eMedicine Specialties > Sports Medicine > Introductory Topics in Sports Medicine
Author: Luis E Palacio, MD, Director of Primary Care Sports Medicine,
Coauthor(s): Jeffrey W R Dassel, MD,, Associate Director, Sports Medicine and Faculty, Department of Family and Community Medicine, Christiana Care Health System
Updated: Oct 31, 2008
Inadequate nutritional intake is more common in female athletes than in their male counterparts. Proper diet is paramount for active individuals to maintain adequate energy during physical activity and for postactivity recovery.
Female athletes who participate in sports that encourage leanness because of a need to wear contour-revealing clothing or because the activities involve scoring on the basis of appearance commonly have inadequate nutritional intake.
Sports that emphasize leanness include the following:
Koutedakis and Jamurtas found that female dancers consume less than 70% of recommended daily energy needs.
An individual's dietary needs depend on his or her sex and body size, on the demands of the activity performed, and the duration for which the person performs the activity.
Dietary components include macronutrients (carbohydrates, protein, and fat) and micronutrients (fluids, electrolytes, vitamins, and minerals). Specific requirements are presented in the Table in the Summary of Nutritional Requirements and Sources section.
Carbohydrates are necessary to meet energy needs, more so in endurance athletes than in strength athletes.
Carbohydrate needs are commonly based on the athlete's body size and activity level. Individuals engaged in moderate-duration, low-intensity exercise require 5-7 g of carbohydrates per kilogram of body weight.
By contrast, those participating in long-duration and high-intensity exercise require 7-12 g of carbohydrates per kilogram of body weight (see the Table).
Fruit, vegetables, brown rice, enriched whole-grain breads, whole grain cereals, rolled oats, beans, legumes, and sweet potatoes are good examples of healthy carbohydrate foods.
Active individuals have a heightened protein requirement because they have a high percentage of lean muscle mass to support, they need protein to repair muscle tissue that is damaged during exercise, and they require additional protein for energy during exercise.
The amount of protein required depends on the type of activity being performed. Researchers recommend protein intakes of 1.2-1.4 g/kg/d for individuals participating in endurance sports and 1.6-1.8 g/kg/d for those involved in anaerobic activities (see the Table).6
Benefits of substituting carbohydrates with protein include the following:
•Enhanced weight loss
•Reduction in truncal adipose tissue
•Optimal maintenance of blood glucose levels
•Improved lipid profile
Protein-rich foods include lean pork and beef, poultry, fish, eggs, beans, tofu, and low-fat dairy products. Women at risk for having a low protein intake are those who restrict their energy intake to achieve weight loss or those who eat a vegetarian diet.
In the past, some investigators expressed concerns that a high-protein diet can cause renal damage. However, no conclusive evidence suggests that a high-protein diet negatively affects healthy adults with normal renal function. In addition, some researchers have raised questions about whether a high-protein or low-carbohydrate diet may increase the all-cause mortality risk in women.8 Further research is necessary to determine if this is the case.
Fat provides essential elements for the cell membranes and is essential for the absorption of fat-soluble vitamins. Fat should account for 25-30% of a person's energy intake.Diets should be limited in saturated and trans-fats, while providing adequate amounts of essential fatty acids (linoleic and alpha-linoleic acid). In women, the following intakes are advised (see the Table):
•Linoleic acid intake 11-12 g/d
•Alpha-linoleic acid intake 1.1 g/d
Functions of essential fatty acids include regulation of blood clotting, blood pressure, heart rate, and immune responses.
Dietary fatty acids should come from naturally lean protein foods, nuts, seeds, nut butter, fatty fish (eg, salmon, trout), fish-oil supplements, flaxseed oil, safflower oil, canola oil, sunflower oil, corn oil, avocados, and egg yolks. Women should avoid consuming fats found in processed foods because of their highly saturated nature.
Low-fat diets are not recommended for active individuals.
Low-fat diets decrease energy and nutrient intake, reduce exercise performance, and decrease oxidation of body fat stores. Fat provides the most energy per gram of all the macronutrients and can help in achieving a positive energy balance. Dietary fat maintains concentrations of sex hormones and may prevent menstrual disturbances.
Fluids and electrolytes
Dehydration impairs performance; therefore, athletes must remain well hydrated. Adequate fluid intake is approximately 2.2 L/d for women aged 19-30 years, and increased drinking is required for active individuals or those in hot environments (see the Table).
