Let Them Eat Dirt. B. Brett Finlay

Чтение книги онлайн.

СКАЧАТЬ bacteria, so where could it get this information? It turns out this very important aspect of the training doesn’t occur in the thymus—it happens in our gut.

      Before we’re born, the lining of our gut is full of immature immune cells, and as soon as we come into the world and bacteria start moving into their new home, these immune cells “wake up” almost magically. They start multiplying, they change the type of activities they do, and they even move to other parts of the body to train other cells with the information they just received. Experiments with germ-free mice, which are mice that are born into and kept in a completely microbe-free environment, show that without microbes the immune system remains immature, sloppy, and unable to fight off diseases properly.

      Scientists haven’t figured out exactly how microbes do this at the molecular level, but it is known that most bacteria will teach these immune cells to tolerate them, whereas some bacteria—the pathogens that cause disease—have the opposite effect. This makes sense; if our immune cells started fighting off all bacteria indiscriminately, there would be an out-of-proportion inflammatory battle between the small quantity of immune cells and the vast numbers of bacteria right after we’re born. In reality it’s quite the opposite; despite the enormous amount of bacteria living in the intestine, it’s a relatively controlled and harmonious place. The way this is achieved is by the microbiota modulating the immune system, allowing most microbes to be tolerated.

      Many inflammatory diseases, such as asthma, allergies, and IBD, are characterized by an overreactive immune response. Knowing what we do now about the importance of microbiota in immune system development, it’s not surprising that these diseases are being diagnosed in more and more children. They are, to a great extent, a consequence of the modern lifestyle changes that are altering the types of microbes that affect the immune system. There’s a reason immune cells wait for microbes to come and train them right after we’re born: because this is the way it has happened for millions of years and is the way it will always be. We need to find ways to modify our modern behavior so that immune cell school can function properly.

Feeding Our Microbes So They Can Feed Us

      Another fundamental function of microbes is to aid in the regulation of our metabolism. Humans, just like any other living animal, obtain energy from food that is digested and absorbed in the intestines. Besides helping us digest certain foods that the intestines can’t handle on their own, bacteria produce energy for us, and the amount they produce is noteworthy. Germ-free mice weigh significantly less than conventionally raised mice, but once bacteria begin to colonize them they have a 60 percent weight gain, despite not eating more food than regular mice. One of the mechanisms by which they accomplish this is a process known as fermentation. Think of the intestine as a bioreactor where bacteria ferment fiber, carbohydrates, and proteins that were not digested and absorbed in the small intestine. The end-products of this process are called short-chain fatty acids (SCFA), and three of them are very important to different aspects of human energy metabolism: acetate, butyrate, and propionate. Intestinal cells rapidly absorb SCFA and use them as an energy source to stay fueled. SCFA are also transported very rapidly to the liver, where they are transformed into critical compounds involved in energy expenditure and energy storage. SCFA help determine how and when we use the energy obtained from food, and, importantly, when to store it as fat. Thus, it’s not surprising that alterations in the production of SCFA have been associated with obesity, both in mice and in humans.

      SCFA 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. СКАЧАТЬ