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A Daily Multivitamin: Dietary Insurance (by the Harvard School of Public Health)

March 18, 2004

It has been asked by millions of people "If you eat a healthy diet, do you need to take vitamins?" 

Not very long ago, the common answer from most experts and professionals would have been an emphatic, "No".

However, today there's quite a bit of evidence that taking a daily multivitamin makes good sense for most adults.

So why have the expert's opinions changed? 

Not only have scientists determined why we need pyridoxine (vitamin B6), but they are also finding substantial evidence that vitamin B6 and others do a lot more than ward off the so-called diseases of deficiency, things like scurvy and rickets. 

It also now believed that intake of several vitamins above the minimum daily requirement, as set forth by so many experts previously, may prevent a variety of cancers, heart disease, osteoporosis, and other chronic diseases.

The following summary focuses on vitamins with newly discovered or suspected roles in promoting health and fighting disease. 

It will do three things. 

First it will present some of the new evidence about vitamins' possible new roles. 

Second, it will point out how you can go about getting more of these in your diet. 

And finally, it will assess the value of taking a good multivitamin on a daily basis.

Vitamin A

Vitamin A does a lot more than simply help you see better in the dark. 

Vitamin A stimulates the production of white blood cells, the activity of white blood cells, and takes part in creating and repairing bone, helps sustain the health of endothelial cells (those cells lining the body’s interior surfaces), and regulates cell growth and reproduction. 

This role of Vitamin A had researchers exploring for years whether insufficient vitamin A caused cancer. 

Several studies have destroyed this hypothesis,(1) as have randomized clinical trials of supplements that contain beta carotene, a precursor of vitamin A. 

Although it's easy to obtain too little vitamin A in your diet, it's also just as easy to get too much vitamin A. Intake of up to 10,000 IU, twice the current recommended daily level, is however thought to be safe. 

There is some evidence that this high an intake of vitamin A may increase the risk of hip fracture (2) or some birth defects in babies.(3)

Recommended Intake: The most commonly recommended intake of vitamin A is 5,000 IU for men and 4,000 IU for women per day. Many cereals, juices, dairy products, and other are fortified with vitamin A. Many fruits and vegetables, whole foods, and some supplements, also contain beta-carotene as well as other vitamin A precursors, which the body can in turn convert into vitamin A the body can use.

The 3 Bs: Vitamin B6, Vitamin B12, and Folic Acid

One of the breakthroughs that has changed the way experts look at vitamins is the discovery that too little folic acid, one of the eight B vitamins, is connected to birth defects like anencephaly and spina bifida. A half a decade ago, no one knew what was responsible for these birth defects, which occur when the early development of tissues that eventually become the spinal cord, the tissues that surround it, or the brain goes awry. 

Twenty five years ago, British scientists found that that mothers of small children with spina bifida had low vitamin levels.(4) Eventually, two extensive clinical trials in which women were randomly assigned to supplement with folic acid or a placebo indicated that obtaining too little folic acid greatly increased a woman's chances of conceiving a baby with spina bifida or anencephaly and that getting adequate folic acid could prevent these birth defects.(5,6)

Getting enough folic acid, at least 400 micrograms daily, isn't necessarily that easy to get from your diet. That's why experts urge women of childbearing age to supplement with folic acid. This is also why the US Food and Drug Administration requires that folic acid be added to many enriched breads, cornmeal, pastas, flour, rice, and other grain products, along with the iron and other nutrients that have been added to these food products for many years.(7)

Another astounding discovery about folic acid and other B vitamins is that they may in fact help fight certain types of cancer as well as heart disease. Experts still don't know if there's merely an association between increased intake of folic acid and other B vitamins and heart disease or cancer, or if high intakes are actually responsible for the prevention of these diseases.

B Vitamins and Heart Disease

In the late 1960's, a Boston pathologist who was investigating the deaths of two children from severe strokes questioned if the high levels of a protein breakdown product called homocysteine found in these children's systems could have been responsible for why their arteries were as clogged with cholesterol as those of an elderly fast food addict.(8) 

Since then, many studies have linked high levels of homocysteine with increased risks of both heart disease and stroke.(9,10)

Vitamin B12, vitamin B6 as well as folic acid play a vital roles in recycling homocysteine into methionine, one of the 20 or more essential building blocks from which our bodies builds new proteins. 

Without sufficient amounts of folic acid, vitamin B6, and vitamin B12, the recycling of homocysteine into methionine becomes inefficient and homocysteine levels can increase. Quite a few studies indicate that high levels of homocysteine are associated with higher than normal risks of both heart disease and stroke.

Increasing consumption of folic acid, vitamin B6, and vitamin B12 is now believed to dramatically decrease homocysteine levels. And some studies also show lower risks of cardiovascular and heart disease among individuals with higher intakes of folic acid, those who use quality multivitamin supplements, or those with higher amounts of serum folate (the form of folic acid found in the body). 

