Exercise and Infertility
The adverse effects of intense levels of physical activity on fertility were observed over 40 years ago in female competitive athletes. Since then, a number of studies have shown that young female athletes may experience delayed age at puberty and lack menstrual cycles. Average age of menarche for competitive female athletes and ballet danc-ers is often delayed by 2 to 4 years. The delay in menarche increases if females begin training for events that require thinness (such as gymnastics) before menarche normally would begin. Very high levels of exercise can also inter-rupt previously established, normal menstrual cycles.
The presence of abnormal cycles reportedly ranges from about 23% in joggers to 86% in female bodybuilders (Table 2.4).34,35
Delays and interruptions in normal menstrual cycles appear to result from hormonal and metabolic changes pri-marily related to caloric deficits rather than intense exer-cise.36 Metabolic and hormonal status generally reverts to normal after high levels of training and caloric deficits end.
Some of the hormones involved in fertility impair-ments perform other important functions in the body, Treatment of underweight women with Clomid
(clo-miphene citrate, a drug that induces ovulation) generally does not improve fertility until weight is regained. Fertil-ity may be improved through the use of GnRH, FSH, and other hormones. However, twice as many infants born to underweight women receiving such therapy are small for gestational age compared to infants born to underweight women who gain weight and experience unassisted conception.15
The eating disorder anorexia nervosa is associated with similar, but more severe, changes in endocrine and hypothalamic functions than those seen with weight loss in normal-weight women. (This topic, as well as hypotha-lamic amenorrhea, is covered in Chapter 3.)
Weight loss decreases fertility in men just as it does in women. In the classic starvation experiments by Keys dur-ing World War II,34 men experiencing a 50% reduction in caloric intake reported substantially reduced sexual drive early in the study. Sperm viability and motility decreased as weight reached 10 to 15% below normal, and sperm production ceased entirely when weight loss exceeded 25% of normal weight. Sperm production and libido returned to normal after weight was regained.
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shown lower intakes of antioxi-dant nutrients in infertile than fer-tile women and men. Other studies have noted that higher average intakes of antioxidants are associ-ated with improvements in levels of oxidative stress in infertile women and improved sperm maturation, motility, concentration, and re-duced DNA and chromosome dam-age in men.39,43,44 Supplemental intakes of vitamin E and selenium appear to improve sperm quality in infertile men.45,46 Regular intake of vitamin C, vitamin E, and beta-carotene supplements have been re-lated to increased sperm number and motility.44 Although the use of antioxidants for the treatment of infertility dis-orders in women and men is being advocated,13,46 addi-tional clinical trials are needed to confirm their beneficial effects.
Zinc Status and Fertility in Men Zinc plays impor-tant roles in the reduction of oxidative stress, in sperm maturation, and in testosterone synthesis, and it has been investigated for its potential role in infertility.53 Lower zinc status in men has been found to be related to poorer sperm quality54 and sperm concentrations, and to abnor-mal sperm shapes.55 Zinc supplementation alone,56 or combined with vitamin E and vitamin C supplementa-tion,57 has been found to improve sperm quality.
which may also be disrupted. Reduced levels of estrogen that accompany low levels of body fat and amenorrhea, for example, may decrease bone density and increase the risk of shortness, bone fractures, and osteoporosis.37
Oxidative Stress, Antioxidant Nutrient Status, and Fertility
A growing body of research suggests that intake of antioxidants such as vitamin E, vitamin C, beta-carotene, and selenium and antioxidant-rich pigments in vegetables and fruits plays an important role in fertility in women and men.38 Antioxidant nutrients are needed to protect cells of the reproductive system, including eggs and sperm, from damage due to oxidative stress. Oxidative stress occurs when the production ofpotentially destructive reactive oxygen molecules (free radicals) exceeds the body’s own antioxidant defenses. Reactive oxygen molecules attack polyunsaturated fatty acids in sperm membranes, and that decreases sperm motility and reduces the ability of sperm to fuse with an egg. Once the membrane surrounding sperm is damaged, reactive oxygen molecules can enter the sperm cell and damage DNA.39 This can result in the passage of defective DNA on to the conceptus.40 Oxidative stress is observed in approxi-mately half of all infertile men.41 In women, oxidative stress can harm egg and fol-licular development and can interfere with corpus luteum function and implantation of the egg in the uterine wall.42 Effects of Antioxidant Intake on Fertility A number of studies have Antioxidants Chemical substances that
prevent or repair damage to molecules and cells caused by oxidizing agents. Vitamins C (see Illustration 2.3) and E, selenium, and certain components of plants function as antioxidants.
Free Radicals Chemical substances (often oxygen-based) that are missing electrons. The absence of electrons makes the chemical substance reactive and prone to oxidizing nearby molecules by stealing electrons from them. Free radicals can damage lipids, cell membranes, DNA, and tissues by altering their chemical structure and functions. They also form as a normal part of metabolism. Over time, oxidative stress causes damage to lipids, cell membranes, DNA, cells, and tissues.
Illustration 2.3 Apple slices exposed to air will turn brown due to oxidation reactions. The oxidation reactions are prevented if the slices are coated with a vitamin C-rich solution such as lemon juice.
