The quick answer to your question is "it depends" – on the nutrients themselves and what is already in your stomach when you consume them. Absorption – which is only one aspect of "bioavailability" – also depends on the combination of nutrients you take in at a given time.
I discussed your question with Tieraona Low Dog, M.D., director of the Fellowship at the Arizona Center for Integrative Medicine, who notes that the amount and rate of nutrient absorption can vary based upon many different factors. For instance, when foods containing iron are eaten in a meal that also provides vitamin C, the vitamin will enhance absorption of the mineral. Eating iron-rich foods with tea or milk has the opposite effect; the calcium and oxalates in those beverages inhibit iron absorption. Dr. Low Dog explains that you also need adequate stomach acid to utilize non-animal forms of iron; if stomach acid levels are low, which can happen as we get older or take medications that inhibit acid production, you are at higher risk for anemia.
Too much zinc can inhibit both iron and copper absorption, Dr. Low Dog says, adding that zinc from plant sources is not absorbed as easily as zinc from animal-derived foods. B-vitamins are better absorbed in the presence of dietary fat and vitamin C. If your diet is deficient in a nutrient, your body will work harder to increase its absorption. Bioavailability of micronutrients from dietary supplements can vary greatly depending upon how the tablet or capsule is made, and how it was stored.
Cooking also plays a role. For some vegetables, cooking actually boosts antioxidant levels. Cooked carrots and tomatoes, for example, have higher available antioxidants than their raw counterparts. But with other vegetables, cooking, especially boiling, tends to lower nutrient content.
Dr. Low Dog and I agree that the best way to insure that you're getting all the nutrients you need is to eat a varied diet that includes fruit, vegetables, whole grains, legumes, healthy fats, and protein. Unfortunately, the widespread reliance on processed and manufactured foods in our society means that many people are not meeting their nutritional needs. Today's mainstream diet is glaringly deficient in the pigments that color vegetables and fruits; the antioxidants in olive oil, tea and chocolate; the novel compounds in ginger, turmeric and other spices and herbs; and the special fats in oily fish, all of which protect our tissues and organs from the inappropriate, unhealthy inflammation that underlies many serious diseases. For guidance on how to choose foods to promote good health, see my anti-inflammatory diet pyramid.
Andrew Weil, M.D.