Notwithstanding, there are other studies that show little or no meaningful association between homocysteine and cardiovascular disease. Upcoming clinical trials, such as the Women's Antioxidant Cardiovascular Study(11) and the Vitamin Intervention in Stroke Prevention Study(12) are believed to provide more concrete answers with respect to relationship between homocysteine, B vitamins, and cardiovascular risk.

Folic Acid and Cancer

In addition to playing a key role in recycling homocysteine, folate (folic acid) also plays an important role in building (and re-building) DNA, the intricate compound that makes up our genetic blueprint. 

Studies indicate that people who have a higher than average intake of folic acid from the foods they eat or from dietary supplements have a lower risk of getting colon cancer(13) and breast cancer.(14) This may be especially true for those who drink a reasonable amount of alcohol, given that alcohol blocks the absorption of folic acid and inactivates folate that is circulating in the body. 

A noteworthy observation taken from the Nurses’ Health Study is that high intake of folic acid may decrease the risk of breast cancer relatively more common among women who have more than one alcoholic beverage per day.(14)

Recommended Intake: The recommended healthy daily intake of B vitamins isn't agreed on by everyone, and is likely to change over the next few years as more reliable data from ongoing trials are evaluated. Since only a fraction of U.S. adults currently receive the recommended daily intake of B vitamins by their diet alone, use of a multivitamin supplement will become more important for many individuals.

Folic Acid: The current suggested intake for folic acid is approximately 400 micrograms per day. There are many good sources of folic acid. These including prepared breakfast cereals, beans, and fortified and whole grains.

Vitamin B6: It is recommended that a health person should include 1.3 to 1.7 milligrams of vitamin B6 in their daily diet. Higher doses have been investigated as a treatment for specific conditions ranging from attention deficit disorder to premenstrual syndrome to and carpal tunnel syndrome. However, to date, there is little supporting evidence.

Vitamin B12: The recommended intake for vitamin B12 is 6 micrograms daily. Not less than 100 years ago, a deficiency of vitamin B12 responsible for the proliferation of a deadly disease called pernicious anemia. Its symptoms include disorientation, hallucinations, memory loss, and excessive tingling in the extremities. Although pernicious anemia is much less common today, it is still often diagnosed in older adults who have getting enough vitamin B12 from their diet. It's also quite likely that some people diagnosed with dementia or Alzheimer's disease are in reality suffering from the curable vitamin B12 deficiency.

Vitamin C

Vitamin C has been under public scrutiny. Even before its was first discovered in 1932, nutrition experts realized that some substance in citrus fruits, such as oranges, could prevent scurvy, a disease that killed millions.(15) In more recent years, Nobel laureate Linus Pauling toted daily mega-doses of vitamin C (the equivalent to eating 12 to 24 oranges) as a means to protect the body from other chronic diseases, as well as the common cold.

Today there is no question that vitamin C plays an important role in fighting and controlling harmful infections. It's also considered a powerful antioxidant that can neutralize free radicals that can harm the body, and vitamin C helps make collagen, a tissue required for healthy teeth, bones, gums, and blood vessels.(16) The question that remains is, do you really need high amounts of vitamin C to keep your body healthy?

The answer is no. Vitamin C's cold-fighting potential certainly isn't what it has be so strongly touted to be. Some clinical trials strongly suggest that the amount of vitamin C in a common multivitamin taken at the beginning of a cold might ease cold symptoms, but there's no compelling evidence that higher than normal dosages make a difference, or that vitamin C can altogether prevent colds.(17) Studies of vitamin C as it relates to cancer, heart disease, and eye diseases such as cataract or macular degeneration also show no clear positive correlation.

Recommended Intake: The dietary intake of vitamin C recommended by most experts in the field is 90 mg a day for men and 75 mg for women (and an extra 35 mg for individuals who smoke). There's no compelling evidence that higher than recommended of vitamin C improve health or fight disease. As the evidence continues to become more apparent, 200 to 300 mg of vitamin C a day appears to be an optimal daily intake. This is amount of vitamin C can easily be obtained through a good diet and a standard multivitamin. Some of the best dietary sources of vitamin C include citrus fruits and juices, berries, both green and red peppers, broccoli, tomatoes, and green spinach. Many breakfast cereals now days are also fortified with vitamin C.

Vitamin D

If you live north of the line connecting San Francisco to Philadelphia, chances are very good that you aren't getting enough vitamin D. The same is also true if you don't, or can't, get outside in the sunlight for at least a 15-minute daily walk. A recent study of individuals admitted to a Boston hospital showed that nearly 60 percent were experiencing vitamin D deficiency at some level of another.(18)

Adequate intake of vitamin D helps ensure that the human body absorbs and retains other vital nutrients including both calcium and phosphorus. Clinical studies also indicate that vitamin D keeps certain types of cancer cells from growing and duplicating.

A few preliminary studies show that insufficient intake of vitamin D may be associated with a higher than normal risk of bone fractures, and that supplementation with vitamin D may help to prevent them.(19) Other preliminary studies indicate a possible association between low vitamin D intake and increased risks of chronic disease including many cancers including prostate, colon, breast and other cancers.

