More Calories

[Men's] calorie needs are greater," says David Heber, MD, director of the UCLA Center for Human Nutrition in Los Angeles. "Those needs are dependent on the amount of lean body mass and average about 14 calories per pound of lean body mass per day at rest with additional calories for exercise."

For example, Heber added, a typical 180-pound man who has 17% body fat would have 150 pounds of lean muscle and would need 2,100 calories per day at rest. A woman who weighs 130 pounds with 100 pounds of lean body mass at about 23% body fat would need 1,400 calories at rest.

"You might add 300 to 500 calories per day for physical activity," Heber notes. "However, the differences are quite large, as you can see."

In large part, these differences are driven by reproductive hormones, Heber says. In men, testosterone is responsible for muscle mass differences from women and this hormone accounts for the extra muscle driving extra protein and calorie requirements.

But there are subtler differences, also.

"Even if you take differences in size and weight out of the equation and express nutritional needs per body weight or lean body mass, there are still differences between men and women," says Paul J. Flakoll, PhD, professor of nutritional physiology and director of the Center for Designing Foods to Improve Nutrition at Iowa State University in Ames, Iowa. "Obviously, there are differences during life events specifically related to women, such as pregnancy and lactation, which men do not experience."

Normal levels of circulating red blood cells are higher in men than in women, which may have nutritional implications, Flakoll says, adding that men do not tolerate low levels of plasma glucose, or hypoglycemia, as well as women.