Here’s a quick pop quiz:
1. Is spinach a good source of iron and calcium?
2. Can you boost the amount of iron you absorb from your food by consuming vitamin C?
3. Is it better to take vitamin/mineral pills with meals?
The answer to all of these questions is “yes, but”—with the emphasis on the “but”…
1. Yes, but spinach also contains substances called oxalates, which bind these minerals so that they are poorly absorbed by the body. I know….interesting…
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 consumed at the same meal.
3. Yes, but it depends on the nutrient and the content of the meal. Fat-soluble vitamins, like 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 of those minerals.
Bioavailability: How nutrients are really absorbed
Just because you consume various nutrients doesn’t mean that 100% of them actually make their way through your GI 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 vary greatly in their bioavailability. For example, studies show that our bodies utilize on average only about 5 percent of the manganese we consume, and 30 to 40 percent of the calcium. (I would bet you didn’t know that!) 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.
There are a lot of different things that 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. Generally, a good rule of thumb is the more processed the food, the less bioavailable the nutrients. Another thing to keep in mind is that certain foods lose a great deal of their nutrient bioavailability in the cooking process (especially leafy greans!) To make things even more confusing (but really, don’t worry!), vitamins and minerals interact in complex, often unpredictable ways. For example, vitamin C and nonheme iron (the type of iron predominately found 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.
Additionally, some substances in foods – sometimes called anti-nutrients – interfere with the body’s use of vitamins and minerals. Meaning, the oxalates found in some dark green leafy vegetables can interfere with the absorption of some minerals (such as calcium, iron, and zinc), as does the phytic acid which is found in some high-fiber foods.
After the nutrients are digested, the actual amount absorbed and retained depends on your body’s own needs, which are mainly determined by age, sex, health, as well as the level of nutrients already in your body. For instance (and this is crazy!): A healthy man absorbs less than 1% 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. Basically, our bodies are extremely intelligent and work to protect and care for us when we don’t even realize it! Age plays an important role, too: People over 60, for example, absorb less vitamin B12, folate (AKA Vitamin B9), and magnesium. Other things that can affect bioavailability include chronic diseases, especially of the digestive tract, and certain drugs, like -some- heartburn medications which can interfere with the absorption of minerals like magnesium, calcium, and iron, along with vitamin B12!
What can you do? Focus on balance, variety, and whole foods
Despite the fact all these various nutrient interactions sound complex and a bit confusing, relax…you don’t need to break out your calculator to keep track of vitamins and minerals. Just follow this simple advice:
- Eat a balanced diet with a variety of different foods. This is the best way to balance all the positives and negatives. Introduce variety at every meal by choosing from fish, plant proteins, and lean meats, whole-grains, low-fat dairy foods, and a wide range of (both cooked AND raw!) vegetables and fruits. Experiment – see what you love and see what makes you feel best!
- Skip fad diets. These not only fail to supply necessary nutrients, but also eliminate beneficial interactions among nutrients, and can amplify any negative ones.
- READ: DO NOT “CLEANSE” or CRASH DIET or ELIMINATE WHOLE FOOD GROUPS or MACRONUTRIENTS
- Do you absolutely best to get your nutrients from foods, not supplements. Foods often contain mutually enhancing nutrients, as well as phytochemicals that may also enhance bioavailability. Remember, nature is smart 😉 Additionally, while you may think you are being helpful to yourself, taking “megadoses” of vitamin or mineral supplements can actually impair the absorption process by introducing imbalances and crowding out important nutrients.