The Interdependency of Nutrients -- the Importance of Maintaining a Balanced Diet
[This is an excerpt from Medscape's CME-course " Nutrition for Health Promotion and Disease Prevention". Most of the rest is however more useful. medscape.com ]
A review of all the functional interdependencies of nutrients is beyond the scope of this CME program. However, the following examples of nutrient-to-nutrient interactions illustrate the complex interrelationships that exist and the importance of promoting healthy, balanced diets based on a variety of food sources.
Vitamin D is integrally involved in the absorption and serum regulation of calcium.[1] It also aids in the absorption of magnesium and phosphate in intestinal brush borders and works cooperatively on a genetic level with vitamin A. Hypervitaminosis of D is related to calcium, magnesium and phosphorus intake, with increased consumption of these minerals resulting in increased symptoms of excess vitamin D.
Consumption of vitamin C and beta-carotene reduces the depletion of vitamin E, but high intake levels of polyunsaturated fatty acids increase the need for vitamin E and the other antioxidants. [1] Vitamin E interacts with zinc to protect skin lipids, helps to protect against iron toxicity, and is necessary for the conversion of beta-carotene to retinol.[2] When vitamin E levels are low, the need for selenium increases and vice versa. However, excess consumption of vitamin E can result in reduced absorption of vitamin A, D, and K resulting in deficiencies of the fat-soluble vitamins. Additionally, excess vitamin A can interfere with vitamin K absorption[2] and vitamin K metabolism is dependent on adequate intakes of niacin and riboflavin.[1]
In addition to its protective role with vitamin E and vitamin A, vitamin C aids in the absorption of iron (especially nonheme iron) by keeping iron in the reduced form.[2] However, excess iron intakes result in greater oxidation of vitamin C, which diminishes its ability to enhance iron absorption. Because of its chelating abilities, vitamin C may increase absorption of heavy metals as well. Other mineral-vitamin interactions include zinc and vitamin A, wherein vitamin A metabolism is disrupted by zinc deficiencies.[2] And, vitamin A deficiency may result in microcytic anemia through the disruption of iron metabolism. Zinc also facilitates the digestion of folate through a zinc-dependent hydrolase.
Vitamin C is thought to be involved in folate metabolism because of its ability to reduce folate to its active form.[2] Folate and B12 are necessary for the conversion of homocysteine to methionine. Vitamin B12 works synergistically with folate to remove the methyl group from N5 methyl tetrahydrofolate, without which the methyl group is "trapped."[2] However, high levels of folate consumption can mask B12 deficiency, which leads to pernicious anemia. Vitamin B6 also is thought to lower homocysteine levels.
The ratios of one mineral to another can be beneficial or antagonistic. The ratios of calcium to phosphorus, iron to copper, iron to zinc, and calcium to magnesium influence absorption and excretion rates.[1] For example, high levels of zinc consumption impair copper absorption, but zinc absorption is impaired by high intake levels of iron. |
