Vitamins are small molecules that the body uses to help speed chemical reactions that are needed to produce energy, new cells and tissues and to repair damaged parts of the body. Although it's critical to get a certain amount of vitamins from your diet, if you ingest abnormally high quantities of vitamins they can actually be toxic. The maximum dose of vitamins is called the tolerable upper limit, or TUL and it represents the highest dose which is unlikely to cause any dangerous side effects.
Vitamin A is important for bone growth, vision, reproduction, the health of the immune system and cell differentiation, the Office of Dietary Supplements at the National Institutes of Health explains. For children under the age of three, the tolerable upper limit for vitamin A is 600 mcg. The tolerable upper limit for children between the ages of four and eight is 900 mcg, and for children between the ages of nine and 13 the TUL is 1,700 mcg. The TUL for ages 14 through 18 is 2,800 mcg, and for adults the tolerable upper limit is 3,000 mcg.
The B vitamins are a group of vitamins which are important for turning carbohydrates, proteins and fats into energy, as well as for making new proteins and DNA. There is no reported tolerable upper limit for many of the B vitamins, including thiamine, riboflavin and vitamin B12. However, the TUL for niacin, a member of the B vitamins which can be used to improve cholesterol levels, is 35 mg per day, the Merck Manual explains. For vitamin B6 the TUL is 100 mg for adults, 80 mg for children between ages 14 through 18, 60 mg for children aged nine through 13, 40 mg for children ages four through eight and 30 mg per day for children age three and younger, VitaminManual notes.
Vitamin C is an important vitamin which is needed for the strength of connective tissue and for the health of the immune system. In adults, the tolerable upper limit of vitamin C is 2,000 mg per day. Children ages 14 through 18 should only take 1,800 mg of vitamin C per day, whereas the TUL is 1,200 mg per day for children aged nine through 13. The tolerable upper limit for children ages four through eight is 650 mg, and children three and younger should consume a maximum of 400 mg of vitamin C per day, according to the Linus Pauling Institute at Oregon State University.
The TUL for vitamin D, which is needed to absorb calcium in the diet, is 2,000 IU, or international unit. The TUL of vitamin E is 1,000 mg, the Merck Manual notes.