TAKE vitamin C to protect yourself from catching cold. Take vitamin E and beta-carotene for heart health. Take vitamin D to help your body absorb calcium. This advice is conventional wisdom but not all of these and other vitamin truisms are true, or always advisable.

In recent years research has shown that.

The importance of sufficient vitamins was recognized long before vitamins themselves were identified. Ancient Egyptians knew that animal liver can help cure night blindness, which can result from a deficiency in vitamin A. As Europeans began to explore the world in the 17th and 18th centuries, sailors began to recognize the importance of fresh fruits and vegetables to prevent and treat scurvy, a vitamin C deficiency. British sailors were given fresh limes for their health, hence the name limey. Many sailors, especially Asian, also suffered beriberi, a deficiency of vitamin B1, because they ate polished milled white rice that lacked the nutritious hulls.

Today many people are aware of the importance of vitamins and good nutrition. But the benefits of large doses of some vitamins and supplements have been misunderstood and exaggerated. Some people rely too much on vitamin supplements - both multiple vitamins and specific vitamins - instead of the fresh foods containing vitamins.

Most people don't need to take supplements if they eat healthy, varied and balanced diets, according to most doctors and nutritionists. And taking too much can be dangerous.

Research in 2009 by the US Women's Health Initiative on 160,000 middle-aged women showed that those who took daily multi-vitamins were no healthier than those who did not; researchers found no link between vitamin intake and cardiovascular disease, cancer or dying.

Because access to fresh fruits and vegetables was once limited, many people took vitamins instead. Though fresh food is now widely available, many people still take unnecessary vitamins.

"As long as we eat a balanced diet, there's little risk of vitamin deficiency," says local nutritionist Yang Feng. "Besides, multi-vitamins cannot supply everything."

A typical multivitamin may contain 20 necessary vitamins, but eating diverse foods provides even more.

Pregnant women may need vitamins, and doctors may prescribe vitamins for certain conditions, but for most people high extra doses burden the body and can cause harm. Too much vitamin C and E, both powerful antioxidants that some people say may prevent cancer, have been linked in some studies to abnormal cell growth.

Research has shown that some highly touted benefits of extra vitamins do not hold up to medical scrutiny.

Studies from the American Journal of Clinical Nutrition (Gomes-Cabrera 2008) have shown that over-saturation of antioxidants (enzymes and other substances, such as vitamin C, vitamin E, and beta-carotene) is unhealthy. Too much prevents the body from naturally adapting to damaging free-radicals and oxidants and from activating enzymes that prevent damage to cells.

Questionable beliefs:



Vitamin C for cold prevention and healing

The book "Vitamin C and the common Cold" by Nobel laureate Linus Carl Pauling in 1970 is probably the earliest major promoter of vitamin C (ascorbic acid) to prevent the common cold and reduce severity of symptoms. Pauling and others also linked megadoses with effective cancer treatment.

Pauling said lack of vitamin C leads to reduced immunity; increasing intake can improve the number and activity of lymphocyte and neutrophil cells, which are important for good immunity, he said. Therefore, many people believe that taking high doses of vitamin C in the cold season (or any season) can largely prevent colds or reduce their severity and length.

But that may only be true for people lacking vitamin C; for most other people without a vitamin C deficiency, extra vitamins may not be effective.

Recent placebo-controlled research by the Cochrane Collaboration (an independent nonprofit organization involving 28,000 volunteers in 100 countries) focused on 11,000 people receiving 0.2g per day or more of vitamin C. It showed that regular ingestion of vitamin C had no effect on common cold incidence in the general population. But it did have a modest but consistent effect in reducing the duration and severity of symptoms.

Around 10mg of vitamin C daily is generally enough to prevent deficiency. Overdose may cause vomiting, heartburn and other symptoms. In severe cases, red blood cells can break down.



More vitamin D for strong bones

Vitamin D is important in calcium absorption for strong bones. Vitamin D also benefits the nerves and reduces inflammation. But that doesn't mean that extra vitamin D is always good for health.

Most people need less than 0.005mg vitamin D daily. Too much vitamin D can lead to too much calcium that interferes in organ function and causes calcium to accumulate on the heart, blood vessels, cornea and lungs.

Children and pregnant woman, who do need extra calcium, should be careful.

Vitamin D occurs in some sea fish, eggs and mushrooms. Besides, Vitamin D is one of the few vitamins that human body can manufacture and is often known as the "sunshine vitamin" because limited exposure to the sun causes production of vitamin D.



Vitamin E and Beta-Carotene for cardiovascular disease

Vitamin E and Beta-carotene are said to protect the heart and blood vessels, but this is not the case, according to research in 2003 by Dr Marc Penn and associates at the Cleveland Clinic Foundation in Ohio.

According to his research on 1,000 patients in randomized controlled trials, vitamin E cannot help heart disease patients while beta-carotene may even aggravate the problem.

"We should really be focusing on healthy diets," he said. "The concept of vitamin supplements to overcome bad dietary habits is not a valid thesis, at least with vitamin E and beta-carotene."

But the Council for Responsible Nutrition disagrees and insists on the potential benefits of vitamin E in maintaining healthy vision and in preventing and treating Alzheimer's disease, cancer and coronary disease.