Menstrual Disturbances and Vegetarianism
By Susan I. Barr, PhD, RDN
Menstrual cycle variations have potential biological relevance in relation to the risks of hormone-related cancers, osteoporosis, and infertility. In the 1980s, several reports suggested that women following vegetarian diets were more likely to experience menstrual cycle disturbances.1-4 Vegetarianism also was common among young women with anorexia nervosa,5,6 (for which amenorrhea is a diagnostic criterion). Over the past two decades, additional research has been published; some of this research contradicts earlier observations. Accordingly, this review will explore whether vegetarianism is associated with disturbances of cycle length (amenorrhea, oligomenorrhea) and cycle characteristics (anovulation, length of the follicular and luteal phases). Dysmenorrhea, premenstrual syndrome, menorrhagia, and symptoms associated with menopause are beyond the scope of this article.
Vegetarianism
The question of whether vegetarianism influences women’s menstrual characteristics is complicated by several factors, beginning with the definition of vegetarianism. Defined as "the theory or practice of living on a diet made up of vegetables, fruits, grains, nuts, and sometimes eggs or dairy products,"7 vegetarianism encompasses great variability in dietary patterns, ranging from the consumption of < 200 g/wk of meat1 to strict veganism (which excludes all animal products, including honey and gelatin). In addition to the different degrees to which those who consider themselves vegetarian exclude animal products, there also is variability in terms of which plant products are included or emphasized. Some individuals simply exclude meat while making few other alterations to the traditional Western diet; others make extensive use of legumes, nuts, seeds, and soy products. There is no single "vegetarian diet," and as in omnivorous diets, dietary intake of fat and fiber can vary widely. This has obvious implications when considering potential mechanisms by which components of a vegetarian diet could affect the menstrual cycle.
Vegetarianism may be a marker for a lifestyle choice as well as a dietary choice: Vegetarians frequently are reported to differ from the general public in exercise habits, smoking, alcohol use, and body weight.8,9 Many of these variables have been associated with women’s menstrual characteristics and must be controlled for in study design and/or analysis. Vegetarians are not a homogeneous group: Lifestyle practices of those who are vegetarian for religious reasons (in North America, predominantly Seventh-Day Adventists) may differ considerably from those who become vegetarian for other reasons, such as animal rights issues or weight concerns, and this also could affect study results.
Potential Mechanisms for Effects of Vegetarianism on the Menstrual Cycle
Vegetarianism could directly or indirectly affect the menstrual cycle by several potential mechanisms.10 These include body weight (menstrual disturbances are more common among both underweight and overweight women); the lifestyle variables mentioned above; fiber, fat, phytoestrogens, and other dietary components that may be more or less common in vegetarian diets; and social cognitive variables. Women with high levels of cognitive dietary restraint (i.e., who consciously attempt to limit their food intake to control body weight), for example, have a higher prevalence of menstrual disturbances,11-14 even when relative body weight does not differ from those with low restraint. High restraint is associated with higher 24-hour urinary cortisol excretion.15 As a biological marker of stress, cortisol is known to be associated with changes in ovarian function.16 Whether psychosocial stress differs between vegetarians and omnivores has received little study.
Given this background, what does the scientific literature find when comparing menstrual cycle characteristics of vegetarians and nonvegetarians? The available literature can be divided into two categories: 1) studies comparing women grouped by menstrual cycle function, in which the prevalence of vegetarianism is compared; and 2) studies comparing women grouped by dietary pattern, in which the prevalence of menstrual disturbances is compared.
Studies Comparing Women Grouped by Menstrual Cycle Function
Studies of this nature were among the first to link menstrual disturbances with vegetarian diets. In 1984, Brooks et al reported on the diets of 26 women runners of similar age, percent body fat, and training volume.1 Eleven women were amenorrheic (£ 1 cycle in the six months preceding the study) while 15 had regular cycles (12 periods per year). The two groups had similar energy intakes, but nine of 11 amenorrheic runners were classified as vegetarian (defined in this study as < 200 g/wk of meat or poultry) compared to only two of 15 eumenorrheic runners. Generally comparable results were obtained in a study of eight amenorrheic and nine regularly menstruating runners who were similar in age, relative weight, and training volume.4 Diets of the women were classified as "vegetarian" if they contained < 200 g/week of meat or poultry; exclusion of red meat was reported separately. The prevalence of vegetarianism was higher in the amenorrheic group (25% vs. 11%), as was the proportion of women excluding red meat (100% vs. 44%). In this study, however, reported energy intakes of amenorrheic women averaged only 64% of the regularly menstruating runners; caloric restriction rather than meat restriction could explain results.
The prevalence of vegetarianism among women with anorexia nervosa also has been interpreted as suggesting that vegetarian diets may contribute to menstrual dysfunction. Among consecutive anorexic patients in two series, 45%5 and 54%6 were vegetarian, which is much higher than the 2-4% prevalence reported among the general public.17 Additional analysis of the data from the study by O’Connor et al, however, suggested that vegetarianism was a consequence of the eating disorder and associated menstrual dysfunction, rather than a cause: It preceded the onset of the illness in only 6% of patients.6 For young women intent on losing weight, vegetarianism may be a socially acceptable way to exclude food (and calories) from the diet.
Studies Comparing Women Grouped by Dietary Pattern
One of the first reports comparing women grouped by dietary pattern was that of Slavin et al, who studied 173 active premenopausal women.2 The prevalence of amenorrhea (< 3 cycles per year) was highest among women who described their diets as "vegetarian" (14/45 or 31%), intermediate among those following a "high-carbohydrate, low-fat" diet (6/44 or 14%), and lowest among those on a "balanced four food group" diet (3/84 or 4%). Actual contents of the diets, however, were not described, nor were other characteristics (age, body fat, training volume) compared among the diet groups.
