Here's a pop quiz:
1. Is spinach a good source of iron and calcium?
2. Can you boost the amount of iron you absorb from your breakfast cereal by consuming vitamin C?
3. Is it better to take vitamin/mineral pills with meals?
The answer to all is “yes, but”—with the emphasis on “but”:
1. Yes, but spinach also contains substances called oxalates, which bind these minerals so that they are poorly absorbed by the body.
2. Yes, vitamin C does help your body absorb iron from nonmeat sources, but only if these nutrients wind up in your intestines at the same time, which usually means they must be eaten at the same meal.
3. Yes, but it depends on the nutrient and the content of the meal. Fat-soluble vitamins, such as A and D, are better absorbed with a meal containing at least a little fat. But if the meal contains lots of fiber, that will block the absorption of a portion of some minerals.
Bioavailability: how nutrients are absorbed
Just because you consume various nutrients doesn’t mean that 100 percent of them make their way through your gastrointestinal tract to your bloodstream and your cells. The body is able to use only a portion of the nutrients it takes in—a principle called bioavailability. Vitamins, minerals, and various phytochemicals (including carotenoids) vary greatly in their bioavailability. Studies show, for example, that our bodies utilize on average only about 5 percent of the manganese we consume, and 30 to 40 percent of the calcium. The recommended dietary allowances (RDAs) take into account the nutritional interactions in the typical American diet and assume that only certain percentages of various nutrients are absorbed.
Many things influence how much of a given nutrient your body can use: its source, for one thing, as well as the other foods you eat at the meal and how the foods are processed and cooked. Vitamins and minerals interact in complex, often unpredictable ways. For example, vitamin C and nonheme iron (the type that predominates in vegetables and grains) are “team players.” Similarly, vitamin D helps with the absorption of calcium. Other nutrients, such as zinc and iron, can act against each other.
In addition, some substances in foods, sometimes called anti-nutrients, interfere with the body’s use of vitamins and minerals. Thus oxalates, found in some dark green leafy vegetables, interfere with the absorption of some minerals (including calcium, zinc, and iron), as does the phytic acid found in some high-fiber foods.
After nutrients are digested, the amount absorbed and retained depends on your body’s needs, which are largely determined by your age, sex, health, and the level of nutrients already in your body. For instance, a healthy man absorbs less than 1 percent of the iron in a balanced diet, but a woman with anemia will absorb as much as 35 percent of the iron in her diet. A pregnant woman, who needs more vitamins and minerals across the board, will absorb and retain even more. Age plays a role, too: People over 60, for example, absorb less vitamin B12, folate (another important B vitamin), and magnesium. Chronic diseases, especially of the digestive tract, can also affect bioavailability. So can certain drugs: For instance, some heartburn drugs (proton pump inhibitors) interfere with the absorption of minerals such as magnesium, calcium, and iron, along with vitamin B12.
Focus on balance, variety, and whole foods
Despite the complexity of nutrient interactions, you don’t need a calculator—or complicated “food-combination” diets—to keep track of vitamins and minerals. Just follow this basic advice:
- Eat a varied and balanced diet. This is the best way to balance the positives and negatives. Introduce variety at every meal by choosing from fish and lean meats, whole-grain cereal products, low-fat dairy foods, and a wide range of vegetables and fruits.
- Skip fad diets. These not only fail to supply necessary nutrients, but also eliminate beneficial interactions among nutrients, and can amplify any negative ones.
- Get your nutrients from foods, not supplements. Foods often contain mutually enhancing nutrients, as well as phytochemicals that may also enhance bioavailability. If you drink milk, for example, you get vitamin D, which will help you absorb more of the calcium and phosphorus in your diet; the milk’s sugar (lactose) will also help you absorb not only calcium but also magnesium. Taking megadoses of vitamin or mineral supplements can impair the absorption process by introducing imbalances and crowding out important nutrients.