Let Them Eat Dirt. B. Brett Finlay

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СКАЧАТЬ times than they can count, forgetting where their keys are while they’re holding them in their hands, falling asleep at work (at 10 a.m.!), feeling full right after a meal, only to feel famished ten minutes later. From differences in her skin and hair to buying pants in three sizes within one year, pregnancy is a time of major changes in a woman’s body. In nine short months, a woman undergoes a series of drastic physiological transformations that nurture a single fertilized cell into a crying, hungry baby. Many of our organs alter their functions to facilitate these new biological needs of both the mother and her developing baby. For example, the liver produces 25–35 percent more fats in order to promote baby growth. Fats, also known as lipids, are formed as a way to store energy. By naturally adjusting liver metabolism to make more lipids, a pregnant mother’s body ensures that there will be enough energy for the baby to grow, and for the future production of milk following delivery.

      Like the liver, a pregnant woman’s microbiota also responds to this new state. In fact, experts believe this change is a normal physiological adaptation to support the growth of the fetus. A recent study showed that the microbiota of a pregnant woman in her third trimester strikingly resembles the microbiota of an obese person (just what every pregnant women wants to hear . . .). Moreover, when the microbiota of a female mouse in late pregnancy was transferred into a germ-free mouse, the latter mouse gained a lot of weight, despite not increasing food intake or being pregnant. This study was carried out in the laboratory of Dr. Ruth Ley at Cornell University in New York, a scientist at the forefront of the microbiota field. She believes that late pregnancy is an energy-thirsty period, during which the body takes advantage of the energy-producing machinery of the microbiome to promote weight gain for the benefit of the mother and her baby. The timing for this large shift in microbiota couldn’t be better, occurring towards the end of the pregnancy when babies start packing on the pounds and when women need to start preparing for the energy demands of breastfeeding.

      This same study, which sampled ninety-one pregnant women (the largest to date), also showed that some species of bacteria that were more predominant in the third trimester of pregnancy were also found in their babies at one month of age. This suggests that another consequence of the big change in microbiota during pregnancy is to pass many of these bacteria on to the newborn. It’s fascinating to think that a woman’s body and her microbiota work together during pregnancy, likely because both benefit from having a new baby. From a genetic perspective, having babies is the only way to propagate our genes; from a microbial perspective, a newborn is brand-new real estate where microbial genes can also multiply and propagate.

      Another recent study showed that the shifts to microbiota during pregnancy reflect the amount of weight women gain. According to the American Institute of Medicine, a woman of normal weight should gain 25–35 pounds during pregnancy, underweight women should gain 28–40 pounds, and overweight women should gain only 15–25 pounds. Women who gain more weight than what is considered standard have distinct changes in their microbiota. Given that a baby inherits many of its mother’s microbes, and that some of these microbes actually promote weight gain, should we worry about passing obesity-associated microbes to our babies? Unfortunately, yes. Women need to watch their weight during pregnancy, especially during the last trimester. Obesity is a complex condition arising from both genetic and environmental (including microbial) factors (discussed in chapter 10), but it appears that even in cases in which obesity is considered genetic, microbes have a role in its development. This makes sense, as microbes are directly involved in the way we break down food and store fats. If you think no one is watching when you give in to that midnight snack craving, that’s sadly not the case—microbes are watching what we eat at all times, since it affects them directly!

      The good news is that, just as we can foster weight-gain microbes through a poor diet, we can promote the growth of beneficial microbes through a healthy diet. Although scientists haven’t identified specific microbes associated with leanness yet, it has been shown that a varied diet that includes fruits, vegetables, and fiber promotes a diverse microbiota, a characteristic of lean (and healthy) individuals. Thus, you, and your microbiota, are what you eat—and there is probably no better time to watch your diet than when you’re pregnant. Bad dietary choices during this stage of life will not only make women gain more weight than what is considered healthy, they also have the potential to influence a child’s future ability to control weight. So, next time you walk by a candy machine, don’t listen to your sugar-loving microbes, and nourish the trillions of microbes that are begging you to grab a piece of fruit instead.

The Vaginal Microbiota

      During pregnancy, microbiota adaptation also occurs in the vagina, an organ that hosts millions of microbes. The composition of this microbiota influences vaginal health tremendously. Many women develop yeast infections after being on antibiotics or oral contraceptives (birth control pills alter the pH of the vagina). Bacterial vaginal infections, also known as vaginoses, are very common. These infections occur when yeast (often Candida) or bacteria overrun a beneficial group of microbes known as Lactobacilli, a type of lactic acid bacteria that is very common in the vagina. Lactic acid bacteria are also used in the dairy industry for the production of yogurt, kefir, cheese, and buttermilk. Many of them have health benefits and are used as probiotics.

      During pregnancy, the number of vaginal Lactobacillus increases dramatically, which is thought to occur for two important reasons. First, by keeping the vagina acidic, the presence of Lactobacillus helps discourage disease-causing microbes such as E. coli, which do not like to grow in acidic conditions. There’s probably no better time to arm the bacterial vaginal defenses than during pregnancy, when a pathogen could track up from the vagina, through the cervix, and into the uterus, where the baby is growing. In fact, it is known that certain vaginal infections during pregnancy are associated with preterm and low-weight births. Second, Lactobacilli are great at digesting milk, as their name suggests (lacto is Latin for “of milk,” and bacillus is the name given to rod-shaped bacteria). By ramping up the levels of Lactobacillus in vaginal secretions, more of these bacteria will reach the baby’s gut (when born vaginally), and facilitate the digestion of the only food the baby will eat for months: her mother’s milk. In this sense, Lactobacilli are probably a baby’s first and best microbial friend.

      The vaginal microbiota plays a very important role during pregnancy and birth, as it is one of the sources (along with the gut microbiota) of the first microbes to set up camp in a newborn. As soon as a baby is born vaginally, she gets covered in vaginal secretions and, yes, with fecal matter, too. Consequently, the composition of vaginal secretions is of utmost importance during pregnancy, and vaginal health should be taken very seriously during this period of time. Just as women should take care of their diet to promote a healthy intestinal microbiota, they should look after their vaginal health, too.

      To promote vaginal health, gynecologists recommend that pregnant women wear cotton underwear, avoid vaginal douching (never recommended), avoid vaginal cleaning products, and use gentle, unscented soaps to clean the outside of the vagina only. The vagina is an organ that cleans itself through the production of secretions and needs little extra hygiene. In fact, cleaning the interior of the vagina is strongly associated with infections, as it alters the balance of the resident microbiota. In addition, it has been shown that the consumption of probiotics containing Lactobacillus acidophilus decreases vaginal infections. Several clinical studies suggest that eating yogurt may help, too, although not to the same extent as probiotics alone. You can even get probiotic preparations in the form of vaginal suppositories, which are used to treat such infections. Safe sex is the best way to avoid sexually transmitted infections (STIs); it is a practice that should always be followed, and especially during pregnancy. An STI contracted during pregnancy can be more dangerous to the mother than an STI contracted at another time, as immune systems are weaker during pregnancy—a physiological adaptation meant to prevent a woman’s immune system from reacting to the fetus. Unfortunately, this makes a mother-to-be more vulnerable to infection.

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