Читать книгу Survival of the Sickest: The Surprising Connections Between Disease and Longevity - Jonathan Prince - Страница 8

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 or red eyes that albinos usually have, you’re actually seeing the blood vessels in the retina at the back of the eye, made visible by the lack of pigment in the iris.

As everybody knows, skin color changes, to some extent, in response to sun exposure. The trigger for that response is the pituitary gland. Under natural circumstances, almost as soon as you are exposed to the sun, your pituitary gland produces hormones that act as boosters for your melanocytes, and your melanocytes start producing melanin on overdrive. Unfortunately, it’s very easy to disrupt that process. The pituitary gland gets its information from the optic nerve – when the optic nerve senses sunlight, it signals the pituitary gland to kick-start the melanocytes. Guess what happens when you’re wearing sunglasses? Much less sunlight reaches the optic nerve, much less warning is sent to the pituitary gland, much less melanocyte – stimulating hormone is released, much less melanin is produced – and much more sunburn results. If you’re reading this on the beach with your Ray-Bans on, do your skin a favor – take them off.

Tanning helps people cope with seasonal differences in sunlight in their ancestral climate; it’s not enough protection for a Scandinavian at the equator. Someone like that – with very little natural ability to tan and regular, unprotected exposure to tropical sun – is vulnerable to severe burning, premature aging, and skin cancer, as well as folic acid deficiency and all its associated problems. And the consequences can be deadly. More than 60,000 Americans are diagnosed with melanoma – an especially aggressive type of skin cancer – every year. European Americans are ten to forty times as likely to get melanoma as African Americans.

As humanity was evolving, we probably had pretty light skin too, underneath a similar coat of coarse, dark hair. As we lost hair, the increased exposure of our skin to ultraviolet rays from the strong African sun threatened the stores of folate we need to produce healthy babies. And that created an evolutionary preference for darker skin, full of light – absorbing, folate-protecting melanin.

As some population groups moved northward, where sunlight was less frequent and less strong, that dark skin – “designed” to block UVB absorption – worked too well. Now, instead of protecting against the loss of folate, it was preventing the creation of vitamin D. And so the need to maximize the use of available sunlight in order to create sufficient vitamin D created a new evolutionary pressure, this time for lighter skin. Recent scientific sleuthing reported in the prestigious journal Science goes so far as to say that white-skinned people are actually black-skinned mutants who lost the ability to produce significant amounts of eumelanin.

Redheads, with their characteristic milky white skin and freckles, may be a further mutation along the same lines. In order to survive in places with infrequent and weak sunlight, such as in parts of the U.K., they may have evolved in a way that almost completely knocked out their body’s ability to produce eumelanin, the brown or black pigment.

In 2000, an anthropologist named Nina G. Jablonski and a geographic computer specialist named George Chaplin combined their scientific disciplines (after already combining their lives in marriage) to chart the connection between skin color and sunlight. The results were as clear as the sky on a cloudless day – there was a near-constant correlation between skin color and sunlight exposure in populations that had remained in the same area for 500 years or more. They even produced an equation to express the relationship between a given population’s skin color and its annual exposure to ultraviolet rays. (If you’re feeling adventurous, the equation is W = 70 – AUV/10. W represents relative whiteness and AUV represents annual ultraviolet exposure. The 70 is based on research that indicates that the whitest possible skin – the result of a population that received zero exposure to UV – would reflect about 70 percent of the light directed at it.)

Interestingly, their research also proposes that we carry sufficient genes within our gene pool to ensure that, within 1,000 years of a population’s migration from one climate to another, its descendants would have skin color dark enough to protect folate or light enough to maximize vitamin D production.

There is one notable exception to Jablonski and Chaplin’s equation – and it’s the exception that proves the rule. The Inuit – the indigenous people of the subarctic – are dark-skinned, despite the limited sunlight of their home. If you think something fishy’s going on here, you’re right. But the reason they don’t need to evolve the lighter skin necessary to ensure sufficient vitamin D production is refreshingly simple. Their diet is full of fatty fish – which just happens to be one of the only foods in nature that is chock-full of vitamin D. They eat vitamin D for breakfast, lunch, and dinner, so they don’t need to make it. If you ever had an enthusiastically caring grandmother try to force cod liver oil down your throat, she was onto something for the same reason – since it’s full of vitamin D, cod liver oil was one of the best ways to prevent rickets, especially before milk was routinely fortified with it.

If you’re wondering how people who have dark skin make enough vitamin D despite the fact that their skin blocks all those ultraviolet rays, you’re asking the right questions. Remember, ultraviolet rays that penetrate the skin destroy folate – and ultraviolet rays that penetrate the skin are necessary to create vitamin D. Dark skin evolved to protect folate, but it didn’t evolve with a switch – you can’t turn it off when you need to whip up a batch of vitamin D. So that would seem to create a new problem for people with dark skin – even if they lived in a sunny climate – because even though they received plenty of exposure to ultraviolet rays, the skin color that protected their supply of folate would prevent them from stocking up on vitamin D.

It’s a good thing evolution’s such a clever sort, because it took that into account – it kept room for a little guy called apolipoprotein E (ApoE4) in the gene pool of dark-skinned population groups. And guess what ApoE4 does? It ensures that the amount of cholesterol flowing through your blood is cranked up. With more cholesterol available for conversion, dark-skinned people can maximize the use of whatever sunlight penetrates their skin.

Much farther to the north, without a similar adaptation, the light-skinned people of Europe would face a similar problem. There, instead of plenty of sunlight that was largely blocked by dark skin, they had to deal with too little sunlight to make enough vitamin D even with the benefit of their light skin. And sure enough, ApoE4 is also common throughout Northern Europe. The farther north you go up the continent, the more you’ll find it. As it does in Africans, the ApoE4 gene keeps cholesterol levels cranked up, allowing its carriers to compensate for limited ultraviolet exposure by maximizing the cholesterol available for conversion to vitamin D.