Nutritional requirements for the active pregnant woman

Although the nutritional needs of active pregnant women are not clearly defined, nutritional needs in pregnancy have been well researched. Energy requirements during the second and third trimesters of pregnancy are an average of 300 kcal a day above prepregnancy requirements [40]. A wide variability in metabolic energy expenditure in pregnancy makes it difficult to set standards for energy requirements [41]. Exercise during pregnancy requires an additional caloric allowance for increased metabolism and greater energy expenditure both during and after the activity. Other factors affecting caloric requirements in pregnancy include prepregnancy body mass index, maternal age, and appetite. Estimation of caloric needs is further complicated by pregnancy changes in maternal extracellular fluid, maternal fat stores, the weight of the fetus and supporting tissue (uterus, placenta, amniotic fluid, and mammary glands), as well as changes in fat-free muscle mass due to variations in activity during pregnancy. Level of activity may either increase or decrease caloric requirements. For example, a competitive athlete who decides to reduce the intensity of the activity may have lower caloric needs in pregnancy compared with prepregnancy needs, while a sedentary person who has started a moderate exercise program may have increased calorie needs above those of normal pregnancy requirements.

Estimation of body composition is more complicated in pregnancy. As gestational age progresses, body water continues to increase, while fat mass stays relatively constant. Changes in hydration, along with errors in measures used to estimate percent body fat, make it more difficult to provide reliable measures of body composition [42].

The Dietary Reference Intakes (DRIs) for macronutrient and micronutrient intakes have not been defined for the active pregnant woman compared with those who are sedentary. Protein requirements in pregnancy have been estimated at 1.1 g/kg/day (71 g/day for someone 163 cm tall, weighing 65 kg.), while in active people there is a slightly higher estimated requirement of 1.2 to 1.4 g/kg body weight per day [43]. The 2005 Dietary Guidelines for Americans recommend 20-35% of calories from fat, with most coming from polyunsaturated and monounsaturated fatty acids, while limiting intake of saturated fats to less than 10% of calories and keeping trans fatty acids as low as possible. Fat intake should not be restricted to less than 15% of energy requirements because fat is important not only as a source of calories, but also to aid in the absorption of fat-soluble vitamins and provides essential fatty acids [44]. Carbohydrate intake of 40-55% of energy requirements is needed to replace the muscle glycogen stores lost during exercise, minimize maternal hypoglycemia, and limit ketonuria. All pregnant women and athletes should strive to consume foods that provide at least the RDA/DRI for all vitamins and minerals in pregnancy and lactation, as discussed in Chap. 1 ("Nutrient Recommendations and Dietary Guidelines for Pregnant Women") [43].

Women who are diet conscious often do not obtain the necessary nutrients required to maintain a normal pregnancy. Inadequate nutritional intake along with the increased energy requirements for exercise may lead to poor weight gain and fetal growth restriction. Although the data linking low birth weight and maternal exercise are conflicting, for pregnant women who exercise, it is unclear if adequate energy intake can offset a decrease in fetal weight [44]. A meta-analysis of 30 research studies concluded that vigorous exercise during the third trimester of pregnancy has been associated with a 200- to 400-g decrease in fetal weight [45]. When deficient energy intake occurs in combination with chronic strenuous exercise during pregnancy, fetal growth may be adversely affected.

Since pregnancy and exercise place higher demands on oxygen requirements, women who exercise during pregnancy should be monitored for suboptimal iron status and inadequate intake. Many women enter pregnancy with depleted iron stores, as discussed in Chap. 16 ("Iron Requirements and Adverse Outcomes"). This, along with expansion of maternal blood volume and increased fetal demand for oxygen, makes it more of a challenge for many women to achieve adequate iron status. If a woman enters pregnancy with iron deficiency anemia, repletion of iron stores may be difficult. Prenatal vitamin and mineral supplements are routinely prescribed to provide additional iron and folic acid. However, these should not replace a healthy balanced diet containing a variety of foods from all food groups so as to ensure adequate intake of antioxidants, fiber, and the necessary nutrients to support maternal health and growth of the fetus [46].

Oftentimes, active women enter pregnancy underweight, with increased awareness of body image and may resort to caloric intake below recommendations to prevent weight gain in pregnancy. To compensate for nutrient deficiencies, women may over compensate by taking large amount of vitamins or minerals. Although vitamin and mineral supplementation may be beneficial, women should be counseled to avoid excessive micronutrient intake, particularly of the fat-soluble vitamins A and D, which can lead to fetal malformations. Excessive amounts of vitamin D can result in congenital anomalies consisting of supravalvular aortic stenosis, elfin facies, and mental retardation [47]. Women taking high amounts of vitamin A >10,000 IU in supplement form showed higher rates (1 infant in 57) of cranial-neural crest tissue defects [48]. The use of dietary supplements is further discussed in Chap. 14 ("Dietary Supplements during Pregnancy: Need, Efficacy, and Safety").

Athletes may choose to consume nutritional ergogenic aids and dietary supplements to enhance athletic performance with hopes of boosting their competitive edge. Nutritional supplements are a multibillion-dollar industry targeting a wide range of populations, including women of childbearing age. Supplement companies are not required to prove supplement safety, effectiveness, and potency before a product is placed on the market as long as the supplement makes the claim that it has not been evaluated by the US Food and Drug Administration (FDA), and that the product is not intended to diagnose, treat, mitigate, cure or prevent disease [43]. Many may believe that since these products are natural and legal that they are safe; however, there is little scientific evidence demonstrating the safety or effectiveness of these products for the general population. Women of childbearing age should be counseled or warned that supplements and nutritional ergogenic aids have not been shown to be safe and therefore should be avoided prior to and during pregnancy. The reader is also directed to Chap. 13 ("Popular Diets").

Water is a critical yet often forgotten nutrient for healthy pregnancies. Exercise induces significant fluid loss and places the woman at higher risk of dehydration. Weighing before and after exercise can help monitor fluid balance. Weight loss of 2 lb is equivalent to approximately a 1-liter fluid loss. Pregnant women should be encouraged to drink 8 to 12 cups of hydrating fluids per day, with water being the preferential source. Sports drinks help replenish carbohydrate, fluid, and electrolyte losses during exercise sessions lasting 30-45 min. Drinking 1-2 cups of water prior to exercise, replacing fluids every 15-20 min. during activity, and replacing fluids lost after exercise helps maintain hydration and keeps body temperature within normal limits.

Physical activity and diet quality are interconnected behaviors. Individuals following a suboptimal diet tend to be more sedentary, less educated, not married, and non-Caucasians [49]. Hormonal alterations during pregnancy have been shown to cause a 1.5- and threefold increase in maternal cholesterol and triglyceride levels, respectively by the mid-third trimester [50]. One study examined the relationship between recreational physical activity in early pregnancy and found reductions in total cholesterol and triglyceride levels in women who spent a greater amount of time (12.7 h/week) on recreational physical activity [51]. Results of this study, along with others conducted in the nonpregnant population suggest that physical activity in pregnancy may lessen pregnancy-associated dyslipidemia.

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