Survival of the Sickest: The Surprising Connections Between Disease and Longevity. Jonathan Prince

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       Chapter Three THE CHOLESTEROL ALSO RISES

      Everybody knows that humanity’s relationship with the sun is multifaceted. As we all learned in primary school, almost the entire global ecology of our planet depends on sufficient sunlight – beginning with the production of oxygen by plants through photosynthesis, without which we wouldn’t have food to eat or air to breathe. And as we all have learned more and more over the last couple of decades, too much sun can be a bad thing on a global level and an individual one, throwing our environment into chaos by causing drought or causing deadly skin cancer.

      But most people don’t know that the sun is just as important on an individual, biochemical level – and the relationship is just as two-sided. Natural sunlight simultaneously helps your body to create vitamin D and destroys your body’s reserves of folic acid – both of which are essential to your health. To manage this can’t-live-with-you-can’t-live-without-you relationship, different populations have evolved a combination of adaptations that, together, help to protect folic acid and ensure sufficient vitamin D production.

      Vitamin D is a critical component of human biochemistry, especially to ensure the growth of healthy bones in children and the maintenance of healthy bones in adults. It ensures that our blood has sufficient levels of calcium and phosphorus. New research is discovering that it’s also crucial to the proper function of the heart, the nervous system, the clotting process, and the immune system.

      Without enough vitamin D, adults are prone to osteoporosis and children are prone to a disease called rickets that results in improper bone growth and deformity. Vitamin D deficiencies have also been shown to play a role in the development of dozens of diseases – everything from many different cancers to diabetes, heart disease, arthritis, psoriasis, and mental illness. Once the link between vitamin D and rickets was established early in the twentieth century, American milk was fortified with vitamin D, all but eliminating the disease in America.

      We don’t have to rely on fortified milk for vitamin D, however. Unlike most vitamins, vitamin D can be made by the body itself. (Generally speaking, a vitamin is an organic compound that an animal needs to survive but can usually obtain only from outside the body.) We make vitamin D by converting something else that, like the sun, has been getting a bad rap lately, but is 100 percent necessary for survival – cholesterol.

      Cholesterol is required to make and maintain cell membranes. It helps the brain to send messages and the immune system to protect us against cancer and other diseases. It’s a key building block in the production of estrogen and testosterone and other hormones. And it is the essential component in our manufacture of vitamin D through a chemical process that is similar to photosynthesis in its dependence on the sun.

      When we are exposed to the right kind of sunlight, our skin converts cholesterol to vitamin D. The sunlight necessary for this process is ultraviolet B, or UVB, which typically is strongest when the sun is more or less directly overhead – for a few hours every day beginning around noon. In parts of the world that are farther from the equator, very little UVB reaches the earth during winter months. Fortunately, the body is so efficient at making vitamin D that, as long as people get sufficient sun exposure and have enough cholesterol, we can usually accumulate enough vitamin D reserves to get us through the darker months.

      By the way, the next time you get your cholesterol checked, make a note of the season. Because sunlight converts cholesterol to vitamin D, cholesterol levels can be higher in winter months, when we continue to make and eat cholesterol but there’s less sunlight available to convert it.

      It’s interesting to note that, just as it blocks the ultraviolet rays that give us a suntan, sunblock also blocks the ultraviolet rays we need to make vitamin D. Australia recently embarked on an anti-skin cancer campaign it called “Slip-Slop-Slap.” The campaign was especially effective at producing unintended results – Australian sun exposure went down, and Australian vitamin D deficiencies went up.

      On the flip side, researchers have discovered that tanning can actually help people who have vitamin D deficiencies. Crohn’s disease is a disorder that includes significant inflammation of the small intestine. Among other things, the inflammation impairs the absorption of nutrients, including vitamin D. Most people who have Crohn’s have a vitamin D deficiency. Some doctors are now prescribing UVB tanning beds three times a week for six months to get their patients’ vitamin D back up to healthy levels!

      Folic acid or folate, depending on its form, is just as important to human life. Folate gets its name from the Latin word for “leaf” because one of the best sources for folate is leafy greens like spinach and cabbage. Folate is an integral part of the cell growth system, helping the body to replicate DNA when cells divide. This, of course, is critical when humans are growing the fastest, especially during pregnancy. When a pregnant woman has too little folic acid, the fetus is at significantly higher risk of serious birth defects, including spina bifida, a deformation of the spinal cord that often causes paralysis. And as we said, ultraviolet light destroys folic acid in the body. In the mid-1990s an Argentinian pediatrician reported that three healthy women all gave birth to children who had neural tube defects after using indoor tanning beds during their pregnancies. Coincidence? Probably not.

      Pregnancy isn’t the only time folate is important, of course. A lack of folate is also directly linked to anemia, because folate helps to produce red blood cells.

      The skin, as you’ve probably heard, is the largest organ of the human body. It’s an organ in every sense of the word, responsible for important functions related to the immune system, the nervous system, the circulatory system, and metabolism. The skin protects the body’s stores of folate, and it’s in the skin that a crucial step in the manufacturing of vitamin D takes place.

      As you might have guessed, the wide range of human skin color is related to the amount of sun a population has been exposed to over a long period. But darker skin isn’t just an adaptation to protect against sunburn – it’s an adaptation to protect against the loss of folic acid. The darker your skin, the less ultraviolet light you absorb.

      Skin color is determined by the amount and type of melanin, a specialized pigment that absorbs light, produced by our bodies. Melanin comes in two forms – red or yellow pheomelanin, or brown or black eumelanin – and is manufactured by cells called melanocytes. Everybody on earth has around the same number of melanocytes – differences in skin color depend, first, on how productive these little melanin factories are and, second, on what type of melanin they make. The melanocytes of most Africans, for example, produce many times the amount of melanin that the melanocytes of Northern Europeans produce – and most of it is eumelanin, the brown or black version.

      Melanin also determines hair and eye color. More melanin means darker hair and darker eyes. The milk white skin of an albino is caused by an enzyme deficiency that results in the production of little or no melanin. When you see the pink СКАЧАТЬ