Table 2.4 Incidence of irregular or absent menstrual cycles in female athletes and sedentary women
Incidence of Irregular or Absent Menstrual Cycles
Joggers (5 to 30 miles per week) 23%
Runners (over 30 miles per week) 34%
Long-distance runners (over 70 miles per week) 43%
Competitive bodybuilders 86%
Noncompetitive bodybuilders 30%
Volleyball players 48%
Ballet dancers 44%
Sedentary women 13%
Christopher O Driscoll/iStockphoto
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of vitamin C–rich fruits and vegetables along with plant sources of iron, iron-fortified cereals, and lean meats.
Caffeine and Fertility
Should women concerned about infertility consume coffee and other foods with caffeine (Table 2.5)? The previous answer to this question of “it looks like they should be concerned” is now in question.
In a study of European women, researchers found that the chance of conception within a 10-month interval of unprotected intercourse was half as likely among women who consumed over 4 cups of coffee per day (.500 mg caffeine) versus the conception rate of women who con-sumed little coffee.60 Another study reported that intake of over 300 mg of caffeine daily from coffee, sodas, and tea decreased the chance of conceiving by 27% per cycle compared to negligible caffeine intake.61 In both studies, the effect of caffeine on time to conception was stronger in women who smoked.
Other studies have failed to find effects of caffeine in-take prior to pregnancy on the amount of time it in-takes to become pregnant,62 on ovulatory infertility,63 or on indi-cators of ovarian function.64 Results of research on the ef-fects of caffeine or coffee intake on fertility in women are conflicting, and overall effects are likely weak.1 Effects of coffee or caffeine consumption on fertility may be due to one or more of the hundreds of other biologically active substances in coffee, or to characteristics of women who consume lots of coffee and other sources of caffeine.
Alcohol and Fertility Alcohol may influence fertil-ity by decreasing estrogen and testosterone levels and by disrupting menstrual cycles and testicular functions.64 In a study of 430 Danish couples attempting pregnancy for 6 months, consumption of from 1 to 5 alcohol-containing drinks per week by women was related to a 39% lower chance of conception. Alcoholic-beverage consumption of over 10 drinks per week was related to a 66% reduc-tion in the probability of concepreduc-tion during the 6-month period.65 Consumption of 7 or more drinks a week has been associated with a doubling of risk for infertility in women over the age of 30 only.66
Not all studies show an effect of alcohol intake on fertility, and some show effects only in women and men with very high intakes of alcohol.67–69 Excess alcohol in-take early in pregnancy is related to impaired fetal growth and development. Consequently, it is recommended that women restrict their alcohol intake while attempting preg-nancy to avoid the possibility of alcohol-related harm to the developing fetus.70
Heavy-Metal Exposure Exposure to high levels of lead is related to decreased sperm production and abnor-mal sperm motility and shape.71 Inhaled or ingested lead is transported to the pituitary gland, where it appears Plant Foods and Fertility Women who regularly
con-sume plant-based, low-fat diets are more likely to have irregular menstrual cycles than omnivores. These results apply to vegans who are thin, normal weight, or over-weight.47,48 Diets providing less than 20% of calories from fat appear to lengthen menstrual cycles among women in general.49
Regular intake of soy foods such as tofu, soymilk, tempeh, and textured soy protein appears to be related to reduced sperm count in men,50 and to a one–day in-crease in menstrual cycle length in women.49 Effects of high plant- and soy-food diets on fertility may be re-lated to the influence of certain phytochemicals in plant foods on levels of estradiol, progesterone, and luteinizing hormone.51 Additionally, some of the effects may be re-lated to the consumption of nutritionally inferior vegan diets.52
Multivitamin Supplement, Folate Intake, and Fertility
Multivitamin intake by preconceptional women has been associated with a lower risk of ovulatory infertility in the large Nurse’s Health Study.58 Intake of folic acid from the supplements appears to account for much of the de-cline in ovulatory infertility observed. Folate status may affect male fertility as well. Higher levels of dietary in-takes of folate from food and supplements in healthy men have been related to the presence of fewer chromosomally abnormal sperm than identified among men with lower intake of folate.59
Preconception Iron Status, Fertility, and Pregnancy Outcomes Iron status prior to pregnancy is related to fertility and pregnancy outcomes. Results of a large pro-spective study of nurses indicate that infertility due to a lack of ovulation is related to iron intake. In this study, women who regularly used iron supplements and con-sumed plant sources of iron were 60% less likely to de-velop ovulatory infertility than women who did not.
Mechanisms underlying the link between iron status and ovulation are not yet established.110
Iron deficiency prior to pregnancy has been shown to increase the risk that iron-deficiency anemia will occur dur-ing pregnancy and that infants will be born with low stores of iron. Iron deficiency before pregnancy is also related to increased rates of preterm delivery.111 Iron deficiency oc-curs in 12% of U.S. women of childbearing age overall, and among 22% of Mexican American and 19% of African American women.7 About one-half of women in the United State enter pregnancy with inadequate iron stores.
It is easier and more efficient to build up iron stores before pregnancy than during pregnancy.111 Iron status can generally be improved by taking modest doses of iron sup-plements (18 mg a day) and by the regular consumption
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mercury in Hong Kong has been associated with decreased sperm count and abnormal semen.73 Consumption of fish from the U.S. Great Lakes does not appear to pose similar problems.74
Exposure to excess levels of cadmium, molybdenum, manganese, boron, cobalt, copper, nickel, silver, or tin may also affect male fertility.75,76 These metals may build up in male reproductive systems through the inhalation of fumes or dust containing particles or through long-term use of dietary supplements, industrial pollution, or con-sumption of contaminated water.75,77