Once menstruation begins, fat levels will even fluctuate during each cycle, with dips in estrogen and a peak in progesterone causing changes in appetite and fat storage. Women crave fats and carbohydrates during the second half of the menstrual cycle, when estrogen levels decrease. In addition to cravings, the simultaneous peak in progesterone promotes fat storage by clearing the blood of triglycerides and storing them in fat tissue. It is believed that this clearance leads to the desire for fatty foods because the blood is devoid of this nutrient. So it goes, month after month-first cravings, then fat. It's a wonder anyone can lose a pound.
And when women do become pregnant, their bodies add still more fat. The weight gain occurs even when their caloric intake is unchanged, and sometimes decreased. Women accumulate anywhere from five to thirteen pounds of fat in pregnancy, even if they are undernourished. The weight gain is not happening by the slowing of metabolism - total energy expenditure goes up when women carry a child. Part of the explanation may have to do with bacteria, our tiny friends in the gut.
Ruth Ley and her team at Cornell University who transplanted microbiota from pregnant women into germ-free mice found that mice with bacteria transplanted from women in the third trimester of pregnancy got much fatter than the mice with bacteria of women in the first trimester. They found the microbiota composition changes profoundly during the gestational period, which may explain some of the gain - women's bacteria increase absorption of food during pregnancy.
However fat gets accumulated during pregnancy, thankfully a portion of it is converted to milk to feed the newborn. Indeed, breastfeeding allows rapid weight loss after pregnancy for many. That women use their body fat to make all-important breast milk underscores the biological necessity of female fat: the survival of the human race depends on it.
Another reason women store more fat than men is a process called "nutrient partitioning," by which the body stores some of the calories ingested as fat, and uses the rest for other purposes such as for immediate energy or for glycogen stores. Depending on your body, you may consistently set aside more or less of your food intake for fat storage.
Partitioning is like getting a paycheck for $100 every week, and automatically diverting $20 of it into a forced savings plan such as a 401k. If the $80 that's left isn't enough to cover your bills, you have to find another source of income, because the $20 will go into your 401k no matter what. In body terms, if you haven't consumed enough calories to fuel yourself after partitioning some as fat, you will feel driven to eat more. Scientists have not yet uncovered a way to significantly alter nutrient partitioning, which means that even when we do manage to eat less, our biology may urge our appetite on.
Michael Jensen explains, "When it comes to excess energy intake, woman and men respond very differently. Women seem to do a better job at partitioning circulating fatty acids into subcutaneous fat than do men."
Men partition calories to fat, too, but at a lower level than women. The difference alone can add up to pounds of extra fat per year for women.
Excerpted with permission of Aleph Book Company from The Secret Life of Fat by Sylvia Tara. Order your copy here.