Athletes should consume 400-600 mL of fluid 2 hours before exercising. During exercise, 150-350 mL (6-12 fluid ounces [fl oz]) should be ingested every 15-20 minutes. For exercise lasting longer than 1 hour or occurring in hot environments, the fluid should be a drink containing carbohydrates and electrolytes.
Postexercise meals should include fluids and foods containing sodium, because diuresis occurs with the ingestion of plain water.
Vitamins and minerals
Female athletes are at increased risk for iron, calcium, vitamin B, and zinc deficiencies. These nutrients are vital for building bone and muscle and for energy production. Vegetarians are particularly at risk for developing deficiencies in these vitamins and minerals.
Iron insufficiency is one of the most prevalent nutritional deficiencies among the female athlete because of menstrual losses (see the Table). Iron deficiency may lead to fatigue. Ferritin values are commonly used to reflect iron stores; however, their reliability in the female athlete is questioned.
Excessive iron ingestion may also cause problems, including gastrointestinal distress, constipation, and iron toxicity.
Energy Needs Total daily energy expenditure (TEE, TDEE)
A female athlete's TEE is calculated by using the following equation:
TEE = REE X PAL + TEA,
where REE is the resting energy expenditure, PAL is the physical activity level, and TEA is the thermal effect of activity. Various means exist to estimate REE, PAL, and TEA.7
Resting energy expenditure (REE)
Calculations of REE that are conducted in a laboratory by means of indirect calorimetry are the most accurate.
An alternative is the use of equations that incorporate anthropometric variables. The Harris-Benedict equation is most commonly applied to athletes. This equation is as follows:
REE = 655 + (9.5 X weight) + (1.9 X height) – (4.7 X age),
where REE is given in kilocalories (kcal) per day, weight is in kilograms (kg), height is in centimeters (cm), and age is in years (y).
Physical activity level
The PAL value can be determined by using accelerometers, heart-rate monitors, activity diaries, or self-reported activity estimates. Depending on their occupation and daily activities, female athletes may be considered to be moderately to extremely active (ie, have PALs of 1.6-2.5).
Thermal effect of activity
The TEA is calculated as follows:
TEA = weight X duration X METs,
where TEA is expressed in kcal, weight is in kg, duration is in hours (h), and METs are the metabolic equivalents of the task being performed, in kcal/kg/h.
METs are found by consulting the Compendium of Physical Activities — a coding scheme developed by Ainsworth et al that classifies specific physical activity by rate — and are determined by dividing the metabolic rate during activity by the metabolic rate at rest.15 A MET value of 1 is defined as 1 kcal/kg/h, which is approximately the energy expended when a person is sitting quietly. Different physical activities are associated with different MET values. For example, weight lifting is equal to 6.0-8.0 METs.15
Complications of Nutritional Deficiencies
Girls and women with low energy and nutrient intake are susceptible to many complications, including those listed below:
•Dehydration (eg, dehydration in girls or female adolescents)
•Delayed growth (see the eMedicine article Growth Failure in the Pediatrics: General Medicine, Endocrinology section)
•Decreased immune response that increases the frequency of upper respiratory tract infections and problems with cell-mediated immunity2 •Irritation
Among athletes, a lack of proper nutrition can have many deleterious effects, including the following:
•Loss of motivation
•Decreased maximal performance
•Increased short- and long-term fatigue
•Preoccupation with food
Inadequate nutrition can lead to amenorrhea (see also the eMedicine article Amenorrhea in the Pediatrics: Surgery, Gynecology section) and a delay or arrest of puberty. Amenorrhea can also occur because of emotional or physical stress, such as intense training. Amenorrhea is more prominent in the athletic population (3-66%) than in the general female population (2-4%).9,10
Female athlete triad
The female athlete triad can lead to severe and long-standing effects. Characteristics of this triad are the following:
In one study, more athletes who competed in leanness sports (70.1%) than those participating in nonleanness sports (55.3%, P < 0.01) were classified as being at risk for the female athlete triad.12
Examples of disordered eating or deficient nutrient intake include the following behaviors:
•Skipping meals (eg, because of busy schedules, training, work, school, and/or desired weight loss)
•Engaging in unsafe weight-loss methods, such as consumption of ultralow-calorie diets, fasting, laxative abuse, self-induced vomiting, or use of diet pills (see the eMedicine articles Anorexia Nervosa and Bulimia in the Emergency Medicine, Psychosocial section)
•Using supplements to compensate for inadequate diets
Healthy Weight Control
Weight loss can be accomplished in a healthy manner. It should be achieved during a period without competitive events.6
The goal of maximizing fat loss while minimizing loss of lean tissue is best accomplished with a gradual weight loss of about 1-2 lb per week.6
Energy intake should not be restricted to less than 1800 kcal/d in active women. Severe caloric restriction can lead to many of the complications described above (see Complications of Nutritional Deficiencies).
