responsible for the fermentation of certain nutrients, such as fiber, carbs, and sugars. Bacteria play other vital roles, such as preventing harmful bacteria from colonizing the gut, removing carcinogens, boosting the immune system, preventing allergies, and preventing inflammatory bowel disease. The gut (your entire body, in fact) is inhabited by trillions of genetically diverse microorganisms. Keeping a healthy diversity of bacteria in the gut is especially important so that your gut can adapt quickly to any diet or lifestyle changes. The composition of bacteria in your gut is dictated primarily by what you eat and how much stress you have in your life. Everyone’s gut bacteria are unique, which could explain why some folks do better on specific diets than others. Research has shown that people can change their food intolerances by eating and supplementing with the right combinations of food and probiotics, ultimately influencing the bacteria in the gut. This means you can potentially improve your lactose or gluten intolerance. Of course, there is still a lot of research to be done, but you can read about this exciting, groundbreaking research on the Human Microbiome Project website (see the Resources section).
NUTRIENT ABSORPTION AND STORAGE
The ultimate goal of the digestive process is to deliver nutrients and energy into the bloodstream so that your cells can soak them up to propel your mountain adventures. If all goes well, nutrients move on to the lymph system and then travel to the liver. They enter the liver via the hepatic portal vein for a quick scan to sort out which nutrients need further processing and which can be passed along to general circulation. Dietary fats (both saturated and unsaturated) are the exception, and they take a different route. The short-and medium-chain fatty acids are absorbed by the lymphatic system before going into the bloodstream and the liver for further processing.
The liver. The liver’s main job is to maintain carbohydrate homeostasis. It does this by grabbing excess glucose and storing it as glycogen in its own cells for eventual use in regulating blood sugar. Other sugars (such as fructose and galactose) are also converted to glucose and stored in the liver. The liver removes excess amino acids and turns them into glucose derivatives and nitrogen. In some cases, amino acids are converted into hormones, or if the body needs glucose or fat in emergencies (such as during long endurance endeavors where you haven’t had the opportunity to eat anything), amino acids will be degraded and converted into glucose or fatty acids and then transported to muscle and fat cells. The liver also processes free fatty acids and forms them into triglycerides that it stores for fuel to carry out liver functions, or they are exported into general circulation. In certain situations, when the liver is processing a lot of fat without the presence of carbohydrates, it will form ketone bodies. Ketones are an alternative fuel for the liver and the brain, and some endurance athletes prefer this type of fuel (for more on the ketogenic diet, see chapter 14, Lifestyle and Food Choices: Special Diets).
The liver filters toxins and drugs before returning the blood to the rest of the body. Once glucose, amino acids, and fatty acids have passed through the liver, they can be used by muscle cells, fat cells, and the brain. In the muscles, glucose is used in ATP production or stored as glycogen; amino acids are used to synthesize new proteins; and fatty acids are used to provide energy, stored as triglycerides, or used for structures such as plasma membranes.
Adipose tissue (fat tissue). Adipose tissue is the body’s long-term energy storage and is drawn upon during extended endurance activities. Before significant mountain ascents, it’s a good idea to pack on some extra reserves in your adipose tissue, for use when you are expending thousands of calories at altitude. When you eat excess calories, those nutrients often get stored as fat in the form of triglycerides (glucose and fatty acids joined together). Other nutrients are also stored in the adipose tissue, such as vitamins, amino acids, minerals, and toxins. Adipose tissue also produces an abundance of hormones.
The brain. The most important organ that receives nutrients is the brain. This is the most delicate organ in the body, so the regulation of what comes into it is quite complex. The brain and the nervous system rely on glucose as their primary source of energy. However, glucose cannot be stored in the brain, so that means a constant stream of glucose needs to be available. When glucose is low, ketones from the liver can be used.
The brain has a border crossing for all cells called the blood-brain barrier that controls what enters it. Only lipid-soluble molecules such as oxygen, carbon dioxide, and steroid hormones can freely cross this border. All other molecules either need to be shuttled in or can’t enter at all. This is important to remember, because many companies claim that their supplements have effects on the brain, but in reality they don’t, since they can’t cross the blood-brain barrier. An example of this is GABA, a neurotransmitter that is involved in elevating mood. Often, those who suffer from anxiety have low levels of GABA and try to increase them with supplements (for supplements that do help alleviate anxiety, see chapter 12, Supplements for Mountain Athletes).
CONTROLLING THE DIGESTIVE SYSTEM AND THE SECOND BRAIN
Two systems control the digestive process: the endocrine system and the nervous system. When the endocrine system senses changes in the body, it sends a chemical message in the form of hormones to tell the body how to respond. For example, back to the digestive journey of the strawberry: once you ingest it, the small intestine detects the number of carbohydrates entering your system and helps your pancreas stimulate an insulin response. The nervous system works to regulate the digestive system by using nerve impulses and releasing neurotransmitters. Some of the control originates from nervous and hormonal connections between the digestive system and the central nervous system.
Hormones. Hormones play key roles in the body; you have hormones that help neutralize stomach acid, hormones that help with muscle contractions of the GI tract, hormones that slow gastric emptying, hormones that are released hours after consuming a meal to suppress appetite, and so on. These hormones work together to orchestrate the digestive process. One of the more critical hormones is ghrelin (a.k.a. the hunger hormone), which is released in response to low food intake or fasting. Ghrelin stimulates the release of growth hormone and encourages appetite. Overall, this hormone is fundamental to the regulation of long-term energy balance (vital for athletes who want to lose fat or gain muscle) and is potentially essential for longevity.
The enteric nervous system, or second brain. In addition to having control systems, the GI tract, and more specifically the intestines, also has what’s called the second brain (the enteric nervous system). It shares many similarities with the brain, such as neurotransmitter activity, signaling molecules, and the number of neurons present, as well as the size and complexity of the system. Communication between the gut and the brain happens in both directions—from the brain to the gut, and from the gut to the brain. This means that your emotions and stress can affect what happens in your gut. Alternatively, what happens in the tissues of your gut can affect your brain.
KEY STATS ABOUT THE SECOND BRAIN
The second brain is wondrously complex. For example, the gut (the intestines) has as many neurons as the spinal cord. The second brain is also affected by your thoughts, feelings, and physical sensations. If you’ve ever had “butterflies” before a big competition or stress-induced heartburn or a sudden urge to use the bathroom before a big race or climb . . . this is how the enteric nervous system communicates. Here are some other important facts:
•The surface area of the intestines is approximately 100 times greater than the surface area of the skin.
•The gut has the largest population of microorganisms of all body surfaces.
•The gut contains two-thirds of our immune system cells and also has thousands of hormone-producing cells, making it the largest endocrine organ.
•The gut uses and locally secretes neurotransmitters to help with digestion, such as GABA, norepinephrine, serotonin, and acetylcholine. It uses most of the body’s serotonin, dopamine, and melatonin.