During the winter, when days are short and the sun is at a low angle, levels of the vitamin in the body tend to dip. Cold temperatures and low vitamin D levels: that may be a bad combination.
If it's not too cold, our bodies adapt to cold temperatures pretty well. When we encounter cold air or water, the lacy network of blood vessels in the skin constricts, and blood is hastily shunted to the interior. That response adds to the insulating power of the skin because there's less heat lost from blood circulating near the surface.
It also protects vital organs against the falling temperature. But we pay a price for the rerouting: diminished blood flow makes fingers, toes, and other peripheral parts of the body the nose, the ears vulnerable to frostbite, which occurs when the fluids in and around tissue freeze. Under the right conditions, blood vessels in the skin will open and close in an oscillating pattern, so skin temperatures rise temporarily, especially in the fingertips.
At Home in the Cold
Shivering is another familiar defense mechanism against falling body temperatures. The rapid, rhythmic muscle contractions throw off heat that helps the rest of the body stay warm. The body may recruit more and more muscles as the temperature drops, so shivering can get intense and very uncomfortable. Voluntary movement — stomping your feet, swinging your arms — is another way to generate heat, and depending on the circumstance, may cancel out the need to shiver. It's not a total gain, though, because exercise also increases blood flow to the skin, so some body heat escapes.
Body type explains some of the varying reactions to cold weather. Taller people tend to get cold faster than shorter people because a larger surface area means more heat loss. And fat's reputation as an insulating material is well deserved, although for warmth during the winter, you want it to be the subcutaneous fat layered under the skin, not the visceral fat that collects in the abdomen.
Most of us spend the winter trying to stay warm and avoid getting cold, but a little bit of exposure may not be such a bad thing. It's been suggested, for example, that moderately cold temperatures could be good for the vasculature because it trains blood vessels in the skin to be responsive. An added benefit: rosy cheeks. Brown fat is the heat-producing, calorie-burning fat that babies need to regulate their body temperatures.
Most of it disappears with age, but PET scans have shown that adults retain some brown fat.
Urban heat island
Years ago, Finnish researchers reported that outdoor workers had more brown fat than indoor workers. Dutch researchers reported findings in The New England Journal of Medicine in that showed that moderately cool temperatures of 61? F activated brown fat in 23 of 24 study volunteers. No one is suggesting that cold weather be used for dieting purposes not yet anyway.
But when we get chilled this winter, we may take some consolation that at least we're firing up those brown fat cells. Using cold temperatures for medical purposes is taken quite seriously in other countries. Patients spend one to three minutes in a room cooled to ? And people in Finland, Russia, and elsewhere are passionate about winter swimming having health benefits. Several years ago, Finnish researchers reported the results of a study of 10 women who for three months took cold-water plunges 20 seconds in water just above freezing and submitted to whole-body cryotherapy sessions.
Remember: Sun damage accumulates over your lifetime. These guidelines are particularly important if you have light skin, a large number of moles, or a family history of melanoma, all of which single you out for greater risk. Scientists believe that skin exposure is particularly risky for children.
Each severe sunburn before the age of 18 ratchets up the risk that skin cancer will develop later in life. So make an extra effort to follow these rules with children and teenagers, keep babies under one year old out of direct sun in the summer, and never leave infants playing or napping in the sun.
Excessive sun exposure and blistering sunburns are clearly a bad idea. But is there anything to fear from the healthy glow of a modest tan?
Chapter 42 - Heat and Cold
Melanin is a pigment found deep in the skin, at the base of the epidermis. Your tan is a defense mechanism that responds to damage caused by the sun. As ultraviolet light strikes your skin, it shatters the DNA in some unlucky cells. Your body has ways of catching and repairing this sort of damage, but when the rate of damage escalates, it presses the panic button and triggers a tan.
In other words, the process that prompts melanin production and tanning is the same process that causes sunburns and may, eventually, cause skin cancer. Races that have traditionally lived in the glare of the equatorial sun have darker skin that offers natural sun protection.
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The light-skinned people of today descended from mostly European races that lost their built-in sunscreen over generations of life in colder, dimmer lands. The obvious question is why. After all, if the sun is so dangerous, surely everyone can use a bit of natural sunblock. The going theory is that when dark-skinned people moved north and south, their natural sunscreen starved them of vitamin D.
But the genetic misfits with light skin absorbed more sun, providing the vitamin D they needed, and they thrived. Races that needed sun protection developed UV-scattering Afro hair to shield their scalps. Races that needed more vitamin D developed straight, kink-free hair that allows light to pass through to the skin below.
In the not-so-distant past, tans were a sign of low-class laborers who had to toil in the fields to make a living. Nobility avoided the sun and strove to keep their skin a perfect shade of porcelain white.
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Today, those views are reversed, and the tan is a symbol of health and wealth. Rightly or wrongly, we assume that someone who has a tan has probably spent the day outside, engaged in vigorous physical activity say, mountain climbing or windsurfing , rather than working all day in an office cubicle. Similarly, someone who sports a tan in the dead of winter clearly has the financial resources to jet away to tropical climes. A melanoma begins life looking like a harmless mole. However, a few red flags suggest the possibility of a problem.
If you catch a melanoma developing in your skin before it goes too deep, the chance of successful treatment is high. Your doctor can help by taking yearly pictures. Under the epidermis is a thicker second layer of skin with a whole lot more going on. This layer is called the dermis. The dermis shapes and supports your skin with tough connective tissue that uses strong, flexible fibers made of collagen and elastin.
These fibers also keep your skin toned and wrinkle-free through the first half of your life, after which they start to loosen and unravel. Unlike the epidermis, which is constantly dying off and renewing itself, your dermis is yours for life. A dense network of blood vessels supplies your dermis with essential nutrients. Tattoo machines work by puncturing the skin thousands of times and inserting small globules of ink. However, the fine detail in a tattoo will fade with time, because the ink droplets drift slightly. The dermis is also home to many more important bits of body hardware.
For example, every square inch of skin has thousands of buried nerve endings that react to different types of sensations, including cold, heat, pain, and pressure.
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Now that you understand the two layers of skin that wrap your body, you can make sense of a whole host of skin insults and injuries. Instead, your skin will just ooze clear liquid. Stretch marks.