Читать книгу Peak Nutrition - Maria Hines - Страница 49

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).