Let Them Eat Dirt. B. Brett Finlay
are not exclusively produced by the microbiota. These compounds are too critical for our metabolism to rely entirely on bacteria for their production. Still, studies performed on patients genetically unable to produce propionate have shown that approximately 25 percent of the propionate in our body is derived from bacterial activity in the gut. The implications of this are significant, considering that treatment with many types of antibiotics severely alters intestinal SCFA production. If antibiotics are given during early childhood, especially in the first few months of life, the risk of experiencing long-lasting metabolic and immune alterations due to abrupt changes to the microbiota increases dramatically.
Scientists haven’t yet figured out all the functions that our metabolism delegates to the microbiota. Immune training and metabolizing energy are two essential things that our microbes do for us, but it’s clear that there are more. Brand-new research shows that the microbiota plays an important role in neurological development (discussed in chapter 15), and even in the health of our blood vessels. These types of discoveries have led scientists to call our microbiome a “new organ,” perhaps the last human organ to be discovered by modern medicine. Although most of this knowledge has just recently emerged and many pieces of the puzzle remain unsolved, it is evident that protecting the initial developmental stages of our microbiota has a significant impact in human health.
In the next four chapters we discuss the life stages that are most influential in the development of the human microbiome, all of which occur during infancy and early childhood. We will explore how some of the actions parents take during pregnancy and birth, as well as through diet, can have profound implications in the communities of microbes that are part of our children’s bodies. With scientific information parents have learned to make better choices when raising their kids, such as limiting sugar intake and even the amount of time spent in front of the TV. With our newfound awareness of how important the microbiome is, let’s explore what we might do as parents to improve our children’s health by caring for their microbes.
THE CAVEMAN DIET
The newest diet fad suggests that eating the way our Paleolithic ancestors did will make us be healthier and live longer. However, evolutionary biologists don’t agree with this because it’s not based on current scientific knowledge. Some assumptions of the “paleo diet” include:
•Our ancestors ate mostly meat, and no legumes or grains. Actually, our ancestors ate incredibly different diets depending on where they lived. One could expect this statement to be close to the truth in Arctic environments, but in more temperate weather this was not the case. Biochemical analysis of dental fossil records from this period show that foragers did eat grains and legumes. Also, the meat we consume today—from domesticated livestock—is completely different than the wild game our ancestors ate.
•Our ancestors did not eat dairy. While this is generally correct, modern humans from many regions of the world where dairy is consumed have genetically modified their metabolism to digest and absorb dairy products. In other words, we have evolved, in a somewhat short period of time, to digest foods that our ancestors didn’t eat. Our genes have changed since we roamed the savannahs.
It is impossible for modern humans to eat the way our ancestors did because our foods today are completely different than before. Carrots, broccoli, and cauliflower did not exist back then, and neither did the leaves used to make salads. All of these are products of agriculture. What certainly is true is that the typical modern human diet has extremely low diversity and is heavily processed, compared to food consumed a hundred years ago.
In addition, only very recently have people stopped eating just what is in season and whole foods. These are the dietary changes that really have an impact on our health, in great part because of the effects on our microbiota. Yes, eating fewer refined carbohydrates and more vegetables will help you lose weight and feel better, but this does not reflect our Paleolithic past in the way “paleo” enthusiasts believe it does.
PART TWO
Raising Babies and Their Microbes
3:Pregnancy: Eating for Two? Try Eating for Trillions
The Pregnant Microbiota: Another Reason to Eat Well
Seeing that positive result on a pregnancy test changes everything for most women. All of a sudden they’re going to the bathroom more 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!
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