Commentaries on Viewpoint: Are there valid concerns for completing a marathon at 39 weeks of pregnancy?

RECENTLY, WIDESPREAD MEDIA attention has been given to Amber Danielle Miller, a 27-yr-old woman (163 cm tall), who, at 39 wk of pregnancy, gave birth following her completion of the Chicago Marathon on October 9, 2011 (17, 19, 21). She completed the Marathon in 6 h, 25 min, and 50 s (Fig. 1), which equates to a brisk walking pace of 14 min, 42 s/mile ( 4.1 m/h or 6.6 km/h) for the full 26.2 mile (42.2 km) marathon distance. The coefficient of variation between her eight 5-km split times was 12% (73 s/mil). Eight hours after completing the marathon, she gave birth to a 7 lb. 13 oz. girl (Fig. 2). In part, what spurred this attention was the public perception that completing a marathon late in pregnancy was either an amazing accomplishment or a hazardous endeavor with potentially harmful effects on the fetus. The Chicago Tribune website has fielded a barrage of criticism in response to this story; some readers accusing her of risking stillbirth and brain damage. Thus the questions arise, to what extent does completing a marathon late (or anytime) during pregnancy in 6.5 h constitute a major accomplishment compared with nonpregnant individuals? To what extent is this dangerous for the mother and/or fetus? Given that she was healthy and was cleared medically to complete the race, we would argue that Amber’s long, brisk walk was probably “no big deal.” Exercise before or during pregnancy has been reported to diminish the risk for gestational diabetes, hypertensive disorders, and abnormal weight gain, especially if the exercise intensity is 21.1 ml·kg ·min 1 (6 METS) (26). Completing a mile in 12 min or a general leisurely swim achieves this level of intensity. In recent reports, we suggested that energy expenditure during pregnancy can be 28 MET-h/wk, even up to 40 wk gestation (25, 26). Given that on race day she weighed 155 lb (70 kg, personal communication), Amber’s marathon energy expenditure equaled 28 MET-h, or 1,984 89 kcal. This was determined by using the American College of Sports Medicine Equation for walking on level ground (the marathon was flat) and substituting the 3.5 ml O2·kg ·min 1 resting component with 4.4 ml O2·kg ·min 1 during pregnancy (3); thus oxygen consumption probably equaled 15.3 ml O2·kg ·min 1 (4.4 METS). The error in the prediction equation for energy expenditure of brisk walking is 3.4 kcal/mile (4) or 89 kcal for the entire marathon. As such, she achieved the target weekly energy expenditure in 1 day (26). A single session expending 1,200 kcal has been shown to improve insulin sensitivity 60% (12). As noted, the question is whether completing a marathon close to date of delivery is appropriate for a healthy pregnant woman? Several newspapers reported mixed responses from physicians (17, 19). For the developing fetus, a potential risk is an abnormal fetal heart rate response ( 110 or 160 beats/ min). However, fetal heart rate abnormality is unlikely when the maternal heart rate is below 90% of maximum (20). Runners who complete a marathon 2 h faster than Amber have an average heart rate that is 82–84% of maximum (10, 15, 22). This suggests that Amber’s average heart rate was substantially less than 84% of maximum. Nonetheless, the guideline to limit heart rates to 90% of maximal is problematic, as only six second trimester pregnant athletes were evaluated. Another problematic issue is that the fetal heart rate response to strenuous maternal exercise is not a predictor of fetal distress (8). Thus monitoring fetal heart rates during exercise is questionable at best. With increasing exercise intensity and duration, uterine blood flow also can decrease progressively by as much as 50% (11, 20). Nonetheless, several mechanisms act to preserve fetal oxygen consumption even during exhaustive exercise (11). At strenuous exercise intensities (above the exercise intensity at which carbon dioxide production increases more rapidly than oxygen consumption) total uterine and umbilical oxygen delivery and oxygen consumption are minimally affected (7). Amber’s marathon was not of high intensity, nor was it exhaustive. Her average pace was a brisk walk, representing a metabolic rate of 4.4 METS, which is only moderate intensity. Neither was it exhaustive, as the major fuel source at moderate exercise intensity is from the supply of fat stores (2), not muscle glycogen, thus prolonging the onset of a substantial drop in running pace. During prolonged exercise 10% the energy expenditure comes from glucose, and 70% comes from intramuscular triglycerides and plasma free fatty acids (18). With prolonged exercise, blood glucose levels may drop; however, glucose ingestion throughout exercise prevents this (1), and blood glucose levels are maintained during a marathon (5). 1 This article is the topic of an Editorial Focus by Peter Wagner (23). 2 A MET is a metabolic equivalent task. One MET is generally said to be equal the resting metabolic rate in a nonpregnant state, which is a resting, whole body, oxygen consumption of 3.5 ml O2·kg 1·min 1 (with a slight variability between individuals). 3 228 MET-h/wk is equivalent to 1,600 kcal/wk in a 120-lb women and increases by 265 kcal for every 20 lb (9 kg) increase in the woman’s prepregnancy body weight. Address for reprint requests and other correspondence: G. S. Zavorsky, Human Physiology Laboratory, Associate Professor, Marywood Univ., 2300 Adams Ave., Scranton, PA 18509 (e-mail: [email protected]). 4 Oxygen consumption for walking on level ground in m; O2·kg 1·min 1 0.1·speed in m/min 4.4. 5 Amber drank approximately 200 ml of glucose/electrolyte/water mixture every 2 miles and ate a nutrient bar ( 240 kcal) midway during the marathon. Two hours before the marathon began she ate a nutrient bar, two pieces of toast with peanut butter, and a banana. After the race, she ate a turkey sandwich, salad, some fruit, and ice cream (personal communication). J Appl Physiol 113: 1162–1165, 2012; doi:10.1152/japplphysiol.01426.2011. Perspectives