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March of Dimes Birth Defects Foundation, California Birth Defects Monitoring Program, Oakland, CA 94606
3 To whom correspondence should be addressed. E-mail: gsh{at}cbdmp.org.
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ABSTRACT |
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 TOP ABSTRACT LITERATURE CITED |
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KEY WORDS: • pregnancy • developing countries • micronutrients • epidemiology • strenuous work
Does strenuous work by women during pregnancy in developing countries influence micronutrient status and thereby increase risks of adverse pregnancy outcomes? Furthermore, does strenuous physical activity in the workplace lead to micronutrient compromise that leads to adverse pregnancy outcomes?
Some data exist about the potential relationship between strenuous work or physical activity and nutrient compromise, strenuous work or physical activity and adverse reproductive outcomes and micronutrient intakes or status and adverse reproductive outcomes. However, no substantial body of data exists that has directly investigated the entire potential causal cascade posed by the above questions.
A search of the literature identified only a few papers from developing countries that provided even remotely related data on the topic. Prentice et al. (1, 2) and Ceesay et al. (3) observed that among Gambian women who were supplemented, an effect of supplementation could be observed on birth weight. The effect appeared to be seasonal and to be among women who performed greater field work activities and who had lower food supplies. The supplement in the Prentice et al. studies consisted of a net energy increase of approximately 431 kcal/d, protein, calcium, riboflavin and vitamins A and C. In the season of greatest food shortage and highest workload, women who were supplemented had a reduction from 28% to 5% in the prevalence of low-birth-weight babies.
Another pertinent example is an investigation of a population in Pune, India (4). Increased physical activity, as measured by work in farming or gathering water, was associated with infants of low birth weight, smaller head circumference, smaller mid-arm circumference and lower placental weight. These associations were observed after maternal energy and protein intake were controlled for. Information about effects that micronutrient intakes may have had on the observed association were not given.
Because of the lack of empirical data, this brief review will piece together some of the available epidemiologic evidence, primarily from studies conducted in developed countries, that may facilitate inferences. The three areas for which some epidemiologic data are available are strenuous work or physical activity and nutrient compromise, strenuous work or physical activity and adverse reproductive outcomes, and micronutrient status and adverse reproductive outcomes.
Strenuous work and nutritional compromise: epidemiologic evidence
A few dietary energy studies were conducted in women from less developed countries (5– 9). A study in Guatemala (5) estimated the energy costs associated with 32 household and agricultural activities performed by peasant women. A study of Gambian women observed that most tasks performed by these women were deemed light or moderate from the perspective of energy expenditures and did not appear to vary based on stage of pregnancy (8). These studies tend to include relatively small numbers of subjects, only a small proportion of whom are pregnant, and do not provide information on the influence of such activities on micronutrient status.
Bendich (10) suggested an indirect effect, although not a biological mechanism, that employment can have on the nutritional status of pregnant women. That is, women who are employed may be at risk of compromised diets because of reduced time for shopping and cooking. Associated with this speculation is the finding that working women who worked standing were less likely to eat three meals per day than were women who worked in occupations that did not require prolonged standing (11).
The implication of physical strain or activity in the workplace on nutritional status of the pregnant woman is subject to a number of considerations. Foremost is what is being measured. For example, energy expenditure averaged over tasks may not be the relevant measure. Peak energy expenditures such as lifting may be more important than averaged expenditures over longer time periods. Measurement of an activity such as lifting and correlation with energy expenditure is a good example of the measurement complexities: lifting may result in a greater intraabdominal pressure, which could be the potential mechanism for the adverse reproductive outcome rather than increased energy or nutrient expenditure. As noted by Eskenazi et al. (12), type of physical exertion, amount of exertion and the context of the exertion are likely to be relevant to this body of literature. Furthermore, not all physical strain is equal. For example, comparing recreational physical exercise versus workplace physical strain (e.g., long work hours) reflects the difference in the degree of voluntarism between recreational and leisure time activity and physical activity at work (13, 14).
The relationships between physical activity, namely exercise, and nutritional effects on pregnant women are complex. For example, a recent study showed that moderate-intensity exercise (weight bearing) reduces fetoplacental growth and size at birth (15).
Strenuous work and reproductive outcomes: epidemiologic evidence
A number of mechanisms have been postulated for how increased physical activity, such as strenuous work, could lead to adverse outcomes of pregnancy (16, 17). Clapp (17) summarized the physiological changes that occur during physical activity: cardiac output is redistributed from visceral circulation to exercising muscles and skin, energy stores are depleted, body temperature increases, the hormonal milieu abruptly changes and a variety of physical stresses occur, all of which are intensified by either increasing the workload or length of the physical activity. These changes have the potential to adversely affect the course and outcome of pregnancy (e.g., infertility, abortion, congenital malformation, cord entanglement, placental separation, membrane rupture, growth restriction, premature labor or fetal hypoxia). Most of these adverse outcomes have been identified in animal models, and two (premature labor, fetal growth restriction) were found in humans in association with physical activity in the workplace (17).