Nutrition in Endurance Athletes
Triathletes and runners have been known to consume 5 or 6 meals per day, whereas cyclists may consume 8-10 meals per day. This frequent eating ensures that the athlete’s high-energy needs are met, while decreasing the gastrointestinal discomfort associated with consuming large meals. Another important issue is that endurance athletes should maintain good hydration, as described earlier and in the Table below.
Nutrition in Strength Athletes
Women who are focused on gaining muscular mass and strength rely less on glycogen during exercise than on other sources, and they are less responsive than others to carbohydrate-mediated glycogen synthesis during recovery. Therefore, to enhance their training and general health, the diet of these women should focus on good-quality proteins and fats rather than on a large amount of carbohydrates.
Female strength athletes should also include high-quality proteins in their diet because they provide essential amino acids, vitamin B-12 and vitamin D, thiamine (vitamin B-1), riboflavin (vitamin B-2), calcium, phosphorus, iron, and zinc.7 These women should consume protein-rich foods every day before and after exercise, as well as between meals to maintain and promote the growth of lean muscle.7
Creatine supplementation has been shown to improve anaerobic performance. Creatine augments strength and gains in lean body mass when it is used during resistance training.7
Summary of Nutritional Requirements and Sources
Important for endurance athletesLess important for strength athletes 5-7 g/kg for moderate- to low-intensity exercise7-12 g/kg for high-intensity exercise FruitsVegetablesBrown riceWhole-grain breadRolled oatsBeansLegumesSweet potatoes
Important for all active individualsNeeded for energy and to repair muscle tissue 1.2-1.4 g/kg/d for endurance athletes1.6-1.8 g/kg/d for resistance or speed athletes Lean pork and beefPoultryFishEggsLow-fat dairy productsBroccoliBeansCorn
Required for active individualsProvides the most energy per gram of all macronutrientsMaintains sex hormones and aids in the absorption of vitamins A, D, E, and K
Linoleic acid is recommended: 11-12 g/d in womenAlpha-linoleic acid intake should be 1.1 g/d for women25-30% of energy intake should be from fat NutsSeedsNut butterFatty fishFish-oil supplementsFlaxseed oilSafflower oilCanola oilSunflower oilCorn oilAvocadosEgg yolksNote: Avoid fats in processed foods
Important to maintain performance 2.2 L/d for women aged 19-30 years400-600 mL 2 hours before exercise150-350 mL (6-12 fl oz) every 15-20 minutes during exerciseUse sports drinks or glucose-containing fluids, if exercise will be >1 hour or in a hot environment Sports drinks containing carbohydrates and sodium
Iron deficiency is common because of menstrual losses or a vegetarian dietIron deficiency leads to long-term fatigue Varies Red meatsSupplementsNote: Closely monitor iron supplementation because of potential adverse effects
•Important for endurance athletes
•Less important for strength athletes
•5-7 g/kg for moderate- to low-intensity exercise
•7-12 g/kg for high-intensity exercise
•Important for all active individuals
•Needed for energy and to repair muscle tissue
•1.2-1.4 g/kg/d for endurance athletes
•1.6-1.8 g/kg/d for resistance or speed athletes
•Lean pork and beef
•Low-fat dairy products
•Required for active individuals
•Provides the most energy per gram of all macronutrients
•Maintains sex hormones and aids in the absorption of vitamins A, D, E, and K
•Linoleic acid is recommended: 11-12 g/d in women
•Alpha-linoleic acid intake should be 1.1 g/d for women
•25-30% of energy intake should be from fat
Note: Avoid fats in processed foods
•Important to maintain performance
•2.2 L/d for women aged 19-30 years
•400-600 mL 2 hours before exercise
•150-350 mL (6-12 fl oz) every 15-20 minutes during exercise
•Use sports drinks or glucose-containing fluids, if exercise will be >1 hour or in a hot environment
•Sports drinks containing carbohydrates and sodium
•Iron deficiency is common because of menstrual losses or a vegetarian diet
•Iron deficiency leads to long-term fatigue
Note: Closely monitor iron supplementation because of potential adverse effects