think about it, having the capacity to lay on fat and conserve it makes sense from an evolutionary standpoint, especially for women. For thousands of years, having a little extra weight helped us remain fertile and deliver a healthy child. Now, even when food is readily available and easy to obtain, we have the same genes that we had when food was scarce and hard to come by, and our bodies still crave calories and defend overweight with a vengeance. That’s why it’s so hard to take off the pounds—and even harder to keep them off.
We’re just beginning to understand how specific genes contribute to differences in body weight—the ways in which variations in these genes can have a profound effect on everything from how we metabolize food to how much energy we expend with physical activity—as well as how these genes operate in families. Deanne knew she was struggling against her own genetic legacy: She came from a background in which overweight was prevalent on both sides of the family.
But there’s another side to the story. Genes do not act in a vacuum; they’re heavily influenced by their immediate environment. Fascinating insights have come from a new field called epigenetics, which explores how environmental factors can affect the behavior of our genes.
In a nutshell: Genes need instructions on what to do and when to do it. They are a little like the keys on a piano. There they sit until the player presses them to make a sound. The player is something called our “epigenome” (meaning in addition to, or above, the genome), which contains chemical switches that instruct genes, helping them decide when to “express” themselves, to switch on or off. As it turns out, diet, exercise, stress, and other factors can activate these switches. Moreover, the responses that are triggered can be passed along through generations.
Consider this surprising theory. Conditions in your mother’s womb may affect your epigenome, and hence, the way your genes behave throughout life. This is known as the Fetal Origins Hypothesis. A pregnant mother’s diet can influence her child’s epigenome, for instance. Some foods (e.g., sesame and sunflower seeds, shellfish, spinach, broccoli, and peppers) contain compounds that become part of the epigenome or affect it, determining the way genes act not just in utero, but into adulthood. Similarly, a pregnant mother’s unhealthy weight gain and obesity during pregnancy can turn on specific existing genes that influence the future body weight of her children. Studies suggest that a child born to an overweight mother is three times more likely to be overweight by age 7 than a child born to a normal weight mother, and may also face a lifelong risk of type 2 diabetes.
Oddly enough, the reverse is true, too. Malnutrition and underweight can have a similar effect. In fact, the first inkling that a mother’s diet and body weight could influence her baby’s genes in utero came from studies of the offspring of parents who suffered through the Dutch famine during World War II.
It’s an intriguing story. In late 1944 and early 1945, parts of the Netherlands suffered severe famine; rations for adults ranged from only 400 to 800 calories a day. Mothers were sorely undernourished, as were their unborn babies. Studies revealed that babies conceived or born during this famine period were predisposed to early heart disease, type 2 diabetes, and obesity. It was as if evolution had devised a mechanism to make offspring conceived or gestated in a time of deprivation more able to conserve energy.
If we perturb the system either way, it seems, our genes automatically go into a kind of default mode toward conserving energy. So if your mother was either underweight or overweight during pregnancy, you may be predisposed to being overweight throughout life.
Our genes also drive our food preferences, what we like to eat. Studies of the dietary habits of twins suggest that genes influence our preference for certain types of foods, including shellfish, flour and grain products, rice, vegetables, fruit, and dairy products. In fact, genetic variance explains between 30 and 50 percent of our leaning toward certain kinds of foods. Tim D. Spector of the Twin Research and Genetic Epidemiology Unit at King’s College, London, where many of these twin studies have been conducted, suggests that much of our preference for foods is set before we are born. “More often than not, our genetic make-up influences our dietary patterns,” he says. One study of Danish and Finnish twins in 2010 found that even our preference for white or rye bread is influenced by genetic predisposition.
Our physical activity levels, too, may be determined in part by genetic variations. One recent study at the University of California, Riverside, found that the drive for daily exercise is a trait that may be passed along genetically to successive generations—at least in mice. The experimenters found that they could selectively breed mice to produce offspring that were “high runners”—that is, they had increased levels of daily activity to about three times the normal level. The female mice raised their levels by running faster; males boosted it by spending more time running. The researchers speculate that this may help us find ways for women and men to more easily step up their activity. Perhaps women would find it easier to boost their workouts by doing more intense exercise, and men, by increasing the duration of their exercise.
In the human domain, evidence from twin studies suggests that genes could be involved in our inclination to be sedentary or physically active. One large study compared identical and fraternal twins, ages 19 to 40 years, from six European countries and Australia. The subjects were classified as “physically active” if they reported at least one hour of exercise each week. It turned out that differences in exercise behavior were about 60 percent attributable to genes. Scientists suggest that a number of genes probably affect this propensity to be physically active or inactive. Right now, they’re poorly understood, but their influence is strong.
YOUR SURROUNDINGS
While our social network and genes play a powerful role in our health, it’s not only who our parents are and the company we keep that makes it hard for us to eat well and stay fit. Our larger environment also plays a part, from our food supply to our physical surroundings.
When Neelam Sharma moved to South Central Los Angeles with her two young children in 1996, she discovered a dismaying problem: “I wasn’t really able to access what I thought was good, affordable, high-quality food.” South Central L.A. is a low-income area with a population of about 1.36 million. Neelam observed that residents in her community had little or no access to fresh fruits and vegetables and other healthy foods. There was no supermarket within walking distance. “I really resented having to drive so far to get basic food items,” she says. She had grown up in England and recalled working in the family garden with her father. “I remember growing berries and spinach; some basic items we used for Indian cooking were always grown in the backyard.” The lack of access to healthy food in her new neighborhood shocked her.
South Central L.A., like other low-income areas in this country, has been what some experts call a “food desert”—a densely populated community with a scarcity of supermarkets but an abundance of convenience stores and fast-food restaurants. In these areas, residents may have to travel miles to reach a well stocked, full-service grocery store where fresh produce and other high-quality, healthy food is readily available at lower prices. (Convenience stores have prices that can be as much as 76 percent higher than those found at grocery stores and they rarely carry fresh produce.)
Easy access to supermarkets is linked with healthier eating. For example, a study of African Americans found that those who lived in neighborhoods with full-service grocery stores ate more fruits and vegetables than those in neighborhoods without these stores. On the flip side: The absence of fresh-food markets in neighborhoods is linked with higher body mass index, especially for low-income Americans. So too is a high density of fast-food restaurants and convenience stores. These “food desert” neighborhoods often have high rates of obesity and diet-related diseases.
The problems in South Central L.A. are not unique. They’re emblematic of the challenges that face us all over this country: Our food supply—fast, cheap, convenient, but often unhealthy—makes it hard for us to eat well and creates what many of
