Most vitamins are water soluble and excess is excreted from the body. Some vitamins are fat soluble and can accumulate in the body. How much of a vitamin one needs depends on age, sex and stage of life. If one is already eating properly, it is not clear whether consuming more vitamins and minerals is good for you. Vitamin consumers do not live longer or have lower cancer rates than those who do not take vitamins. Serious overconsumption of certain vitamins have been linked with health disorders.
The absence of excretory pathways for vitamins A and D and for carotene makes them candidates for excessive intake. Nausea, polyuria and diarrhoea are early symptoms of toxicity of vitamin D. Later symptoms include weakness, the deposition of calcium in soft tissues and depression. Chronic vitamin A intoxication occurs more frequently in children than adults. Depending on the severity of the intoxication, children may develop loss of appetite, weight loss, irritability, fissuring at the corners of the mouth, and cracking and bleeding of the lips. Later signs include liver enlargement, loss of hair and severe bone and joint pains.
It is claimed that very high intakes of vitamin C (a gram or more a day) can cause diarrhoea, kidney stones in sensitive people, and precipitate the deficiency disease scurvy if the high dose is stopped.
Large doses of vitamin E prevent vitamin K from clotting blood correctly. Large doses of calcium limit the absorption of iron.
More than 50% of Americans over the age of 55 believe taking a multivitamin once a day will reduce their risk of cancer. clinicians. Whilst there is strong evidence to suggest that the vitamins and nutrients found in fruit, vegetables and whole grains are vital in helping the body defend itself from malignant cells (more than 190 scientific studies have been able to demonstrate the benefits of a diet rich in plant foods), only a handful have found favour in dietary supplements.
Consumers who buy vitamins and minerals are volunteering for a vast uncontrolled experiment with substances that could be curative, noxious or ineffective. Labels neither point out potentially harmful effects nor the lack of tests before marketing the item.
The American government does not have standards for vitamins and minerals, and so it is hard to obtain interpretable information about these substances even from experts. The government is drawing up these standards.