Interestingly, the biological mechanisms underlying an association between strenuous work during pregnancy and adverse reproductive outcome usually do not include nutritional explanations per se. Explanations have focused on increased sympathetic vasomotor tone to skeletal muscles, decreased plasma volume resulting from prolonged standing, catecholamine release resulting in increased uterine vasoconstriction and hyperthermia (18). Findings support the notion that increased physical activity may be beneficial to the outcome of pregnancy. For example, women who exercise during pregnancy have been observed to have a lower risk of spontaneous abortion (19).
Thus, making an inference that physical or strenuous activity in the workplace contributes to the etiology of adverse reproductive outcomes is complicated by the observation that increased physical activity during pregnancy is likely to be beneficial relative to some pregnancy outcomes. However, the factual basis of that statement can be questioned. For example, a recent study from Denmark indicated that physical activity peaks at critical times (specifically implantation) were associated with an increase in spontaneous abortion (20). This finding is consistent with an alternative interpretation (21).
Physical exertion in the workplace during pregnancy may be predictive of adverse reproductive outcome. Particular occupations, long hours on the job, irregular hours and shift work have been implicated (22– 24). Many studies have suggested that strenuous physical activity, including in the workplace during pregnancy, is associated with reduced infant birth weight, lower pregnancy weight gain, shorter gestations, intrauterine growth retardation, spontaneous abortions, fecundity and some congenital malformations (12, 13, 25– 42). These types of associations, however, have not been observed in other studies (18, 43– 48). Moreover, only a few such studies have been conducted in developing countries (25– 28, 40).
This fairly sizable literature was recently reviewed (49, 50). Numerous methodological differences and explanations likely contribute to the inconsistency of observed results, including variability in the definition of strenuous work activities (e.g., prolonged standing, lifting, working long hours and higher energy expenditures), lack of information on potential confounders, lack of information on actual workplace exposures and failure of most studies to consider the additional potential effects of physical demands outside the workplace.
The literature also reveals some other interesting associations. Women with high-stress jobs, defined as jobs high in demands and low in control, have been observed to deliver babies weighing 190 g less than women who had low-stress jobs or were unemployed (51). Such high-stress jobs have been associated with other pregnancy outcomes as well, such as preeclampsia (52). Thus, to adequately consider potential pregnancy effects of strenuous activity in the workplace, the psychological stress component of women's jobs needs to be considered.
Nutritional deficiencies, compromised or lowered intakes and adverse reproductive outcomes: epidemiologic evidence
Data from developing countries on this topic are limited. Women from one region in Africa with chronic energy deficiency (defined as a body mass index, calculated as kg/m2, of <18.5) were more likely to have a lower male-to-female ratio for offspring (fewer male births) than were women with higher body mass indexes. Short stature and obesity both contributed to the lowered sex ratio (53).
Epidemiologic data indicate that periconceptional vitamin supplementation substantially reduces the risk for neural tube defects, orofacial clefts, selected heart malformations and other congenital malformations (54– 56). This risk reduction may be specifically attributable to folic acid supplementation (57) but is unlikely to be explained by a simple maternal vitamin deficiency or a simple maternal absorption deficiency (58). Evidence points toward disordered folate metabolism in some susceptible individuals, with the metabolic error affecting uptake or metabolism of folate by fetal cells (59). Several epidemiologic studies also investigated the influence of folic acid and multivitamin supplement intake on the occurrences of other reproductive outcomes, such as low birth weight and preterm delivery (60– 62).
Nutritional factors other than folic acid have also been observed to influence risks of congenital malformations (63– 65). For example, increased intakes of methionine (66), zinc (67), vitamin C (68) and dairy products (69) have been associated with decreased risk for neural tube defects. Maternal factors such as diabetes (70), prepregnancy obesity (71), hyperinsulinemia (72) and intakes of sweets (73) also have been associated with increased risks.
Conclusion
The available data are insufficient to draw firm inferences that strenuous work, in either a developing country or a developed country, alters a pregnant woman's nutritional status and therefore affects her risk of an adverse pregnancy outcome. Effects on the nutritional status, micronutrients in particular, of pregnant women from strenuous physical activities at work or in other lifestyle events require further study in developing countries. In addition, how nutritional status effects might influence risks of adverse reproductive outcomes also requires further study, particularly in developing countries.
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FOOTNOTES |
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2 This research was partially supported by funds from the Centers for Disease Control and Prevention, Centers of Excellence Award No. U50/CCU913241.
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LITERATURE CITED |
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