The Female Athlete

A quick review of the Dietary Reference Intakes (DRIs) demonstrates clearly that females have different nutrient requirements than males. Many of the requirement differences are based on body size (males being larger than females), but some are due to clear physiological differences, as is the case with iron (females require twice as much).

Energy intakes, for all athletes, are based on total weight, weight of the metabolic mass, and duration and intensity of exercise. Surveys of female athletes commonly report an underconsumption of energy, leading many to conclude that female athletes are at elevated risk of developing eating disorders regardless of the type of sport they are participating in.³ In addition, the literature is filled with reports of the impact that intense exercise has on the female reproductive system, with amenorrhea or oligomenorrhea a common outcome. These reports suggest that increasing caloric intake to offset the high energy demand may be sufficient to reverse the menstrual dysfunction and halt the associated reduction in bone mass.⁴ The reduction in bone mass caused by menstrual dysfunction is clinically relevant for female athletes because it places them at current increased risk for stress fractures and later increased risk for osteoporosis. In one study of 46 female athletes (31 with multiple stress fractures and 15 without stress fractures), nearly half of all athletes with stress fractures had menstrual irregularities, with a particularly high prevalence observed in endurance runners with high weekly training mileage.⁵ Although consuming sufficient calories and calcium will not correct the biomechanical factors associated with stress fractures, including a high longitudinal foot arch and leg-length inequality, it will substantially reduce risk if this strategy helps females return to normal menstrual function.⁶

The energy substrate distribution is of interest to female athletes. Studies indicate that females have a higher lipid, lower glycogen (carbohydrate), and lower protein utilization than do male athletes in endurance exercise.⁷ Because glycogen storage is limited, the lower rate of glycogen utilization gives female athletes what appears to be a clear advantage over men in long-duration, lower-intensity athletic events.⁸ This also gives rise to the following question: Should female endurance athletes have a different energy substrate consumption pattern than male endurance athletes given the difference in the pattern of substrate utilization? No solid evidence indicates that there should be a difference in intake, and the nature of endurance and ultraendurance events still makes carbohydrate storage (glycogen) the limiting substrate in performance. Whether an endurance athlete is male or female, when glycogen is depleted the athletic performance will drop (or stop). A series of studies assessing the carbohydrate consumption pattern of female athletes involved in different sports indicated a wide range of intakes (see table 11.1). Few of the assessed female athlete groups meet the recommended carbohydrate intake of 5 to 7 grams per kilogram per day for general training and 7 to 10 grams per kilogram per day for endurance athletes.⁹

The general (nonathlete) recommendation for protein consumption in adults is .8 grams per kilogram per day. The athlete recommendation is approximately double this and ranges between 1.2 and 1.8 grams per kilogram per day, depending on the degree to which the athlete is involved in endurance activity.¹⁰ This recommended level is likely to be greater than actual needs, provided adequate total energy consumption is obtained. It should be noted that no specific protein requirement data are available for female athletes, so these values are derived from mixed-athlete or male studies. Until female-specific protein requirement data are determined, female athletes should try to consume a protein level within the currently established range.

Fat consumption is targeted by female athletes wishing to lower body weight, as indicated in a study showing amenorrheic athletes have a fat intake that is 6 percent lower than eumenor rheic athletes.¹¹ To obtain a sufficient energy intake, fat consumption should not be eliminated from the diet. Given the high energy needs of athletes, plus the fact that female athletes have an excellent system for catabolizing fat for energy, fat intakes should range in the area of 20 to 25 percent of total energy intake.

Female athletes appear to have less than adequate vitamin B₆ intake, assessed either in absolute values or in ratio to protein consumption.^12,13 With the exception of this vitamin, female athletes who are not on energy-restrictive food intakes appear to obtain adequate vitamins to sustain health and physical activity.

There is no question that calcium and iron intakes are of concern in the diets of female athletes. Adequate calcium consumption is necessary to develop and maintain high-density bones that are resistant to fracture, and iron is necessary for oxygen delivery to working cells. For athletes concerned about dairy product consumption, calcium-fortified orange juice is an excellent alternative and, per equal volume, has the same calcium concentration as fluid milk. It should be understood that calcium intake, by itself, does not guarantee good bones. Although calcium, vitamin D, estrogen, and physical stress are all needed for bone development, ensuring an adequate calcium intake is within easy reach of every athlete.

Surveys have found low storage iron (ferritin) in female runners and other studies have found that female athletes with anemia can improve aerobic performance through a program of iron supplementation.^14,15 Unnecessarily taking iron or other supplements in the absence of iron deficiency is not desirable. Given the very real risks that iron depletion poses, female athletes should regularly (at least yearly) have iron status assessed, with the inclusion of ferritin in the assessment protocol.

General Recommendations for Female Athletes