Recommended Intake: The current suggested optimal intake of vitamin D is 5 micrograms daily up to age 50, 10 micrograms daily between the ages of 51 and 70, and 15 micrograms daily after age 70. It is important to know that very few foods naturally contain vitamin D in sufficient quantities. Good dietary sources of vitamin D include dairy products and fortified breakfast, as well as fatty fish such as salmon and tuna. For many individuals, the best way to obtain the recommended daily intake is by taking a good multivitamin.

Vitamin E

For quite a long time, vitamin E supplements have appeared to be an easy and effective means to prevent heart disease. Preliminary studies, including the Nurses' Health Study(20) and Health Professionals Follow-up Study,(21) indicated that individuals who supplemented with vitamin E (400 IU or more daily) for least two years experienced 20% to 40% reductions in risk of coronary heart disease.(22)

However, the finding from a number of randomized clinical trials have dampened enthusiasm for vitamin E's proclaimed ability to prevent heart attacks or heart disease related deaths among individuals with heart disease or those at higher risk for it. In the Gruppo Italiano per lo Studio della Sopravvivenza nell’Infarto miocardio (commonly known as the GISSI Prevention Trial), 3 years of treatment with vitamin E showed little if any effect on the rate of strokes, heart attacks, or deaths from any cause among over 10,000 survivors of heart attacks, although it did appear to reduce the incident of sudden deaths and deaths caused by cardiovascular disease.(23) Results from the Heart Outcomes Prevention Evaluation (HOPE) trial also indicated no substantially benefit of four years worth of vitamin E supplementation among many thousands of individuals already diagnosed with heart disease or at high risk for heart disease.(24)

However, it is very possible that in secondary prevention trials, the use of common drugs such as beta blockers, aspirin, and ACE inhibitors masked the effects of vitamin E, and that vitamin C may have meaningful benefits among healthier individuals. Ongoing trials of vitamin E will teach us more about its possible benefits in the years to come.

Recommended Intake: The most commonly recommended intake of vitamin E from food is 15 milligrams from dietary sources daily. That's equal to 22 IU from natural-source vitamin E or 33 IUs of synthetically created vitamin E. Scientists are still in the midst of discovering all that vitamin E is and has to offer. Evidence from studies indicate that at least 400 IU of vitamin E daily, and possibly more, may be needed for optimal health. However, since many standard multivitamins only contain around 30 IU, a separate vitamin E supplement is needed to reach the optimal intake level.

Vitamin K

Vitamin K assist in making six of the 13 proteins required for blood clotting. Its role in maintaining the clotting cascade is so vital that individuals who take anticoagulants such as warfarin (also known as Coumadin) must be extremely careful to maintain their vitamin K intake at a stable level. Recently, both scientist and industry researchers have demonstrated to reliable degree that vitamin K is also important to bone building in the human body. Lower than average concentrations of circulating vitamin K are connected with low bone density, and supplementation with vitamin K shows noticeable improvements in biochemical measures of bone health and density.(26) A relatively recent report from the Nurses' Health Study indicates that women who take at least 110 mgs of vitamin K daily may be up to 30% less likely to break or fracture a hip as women who get less than 110 mgs of vitamin K per day.(27)

Recommended Intake: Experts suggest that the optimal intake of vitamin K is 80 micrograms for men and about 65 micrograms for women. Because vitamin K is so readily available in many dietary sources, especially green leafy vegetables and cooking oils, most adults get enough vitamin K in their diets. However, on the flip side, a 1996 survey, indicates that substantial number of American citizens, especially children and younger adults, are not getting the vitamin K they need.(28)


Cells in the human body must constantly fight against harmful substances called free radicals. Free radicals can damage the body's DNA, the inside lining of artery walls, eye proteins and just about any substance or tissue in the body. Many free radicals are created by the body as an inevitable byproduct of processes that occur on a regular basis in the body. Free radicals also come from the foods we eat and even the air we breathe.

However, the human body isn't completely defenseless against free radicals. Antioxidants we extract from the foods we eat can help protect us against harmful free radicals. Hundreds of antioxidants are found in fruits, vegetables, and other plant-based foods that we can east. The most common antioxidants are vitamin C, vitamin E, beta-carotene and other similar carotenes. Minerals such as selenium and manganese which are needed by enzymes that destroy free radicals can also be found in many whole food products.

The Logical Conclusion

A regular run-of-the-mill multivitamin supplement isn't going to make up for a bad diet. While it will provide a dozen or so of the vitamins known to maintain health, it only is going to offer a mere shadow of what's made available to the body from eating plenty of good foods, including fruits, vegetables, and whole grains. Instead, a daily multivitamin provides a sort of nutritional safety net, not a solution. While a majority of the populations gets enough vitamins to avoid common deficiency related diseases, very few people get enough of five essential vitamins that may be key in preventing many chronic diseases. These include:

  • Folic acid
  • Vitamin B6
  • Vitamin B12
  • Vitamin D
  • Vitamin E

A RDA-level multivitamin can supply you with enough of these vitamins. And it's about the least expensive health insurance you can buy.


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