle tissue.
What can elevated testosterone levels do? In addition to building muscle, increased testosterone can have favorable effects on a number of other factors including sexual interest, erectile function, mood, and bone density.
Three absolute requirements to build muscle
One way to naturally manipulate testosterone levels is through diet. Dietary nutrients and hormones (like testosterone) work together to regulate repair of skeletal muscle proteins that are broken down during training. Manipulation of these variables to favor anabolism (building up) over catabolism (breaking down) during recovery enhances the development of muscle strength and size. Resistance training provides the stimulus for increasing muscle size and strength but without the right building blocks supplied from diet and the right hormonal environment, optimal gains in size and strength will not be achieved.
The dietary fat trigger The primary nutrient shown to affect testosterone to the greatest extent is fat. In general, scientific studies indicate that a diet low in fat is associated with lower testosterone values compared to a diet higher in fat. Individuals consuming a diet containing ~20% fat compared to a diet containing ~40% fat have significantly lower concentrations of testosterone. Men consuming a vegetarian or meatless diet have lower circulating concentrations of testosterone compared to men consuming a mixed Western or a high meat diet. In another study, male endurance athletes who switched from a meat-rich diet to a lacto-ovo vegetarian diet had a significant decrease in resting testosterone concentrations and a lessening of the exercise-induced increase in testosterone. The diets in this study contained equal percentages of calories derived from protein, carbohydrate, and fat, however the source of protein in the vegetarian diet was derived mainly from vegetable sources (83%) whereas the mixed diet contained significantly less vegetable protein (35%). Thus, quality not just quantity of the diet appeared to influence testosterone in this study. A study in our laboratory showed that testosterone levels were higher in resistance-trained men who consumed higher amounts of fat, especially monounsaturated fat (rich in canola and olive oils).
In summary, cross-sectional and diet intervention studies indicate that low-fat diets (20-30% of calories) are associated with lower testosterone levels compared to diets higher in fat (about 40% of calories). The type of protein is also important. Reducing vegetable protein in favor of animal protein can increase the resting testosterone levels and complement, not blunt, the normal exercise-induced increase in testosterone.
Testosterone killer - calorie restriction In addition to dietary fat, the total amount of calories can also have an important impact on testosterone as well. Several studies indicate that reducing calories can negatively affect testosterone levels. Kiddy et al examined the effect of a very-low-calorie diet (330 kcal/day) on testosterone levels in overweight women. After 2 weeks there was a -40% decrease in free testosterone levels. Strauss et al measured testosterone levels in wrestlers during the competitive season and two months after the season. Large reductions in body fat induced by caloric restriction and exercise during the season resulted in the largest decreases in testosterone. The wrestlers with an extremely low body fat demonstrated abnormally low testosterone levels that returned to normal after the season.
Other studies have also shown decreases in testosterone levels in wrestlers while consuming a low caloric energy diet. Guezennec et al (1994) measured testosterone in soldiers consuming low (1800 kcal/day), moderate (3200 kcal/day), or high (4200 kcal/day) calorie diets with similar ratios of carbohydrate, protein, and fat. The diets were consumed during 5 days of prolonged exercise and sleep deprivation (4 hrs/day). After 5 days, testosterone had decreased by ~50% on the low calorie diet and only about ~20% on the moderate and high calorie diets. These findings indicate that exhaustive exercise can decrease testosterone levels and that inadequate energy can speed up the decrease in testosterone.
Collectively the studies that have examined the effects of energy intake on testosterone indicate that large reductions in calories and body weight can reduce testosterone levels. This is especially true if a large reduction in calories is combined with prolonged and exhaustive physical activity. If weight loss is a goal, a much better approach would be to reduce calories slightly (about 300-500 kcal/day) and incorporate heavy resistance training into your exercise program. This should eliminate the decrease in testosterone that typically occurs when calories are reduced.
The bottom line
It's desirable for men and yes, even women, to maintain healthy testosterone levels.
- Engage in resistance training.
- Don't eat a low-fat diet. Instead consume monounsaturated fats.
- Favor animal protein over vegetable protein.
- Only reduce calories slightly while reducing weight.
In order to enhance development of muscle strength and size, you need to be engaged in resistance training. This is mandatory. In order to get the most bang for your resistance training buck, you need to have adequate testosterone levels. Testosterone is a steroid hormone secreted from specialized cells in the testes in men and in smaller amounts from the ovaries in women. Testosterone has potent anabolic (muscle building) effects on muscle tissue. 
