How much vitamin D is enough?

Vitamin D is a bit mystifying -- the only vitamin that's also a hormone, the only vitamin you don't have to consume in food or supplements. You manufacture your own when your skin is exposed to the sun. See how you do on this quick quiz:

True or false:
1. For bone and muscle strength, vitamin D is as vital as calcium.
2. Cod liver oil is your best source of D.
3. Two cups of milk supply all the vitamin D you need daily.
4. Yogurt is also a good source.
5. The latitude you live in can make you deficient in D.

Answers:
1. True. Without vitamin D, you cannot absorb calcium.
2. False. It's very high in D, but poses health problems.
3. True, but only if you are under 50. Milk in the U.S. is fortified with 100 IU of vitamin D per cup and is the primary dietary source.
4. False. Yogurt and cottage cheese are not made from fortified milk. They have no D.
5. True. In Canada and the northern U.S., there isn't enough sun in the winter months to stimulate human skin to manufacture D.

The hormone your bones need

First, a few basics. The current recommended intake of vitamin D is 200 IU (international units) for those under 50; 400 IU for people aged 50 to 70; and 600 IU for those 70 and older.

Unlike the other vitamins, D is a hormone, which controls such important biological processes as calcium absorption, thus enabling the body to build strong bones and teeth. In so doing, D also helps maintain muscle strength -- and a lack of it may be partly responsible for leg-muscle weakness and a tendency to fall in older people.

A deficiency of D produces weak bones, known as rickets in children and osteomalacia in adults. Vitamin D deficiency may also cause aches and pains, which are sometimes misdiagnosed as fibromyalgia.

The latest research finds that D is linked to good health in many other ways. It regulates cell growth and the immune system. People who get little or no exposure to the sun tend to have higher rates of breast, colon, and prostate cancer, as well as multiple sclerosis, which is more common in northern regions. Vitamin D may play an important role in all this. Some research has linked chronic D shortages to diabetes and rheumatoid arthritis.

Increasingly this vitamin looks like a key to preventing some chronic diseases. Yet studies show that here and around the world adults over 50 tend to fall short of vitamin D -- not only in the north but in Mediterranean countries like Spain. Black Americans are particularly at risk.

Where D comes from

As noted, it's the only vitamin you make yourself. (Some vitamin K and some B12 is produced in our bodies, but by intestinal bacteria). You manufacture some or all of your own D just by being out in the sun, though it's hard to measure how much.

Vitamin D is not really plentiful in the diet. Fatty fish, such as salmon, tuna and sardines, are rich sources. Milk is the major dietary source. Milk (including some soy milks but not yogurt or most cheeses) and sometimes orange juice are fortified with D. Some margarines and breakfast cereals are also fortified. D is a fat-soluble vitamin, meaning that it is easily stored in the body. Extra amounts don't go to waste. That's good -- but also the reason that overdoses are dangerous.

If you get 10 to 15 minutes of sun exposure on your face and arms at least twice a week, you might not need to consume any D in foods at all. And if you drink a quart of milk a day, that's 400 IU right there. A glass of fortified orange juice (100 IU), and a small serving of tuna (200 IU) would boost you up to 700 IU, even if you got no sun.

But it isn't as easy as it sounds

Few people get enough D from food. Most of us don't drink a quart of milk a day, or anything near it. You may eat salmon and tuna for the heart benefits, but two or three servings of fish a week (12 ounces in all) is the maximum you should eat, and it shouldn't all be salmon and tuna.

If you have dark skin and/or live in the north, and/or are over 60, you may need more D. In Canada and the northern U.S., there isn't enough sunshine in winter to stimulate D production in the skin. In Boston, Detroit and Chicago, for example, there's enough sunshine only from April through October. (In the southern U.S. there's enough sunshine year round.) But many people stay indoors or use sunscreen when outside. Experts now argue, sometimes angrily, over whether total sun-avoidance is such a good idea after all.

Older people tend to eat less and have poorer diets; they may not drink any milk. In addition, they may not be out in the sun at all, and they produce less D from the sun exposure they do get.

Scientific evidence is piling up that you may need more than the current recommended levels. New government guidelines recommend 1,000 IU a day for older people and those with dark skin and/or the housebound. (Sitting near a window is no help -- glass blocks the kind of light that stimulates D production.) Some researchers think levels of up to 2,000 IU a day should even be considered.

Not enough, too much

Vitamin D can be harmful in large doses, probably starting at 2,000 IU a day or higher, which can lead to kidney stones, kidney failure, and the deterioration of muscle and bone. But a daily intake of 1,000 IU is known to be safe.

In our last article on D two years ago, we thought that the current recommended intakes were fine. We now believe that 1,000 IU is a better goal for those 70 and older, and for adults with dark skin, especially blacks. It's nearly impossible to get this much without taking a supplement. The housebound or institutionalized as well as those living in the northern third of the U.S. and in Canada should also consider supplementation. So should anyone who consumes no fortified milk or fish.

The table below shows good food sources of D. Most multivitamin/mineral supplements have 400 IU. That's an economical way to get extra D. If you take calcium, consider a calcium supplement with D. Many brands contain 200 IU along with 300 milligrams of calcium per tablet.

Sunscreen note: Sunscreen can reduce or shut down the synthesis of vitamin D if you coat all exposed skin. But this doesn't mean you should do without sunscreen. Just get 10 to 15 minutes of sun on your face and arms -- without sunscreen -- a few times a week (dark-skinned people need longer exposure), and then apply sunscreen.

Cod liver oil note: Cod liver oil is very rich in D, one tablespoon supplying about 1,400 IU. This on top of what your body manufactures and what you get from food might well constitute an overdose. Furthermore, cod liver oil also contains very high levels of vitamin A, which can weaken bones. And it may contain contaminants. You should avoid it.

FOOD	VITAMIN D (IU)
Salmon (3.5 ounces)	360
Tuna, canned (3 ounces)	200
Sardines, canned (1.75 ounces)	250
Milk, cow's (1 cup)	100
Milk, soy (1 cup)	100
Margarine (1 tablespoon)	60
Fortified cereal (about 1 cup)	40
Egg (1 whole)	20