Are girls entering puberty earlier than they used to? This question, which has been debated recently by doctors and scientists in the pages of Time magazine and the New York Times, proves that there is still a great deal to learn about women's reproductive health. Female Fertility and the Body-Fat Connection is the record of one scientist's groundbreaking and decades-long work on the connections among fertility, body fat, and reproductive health in women.
Rose E. Frisch explains here how, in women, a certain amount of body fat is crucial to the reproductive system and sexual maturation. Women who are too lean are infertile and cannot conceive children; young girls who are too thin have a delayed onset of their first period. Female Fertility and the Body-Fat Connection illuminates how and why a "critical fitness" level underlies a woman's reproductive health. In the process Frisch gives readers a comprehensive view of the research done to date on the relationship between body composition and fertility and also describes her own journey as a woman scientist working to advance her critical-fitness hypothesis both to the general public and the scientific community. Frisch answers the questions every woman has about the desirable weight for health and fertility and even includes tables to help women find their own best weight. She also demonstrates how important diet and exercise are for the long-term reproductive health of women, and shows what factors influence the onset of puberty in girls.
Each milestone of the reproductive life span is affected by food intake and energy output, the factors affecting the storage of fat. Female Fertility and the Body-Fat Connection is a cornerstone to understanding the health of girls and women.
About the Author
Rose E. Frisch is an associate professor of population sciences emerita at the Harvard School of Public Health and a member of the research faculty of the Harvard Center for Population and Development Studies. She is a fellow of the American Academy of Arts and Sciences and the John Simon Memorial Guggenheim Foundation. She is the editor of Adipose Tissue and Reproduction and has written widely on female fertility and on the natural fertility of populations.
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Female Fertility and the Body Fat Connection
By Rose E. Frisch
University of Chicago PressCopyright © 2002 Rose E. Frisch
All right reserved.
1 - Female Body Fat
Celebrating the Difference
Since the dawn of human history, the symbols of female fertility have been fat, very fat--particularly in the places where the female sex hormone, estrogen, stores fat. Figure 1 below shows a typical fertility goddess, the Venus of Willendorf, dating from 30,000 b.c.; fortunately, she is only 4-3/8 inches high. African, Asian, and Indian fertility goddesses in past centuries were also archetypal curvaceous females, a far cry from our current cult of the pencil-slim woman.
Why did the ancients have fertility goddesses? We can deduce that fertility was not an easy thing to achieve in the early times of our species because food was often in short supply. This ancient recognition of the connection between fatness and fertility proved to be a sound strategy for the maintenance of the species.
During the centuries up to our own times, artists have celebrated female body fat, but with considerably more restraint, as the Venus de Milo and the lush ladies in Peter Paul Rubens's paintings--two examples of many--illustrate. The historical linking of fatness and fertility makes sense to me biologically because, more than two decades ago, I discovered that women need a certain amount offat to become fertile and remain fertile. I found that a girl does not have her first menstrual cycle (menarche) until she has a predictable minimum amount of body fat, and that a grown woman requires a larger minimum amount of fat to maintain ovulation and regular menstrual cycles.
She and He: Diverse Bodies
When I began researching why differing amounts of body fat could apparently turn menstrual cycles on or off, I had the entire field to myself. Body fat was not high on the agenda of reproductive endocrinologists. I began by reading everything I could find about body fat and the differences between the body fat of women and men.
Women at all ages have more body fat than men. It is at puberty, however, that the female attributes of rounded breasts, hips, buttocks, and thighs develop. Estrogen, the female hormone, begins to rise at this time, eventually reaching adult levels. Males, in contrast, become leaner as they become sexually mature at puberty.
The differing amounts of female and male fat are also distributed differently, as is normally very easy to observe. Men have more fat on the trunk and above the umbilicus (belly button). When men get fatter as they get older (which they do, just as women do, unless they work at avoiding it), that's where most of the new fat is added. Another male characteristic, not as well known, is the fat at the nape of the neck: it is thicker in men. In fact, this is the only area where fat is known to be thicker in men than in middle-aged women (that is, women about 40 years old). As women become fatter with age, they add fat below the umbilicus rather than above. Women may also have an increase of fat on the trunk after menopause.
A woman's body and a man's body differ internally as well as externally, a very important but not commonly observed fact. "Man," the medical textbooks state, is 60 percent water. Woman, as most textbooks neglect to state, is 50 percent water. And thereby hangs a tale. A 10 percent difference in body water may not seem significant, but it is. Fat contains very little water (5 to 10 percent), while muscles and internal organs contain a great deal of water (about 80 percent). The 10 percent difference in body water translates into a much larger difference in body fat: in men body fat is 12 to 14 percent of body weight, and in women body fat is 26 to 28 percent of body weight.
The difference in body composition of a young woman and a young man of the same height and weight is shown in figure 2. The numbers are astonishing. By the time girls finish their growth in height and weight at about age 16 to 18, they have stashed away about 35 pounds (16 kg), 28 percent of their body weight, as fat: 144,000 calories. Boys complete their height and weight growth later than girls, at about age 18 to 20, and by that time they have stored only about 15 pounds (7 kg), 12 percent of their body weight, as fat: 63,000 calories. This large difference in body composition may be one of the most important differences characterizing women and men, because, as I found, all of that fat in women is related to their reproductive ability.
When I first calculated that a nonoverweight young woman had 35 pounds of fat in her body, I didn't believe it. Where is it all? I looked for direct dissection measurements in autopsy reports; there weren't many that recorded fat content, but the few that did confirmed the large amount of fat in adult women. Much of the fat is inside the abdomen, surrounding the internal organs. I asked my research assistant who had dissected a female corpse about it. "I removed the fat in buckets," she assured me. As women age, the proportion of body weight that is fat rises to 30 or 40 percent if they are well nourished. As men age, they also gain rapidly, until fat is 20 percent or even 30 percent of body weight.
