Suppose you have two groups of pregnant female rats. Rats in the first group can either eat as much regular lab-rat chow as they like, or they can eat their fill of human junk food — cookies, doughnuts, marshmallows, potato chips, muffins, chocolate. Rats in the second group only get chow, but again, can eat as much as they like. After the rats have given birth, continue the different regimens while the pups are suckling. Then give both groups of pups access to the chow and the junk food.
Experiments like this have found that pregnant females with access to junk food ate, on a daily basis, roughly 40 percent more food (by weight) and 56 percent more calories than rats that just had chow. Moreover — and this is the interesting bit — pups whose mothers ate junk food while pregnant and lactating had a greater taste for food high in fat and sugar than those whose mothers did not. The junk-food pups ate more calories and were more prone to gaining weight.
What goes for rats does not necessarily go for humans. Nonetheless, such results are thought-provoking. As everyone knows, humans are getting fatter and fatter. According to the World Health Organization, 400 million adults around the world weighed in as obese in 2005. In the United States, more than a third of women between 20 and 39 are obese, some of them extremely so. For the first time in history, large numbers of obese women are having children.
Being obese during pregnancy is dangerous for the mother and expensive for the health care system. But does it affect the babies?
There are reasons to think it might. The period between conception and birth is crucial — after all, you’re growing from a single cell into a baby. Your heart is being built; your brain is being wired. Exposure to alcohol during this time can disrupt brain development; lack of iodine may permanently stunt growth. Being starved in the womb can lead to health problems such as heart disease later in life, especially if food becomes abundant. So what about overnourishment? Does an “obese” environment in the womb somehow predispose babies to obesity later on?
At the moment, such questions are difficult to answer. Humans are much harder to study than rats, and the phenomenon of obesity in pregnancy is relatively new, so we don’t know much about it yet. Moreover, many factors contribute to someone’s becoming obese, and picking them apart is tricky. Added to that, an “obese” environment in the womb has two separate elements: the nutrients provided by the mother via the food she eats, and the hormonal environment of someone who is overweight. (Being obese can profoundly alter a woman’s hormonal profile.) Again, picking these apart is hard.
But the results of several studies suggest that the very fact of a woman being obese during pregnancy may predispose her children to obesity. For example, one study found that children born to women who have lost weight after radical anti-obesity surgery are less likely to be obese than siblings born before their mother lost weight. Another study looked at women who gained weight between pregnancies; the results showed that babies born after their mothers put on weight tended to be heavier at birth than siblings born beforehand. Since the mother’s genes haven’t changed, the “fat” environment seems likely to be responsible for the effect.
Why might this happen? Perhaps an “obese” environment in the womb alters the wiring of the developing brain so as to interfere with normal appetite control, fat deposition, taste in food, or metabolism. Studies on other animals suggest that parts of the brain that control appetite develop differently under “obese” conditions. And in humans, one study has found that babies born to obese mothers have lower resting metabolic rates than babies whose mothers are of normal weight.
For most of our evolutionary past, the problem has been avoiding starvation. An environment awash with sugars and fats is, therefore, an evolutionary novelty: in hundreds of millions of years of evolution, this is the first time such foods have been abundant. Giant quantities of fats and sugars have not, historically, been available to a developing fetus, so it wouldn’t be surprising if they do have a harmful impact.
If this is right, it raises the alarming possibility that the obesity epidemic has a built-in snowball effect. If children born to obese mothers are, owing to the environment in the womb, predisposed to obesity, they may find staying thin especially hard. Reversing the epidemic may thus rest on helping women to lose weight before they conceive and helping them to eat a balanced, non-junk-food diet while they are pregnant. The well-being of the next generation may depend on it.