Vegan for Life (11 page)

Zinc is a tricky issue for vegans—and for nutritionists. This mineral is needed for at least 100 different enzymatic reactions in the body. It’s required for protein synthesis, cell growth, blood formation, and immune function. While overt zinc deficiency is rarely seen in western countries, it’s possible that some people—especially children in lowincome families—suffer from marginal deficiency. Poor growth in children is one sign of a marginal deficiency. But because zinc is used for so many functions, there might be other suboptimal health conditions related to low zinc intake that we don’t really understand yet. It’s also somewhat difficult to measure zinc status accurately.

As a result, we have some unanswered questions about zinc. And one of those questions is: How much zinc do vegans need?

The adult RDAs for zinc are 11 milligrams for men and 8 milligrams for women. Some studies show that vegans consume around 10 to 13 milligrams per day, which is slightly less zinc than omnivores—but for the most part, intakes are pretty comparable.

However, absorption from plant foods is quite a bit lower than from animal foods, and the Food and Nutrition Board suggests that zinc needs could be as much as 50 percent greater for vegans. That means that vegan zinc needs would be 16½ milligrams for men and 12 milligrams for women. While there is no evidence that vegans and other vegetarians suffer from zinc deficiency and there is evidence that they could adapt to lower absorption, it’s important to optimize zinc absorption. As with iron, phytate is one of the important factors affecting zinc bioavailability, so many of the food preparation techniques that boost iron absorption also work for zinc. See “Maximizing Iron and Zinc in Vegan Diets” on page 70 for guidelines on getting enough zinc. If you think that your diet may fall short on well-absorbed zinc, consider taking a multivitamin that contains zinc. Or if you are already taking calcium tablets, choose one with zinc.

Most people don’t give a second’s thought to iodine, a mineral that is needed for healthy thyroid function. But throughout the world, iodine deficiency is a serious public health problem. Deficiency in pregnancy is especially serious since it can impact brain development in the fetus.

Eating either too much or too little iodine can cause the thyroid gland to become enlarged, which is called a goiter. When iodine intake is too low, it causes hypothyroidism, resulting in slowed metabolism, elevated cholesterol, and weight gain. Too much iodine can cause either hypothyroidism or hyperthyroidism.

In the United States, most people get enough iodine by using iodized salt or eating fish or dairy products. Milk and other dairy foods aren’t necessarily good sources of iodine, but iodized solutions are used to clean the cows’ teats and dairy equipment, and the iodine ends up in the milk itself. In some parts of the world, sea vegetables (seaweeds) provide iodine.

The iodine content of most plant foods is variable, though, because it depends on the iodine content of the soil. Foods grown closer to the ocean tend to be higher in iodine. (In fact, even ocean mist can provide iodine, although it’s not a reliable or measurable source.) In some parts of Europe, where salt is not iodized at high enough levels (or at all) and the iodine content of plants is poor, vegans who don’t use supplements could have abnormal thyroid function.
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Naturally occurring compounds known as goitrogens, which are found in soybeans, flax seeds, and raw cruciferous vegetables (broccoli, brussels sprouts, cauliflower, and cabbage) counteract the activity of iodine. A diet high in goitrogens can cause hypothyroidism if the diet is too low in iodine. But as long as your diet is adequate in iodine, there is no reason to avoid soyfoods or other sources of goitrogens. See Chapter 15 for more discussion on the safety of soyfoods.

The recommended iodine intake for adults is 150 micrograms per day. Vegans can get adequate iodine if they do any one of the following:

The active form of vitamin A is retinol and it is found only in animal products. But plants have more than fifty compounds called carotenoids that the body can convert to vitamin A. The most common is beta-carotene. Because there are so many forms of vitamin A, the vitamin A content of foods is stated as retinol activity equivalents (RAE). Think of these as the amount of potential vitamin A activity in a food. The RDA for vitamin A is 900 RAE for men and 700 RAE for women.

In addition to their role as vitamin A precursors, the carotenoids have antioxidant properties and other potential benefits for reducing chronic disease. The preformed vitamin A in animal foods doesn’t have those advantages.

In 2000, based on new evidence about the conversion of beta-carotene into active vitamin A, the FNB doubled their estimate of how much beta-carotene it takes to produce adequate vitamin A. That means that the RAE content of plant foods is only half of what was previously thought. Where we once thought that vegan diets automatically provided enough vitamin A, it’s now clear that getting enough requires at least some diligence.

As you can see from the table on page 74, carrot juice is very high in vitamin A. If you like carrot juice, consider drinking ¼ cup per day to give your vitamin A intake a boost. A varied diet that includes plenty of brightly colored vegetables should make up the rest of your requirements. Both cooking and added fat increase the absorption of beta-carotene, so there is a benefit to eating some of these vegetables cooked rather than raw. And it’s a good idea not to let the fat content in your meals drop too low.

Although vitamin K was discovered in the early part of the twentieth century, its exact function in the human body wasn’t understood until 1974, which is pretty recent in the world of nutrients.

Vitamin K is essential for blood clotting, and most people get enough to support that function. But research suggests that vitamin K also contributes to bone health. Older people with higher vitamin K intakes and higher blood levels of vitamin K appear to be at lower risk for hip fracture.
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Because of some errors in measuring vitamin K content of foods, there is evidence now that people have lower vitamin K intakes than previously thought. This isn’t an issue for blood clotting but it may be for bone health. The best sources of vitamin K are leafy green vegetables.

Soy, canola, and olive oils are also good sources. Since vitamin K is fat soluble, cooking greens in a small amount of oil can help your body absorb more.