Pirke et al monitored menstrual function during weight loss in 18 healthy women who initially had normal ovulatory cycles.3 Nine women were assigned to a vegetarian diet and nine to a nonvegetarian diet. During the six-week intervention, both groups lost approximately 1 kg/week. Seven of the nine women on the vegetarian diet experienced an anovulatory cycle during the intervention, compared to only two of the nine women on the nonvegetarian diet. Although one could speculate that changing to a vegetarian diet might be more stressful than maintaining a nonvegetarian diet, global mood score actually tended to be somewhat worse among the women on the nonvegetarian diet.
One study that compared hormone levels in vegetarian and nonvegetarian teenage girls also provided relevant data on menstrual characteristics.18 Vegetarian teens (n = 35) were students at a Seventh-Day Adventist boarding school and nonvegetarians (n = 40) were students at a private boarding school. There were no significant group differences in age, weight, body mass index, caloric intake, exercise, age at menarche, or the percentage of girls with ovulatory cycles. Minor differences were observed in hormone levels (vegetarians had higher follicular phase log estradiol levels and higher luteal phase dehydroepiandrosterone sulfate levels than nonvegetarians). However, this study’s results suggested that menstrual characteristics were similar in well-matched groups of lifelong vegetarians and nonvegetarians.
Pedersen et al conducted one of the first studies specifically designed to assess the effects of vegetarian diets on menstrual regularity.19 Vegetarian (n = 34) and nonvegetarian (n = 41) women reported whether they had regular cycles (11-13 menses per year), irregular cycles (3-10 cycles per year) or were amenorrheic (£ 2 cycles per year). The women were similar in age and relative weight, but irregular cycles were significantly more common among vegetarians (26% vs 5%). This study had two potential limitations. A crucial limitation is that current oral contraceptive (OC) users were not excluded. The proportion of women currently using OCs in the two groups was not reported, but nonvegetarian women had used OCs for significantly longer. Because OCs impose "regular" cycles, OC use would be expected to contribute to fewer "irregular" cycles. Recruitment bias also may have existed: If recruitment notices described the study as exploring whether menstrual disturbances were more common among vegetarians, vegetarian women with irregular cycles may have been more likely to volunteer. A second study by this group, in which OC users were excluded had similar results.20 Four of 27 vegetarians (15%) and none of the nonvegetarians reported irregular menses or amenorrhea. The groups were well matched for variables that could confound the results; however, the possibility of a recruitment bias remained.
Barr et al attempted to control for recruitment bias by studying regularly menstruating vegetarian and nonvegetarian women. The occurrence of subclinical menstrual disturbances was assessed prospectively during a six-month study.12 It was reasoned that if clinical disturbances of menstrual function were more common among vegetarians, subclinical disturbances (anovulation and cycles with a short luteal phase) also would be more common. Other recruitment criteria were intended to reduce the influence of confounding variables, and included no OC use for at least six months, stable weight with body mass index 18-25 kg/m2, no history of an eating disorder, nulliparous, exercise £ 7 hours/week and alcohol intake £ 1 drink daily. Participants were similar in age, gynecologic age (years since menarche), exercise level, energy intake, and percent energy as dietary fat, although relative weight of the vegetarian women was lower than that of nonvegetarians. Over the six months of the study, vegetarian women experienced significantly fewer anovulatory cycles (4.6% vs 15.1% of cycles) and also had significantly longer mean luteal phase lengths, although cycle lengths were similar. Thus, in this highly selected group of women, vegetarians had fewer subclinical menstrual disturbances.
Although the results of the above study strongly suggest that vegetarianism is not associated with an increased prevalence of menstrual disturbances, they cannot be generalized to the population level because of the highly selected sample. In the "real world," some women may become vegetarian because of concerns about body weight, and these women could have been excluded from Barr’s study.12 Given that concern about body weight (as reflected by scores for cognitive dietary restraint) has been associated with subclinical menstrual disturbances, population studies are needed to address this question at the broadest level. Results from a study of 666 university women provide some insight into this issue.21 The study assessed nutrition-related attitudes and behaviors but did not address vegetarianism specifically, thus avoiding a selective recruitment bias. Questionnaires were distributed to all women students in various university classes, and 52 respondents were vegetarian. Compared to the nonvegetarians, the vegetarian women weighed more, exercised more, had higher dietary restraint scores, and were more likely to smoke cigarettes, all of which suggest that concern about body weight is common among this group. Furthermore, they were more likely to have a history of an eating disorder (18% vs 3%) and to ever have tried to lose weight (90% vs 72%). Although the difference was not statistically significant, they tended to be more likely to report irregular menstrual cycles (31% vs 20%, P = 0.096).
Conclusion
Although there is no convincing evidence that a vegetarian diet per se causes menstrual disturbances, some studies do report a higher prevalence of menstrual disturbances in vegetarians. Results appear to vary with the characteristics of the vegetarian group studied, suggesting that factors associated with vegetarianism (e.g., in some young women, the desire to control body weight) are at least as important as components of a vegetarian diet. Nevertheless, results of a carefully controlled prospective study showed clearly that vegetarian women did not have an increased prevalence of subclinical menstrual disturbances.
Dr. Barr is Professor of Nutrition at the University of British Columbia in Vancouver.
References
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17. Vegetarian Resource Group. How many vegetarians are there? Available at: www.vrg.org/journal/vj2000may/2000maypoll.htm. Accessed August 10, 2000.
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