Body Fat, an Active Metabolic Tissue
When researchers discovered the hormones controlling human reproduction about seventy years ago, body fat was considered to be an inert storage layer that insulated the body surface and acted as a protective cushion for delicate organs like the kidneys. The large difference in the relative fatness of women and men was not regarded as a potential determinant of or contributor to reproductive ability or sexuality any more than an electric blanket would have been.
In about 1940, however, researchers showed that body fat, or adipose tissue (the medical term, from the Latin adeps, "fat"), is metabolically active. Body fat actively participates in the maintenance of the body's energy balance; body fat cells (adipocytes) store fat as a potential source of energy and mobilize the fat into a fuel molecule when needed. Body fat, as I learned, is an amazing tissue, aside from its connection to fertility. Here are some basic facts.
To be of use to the body, the energy must be easily accessible for withdrawals or deposits. Although fat looks white or pale yellow in both humans and animals, it has a network of capillaries (small blood vessels) equal to that of muscle, so interactions with the circulatory system for the deposit and withdrawal of energy are efficient. Each fat cell has a connection to the sympathetic nervous system (the system of nerves that carries messages to the blood vessels and organs). The adrenal gland, a small gland located on top of each kidney, secretes a hormone that controls the flux of energy from the fat cells. Insulin, a hormone secreted in the pancreas, is the overall regulator of the input and outflow of free fatty acids from the fat cells that are used as energy, in addition to the sympathetic adrenal regulation.
Free fatty acids continually leave the fat cells as energy-rich materials to be used as fuel, or oxidized, by the heart, muscles, and the other organs of the body. The brain, however, can use only glucose, a carbohydrate, as a source of energy. During times of hunger, muscular activity, cold, or acclimation to cold, the flow of energy from the fat cells increases. After starvation and refeeding, fat cells accumulate fat. They can also accumulate fat after normal feeding.
Carbohydrates (starches and sugars) in the diet are converted into fat by the fat cells. How much of the glucose is stored as fat is regulated by your nutritional state: When not much carbohydrate is present, the conversion to fat is reduced; when a lot of carbohydrate is available, the storage is accelerated. In the steady state, about a third of the carbohydrate you eat is temporarily converted to fat.
Why We Store Energy as Fat
Why do we have this elaborate system for storing energy as fat? Why not store carbohydrates directly? There are very good reasons. If you stored energy as carbohydrates, you might not be able to move very quickly, and in prehistoric times your predators would have caught you. Calories from fat are stored directly in the fat cells without any conversion. In contrast, carbohydrate (glucose) is stored as glycogen in the liver and muscle, which requires the addition of water (which is heavy) and other substances. Fat stores energy more efficiently both because it does not require additional water and because it has a greater caloric density per gram (9 kilocalories) compared to carbohydrate (4 kilocalories).
A "Thrifty" Genotype
Centuries ago, in populations with marginal and fluctuating food supplies and a low caloric intake, women and men who stored energy as fat had a better chance of surviving when food supplies were low. They developed a "thrifty" genotype, which allowed them to store calories easily. But this ability, once an asset, has become a liability; today it means the easy accumulation of fat. The Pima Indians, for example, inherited this genotype, and as a result they have developed a high incidence of obesity and of diabetes associated with obesity. In the past, Pima Indians ate a low-fat, high-fiber diet of corn, beans, and indigenous plants, so obesity was not a problem. With a modern, high-fat diet, however, there is too high of an energy intake, and the energy is stored too easily.
Body Fat and Estrogen, the Female Hormone
One of the surprising scientific discoveries of the twentieth century was that body fat converts androgens, the male hormone, to estrogens, the female hormone, by means of an enzyme called aromatase. With the help of this enzyme, which is found in the endoplasmic reticulum (intercellular mechanism) of the fat cells, the weak androgen, androstenedione, made mainly by the adrenal cortex (the outside layer of the adrenal gland), is converted to an estrogen.
By this process, called aromatization, all the estrogen produced by a woman after menopause comes from her body fat. About a third of the circulating estrogen of a premenopausal woman comes from aromatization of androgens to estrogen by her body fat.
If young women or men become obese, this extra-ovarian source of estrogen may be directly involved in reproductive disorders that accompany obesity. Some women who suffer from polycystic ovary disease, which is often associated with obesity, have a chronic lack of menstrual cycles (amenorrhea). Men who are obese may have gynecomastia (enlargement of the breasts). Even in nonobese women, the production of estrogen by fat tissue in the breast may contribute to the risk of breast cancer.
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Table of Contents
Foreword, by Robert L. Barbieri
1. Female Body Fat: Celebrating the Difference
2. Too Little and Too Much Body Fat
3. Female Adolescence: Puberty and Growing Up
4. Eggs, Sperm, "Female Testes," and Other Fancies and Facts about the Reproductive System
5. Historical Guesses: What Hastened or Slowed Menarche?
6. Predicting Menarche: Critical Fatness
7. Pubertal Body Fat—Sex Fat?—A Neat Mechanism for Reproductive Success
8. Physical Activity and Too Little Fat: Ballet Dancers, Swimmers, Runners, and Other Athletes
9. Exercise and Lower Risk of Breast Cancer: The Alumnae Health Study
10. Leptin: A New Hormone Made by Body Fat
11. Population, Food Intake, and Fertility: Old and New Perspectives
12. Fatness, Fertility, and the Body Mass Index: Finding Your "Desirable Weight"
Suggestions for Further Reading