This is an excellent topic to fit with the last two on supplements and multivitamins. I do not think about it generally because I am aware of the difference between fat and water-soluble nutrients. I do not consciously think about this because of the few supplements I do take, but I do tend to be more careful with fat-soluble because the body does not automatically flush excess out of the body.
The body sometimes does not automatically flush excess water soluble, but in general, it does. Water-soluble excess is generally flushed and not stored by the body. There are exceptions that you need to be aware. Too much vitamin B6 can cause nerve damage. Excess niacin can cause flushing and excess vitamin C may cause kidney stones. A serious problem may be caused by excess folic acid, which may mask vitamin B12 deficiency and is most common in people over the age of 50.
Fat-soluble is dissolved in fat and stored by body tissues for use later. However, fat-soluble is not equal as Vitamin D is a hormone and apparently allows higher tolerances than other fat-soluble vitamins. Vitamin D is one of the more controversial vitamins as many think the Institute of Medicine (IOM) has set the limit too low for the needs of the body. Many are claiming vitamin D has more health benefits than it may have.
Vitamins A, E, and K are also fat-soluble. Because they are stored, over time they can rise to dangerous levels and lead to a condition called hypervitaminosis. This means excess amounts of a vitamin in the body that are unhealthy and can cause health consequences. Excess vitamin A in women can cause birth defects and too much vitamin E may increase the risk of hemorrhaging. Excess vitamin K can reduce the effect of blood thinner medications and prevent normal blood clotting.
Exceeding the recommended daily allowance (RDA) of many nutrients can cause the vitamins to act like drugs and may lead to health problems. Research has shown at what levels nutrients can cause potential problems and these levels do take into account all sources of vitamins and minerals from food, fortified food and supplements.
Fortified foods is a way for people to help fill the nutrient gaps and has resulted in production practices of adding iodine to salt, enriching grains with vitamins B and iron, and milk fortified with vitamins A and D. In combination with whole foods, supplements, and fortified foods is raising concerns from the experts. These concerns are that the supplements may cause the diets of people to exceed the safe upper limits and potentially lead to a toxic buildup of certain nutrients.