May 3, 2012
Beta-Carotene: Why Most of Your Vitamins Should Come From Food, Not Supplements
One mantra we often hear from the natural health industry is "take your supplements." While a high quality supplement may provide you with some vitamins and minerals that you may be deficient in, it will never replace a well balanced diet and the delivery system that nature intended. A perfect example is the dark side of beta carotene intake.
Consumption of high amounts of beta-carotene could pose a health risk to people by blocking the action of vitamin A rather than enhancing it, according to new research.
"The kind of dosages of beta-carotene we see in health supplements could be harmful, especially since most people believe that more is better," said nutritionist Albert Meades. "However, even natural foods containing high-amounts of beta-carotene can also cause harm if consumed in excess, for example juicing carrots multiple times per day...everyday," he added.
It's another reason more people are moderating their health supplements and choosing to emphasize a balanced diet filled with more organic fruits and vegetables (rather than conventional), especially due to a more dense concentration
and availability of antioxidants. For example organic apples have been proven to beat
conventionals on antioxidants.
High intake of beta-carotene, found naturally in carrots, could actually block certain functions of vitamin A function.
Writing in the Journal of Biological Chemistry researchers from the Ohio State University, USA, revealed that the natural pigment beta-carotene -- perhaps best known as a precursor of vitamin A -- could also have a 'dark side'.
Vitamin A, and by association beta-carotene, is important for normal human growth, essential for good vision, it helps develop and maintain the tissues that line the external and internal surfaces of the body (skin and mucous membranes), and it is a necessary component for the immune function.
Besides it’s function as pro-vitamin A, beta-carotene acts as an antioxidant and may help prevent chronic diseases including heart disease, cancer, age-related eye diseases, brain disorders and many more. beta-carotene is also known to protect skin exposed to sunlight as it absorbs hazardous ultraviolet radiation.
But now a team of scientists led by Professor Earl Harrison have found that certain molecules derived from beta-carotene have an opposite effect in the body -- by blocking certain actions of vitamin A, which is critical to human vision, bone and skin health, metabolism and immune function.
"We determined that these compounds are in foods, they're present under normal circumstances, and they're pretty routinely found in blood in humans, and therefore they may represent a dark side of beta-carotene," said Harrison.
"These materials definitely have anti-vitamin-A properties, and they could basically disrupt or at least affect the whole body metabolism and action of vitamin A."
Because the anti-vitamin-A compounds are derived from beta-carotene at the same time as vitamin A, Harrison predicts that higher intakes of the antioxidant will inevitably lead to a larger amount of the potentially harmful molecules as well.
However he said that more work is needed, adding: "We have to study them further to know for sure."
Harrison explained that because vitamin A provides its health benefits by activating hundreds of genes: "This means that if compounds contained in a typical source of the vitamin are actually lowering its activity instead of promoting its benefits, too much beta-carotene could paradoxically result in too little vitamin A."
He said the findings also might explain why previous clinical trials have found that people who were heavily supplemented with beta-carotene had a higher incidence of lung cancer than participants who took no beta-carotene at all.
"Those trials are still sending shockwaves 20 years later to the scientific community," said Harrison. "What we found provides a plausible explanation of why larger amounts of beta-carotene might have led to unexpected effects in these trials."
However, the authors were keen to stress that they are not recommending against eating foods high in beta-carotene.
The US based research team manufactured a series of beta-carotene-derived molecules in the lab that match those that exist in nature. They then exposed the molecules to conditions mimicking their metabolism and action in the body.
Of the 11 synthetic molecules produced, five appeared to function as inhibitors of vitamin A action based on how they interacted with receptors that would normally launch the function of vitamin A molecules.
"The original idea was that maybe these compounds work the way vitamin A works, by activating what are called retinoic acid receptors," said Robert Curley, who co-authored the study.
"What we found was they don't activate those receptors. Instead, they inhibit activation of the receptor by retinoic acid," he explained.
Once that role was defined, the researchers sought to determine how prevalent these molecular components might be in the human body. By analyzing blood samples obtained from six healthy human volunteers, the scientists in the lab found that some of these anti-vitamin-A molecules were present in every sample studied -- suggesting that they are a common product of beta-carotene metabolism.
The researchers are continuing to study these compounds, including whether food processing or specific biological processes affect their prevalence.
Food Sources Are The Best Option
Beta-carotene is found primarily in plant foods, specifically fruits and vegetables. Yellow and orange vegetables contain significant quantities of carotenoids. Green vegetables also contain carotenoids, though the pigment is masked by the green pigment of chlorophyll.
Estimates of fruit and vegetable intakes in Europe show as many as 60-80% of adults in France, Switzerland and the U.K. consume few daily servings, defined as less than 1.5 times per day for fruits and less than 2.5 times per day of vegetables. The U.S. is equally short on consumption in all age classes.
Some of the richest dietary sources of beta-carotene are orange/yellow fruits and orange/yellow vegetables, as listed in the table below. They include:
Dark green lettuces,
The rate of beta-carotene uptake after consumption (bioavailability) depends strongly upon the method of food preparation. Since beta-carotene is a fat-soluble component, its bioavailability can be improved by enriching and heating beta-carotene containing foodstuffs with fat.
Its bioavailability in fruits is generally better than that in vegetables.
Beware of Genetically Modified
Sources of Engineered Vitamins
Bio-engineers are now collaborating with the biotech industry to create genetically modified crops that contain excess beta-carotene. The claim is to create a sustainable way to provide this carotenoid to developing countries.
However, "a concern is that if you engineer these crops to have unusually high levels of beta-carotene, they might also have high levels of these compounds," Harrison said.
Journal of Biological Chemistry
Natasha Longo has a master's degree in nutrition and is a certified fitness and nutritional counselor. She has consulted on public health policy and procurement in Canada, Australia, Spain, Ireland, England and Germany.