Vitamin A is commonly known as the anti-infection vitamin. It plays an important role in vision and bone growth, and maintains the surface linings of the eye and respiratory, urinary and intestinal tracts (Pediatr Nurs, 1996; 22: 377-89, 456; Proc Nutr Soc, 1999; 58: 289-93).
Retinol, one of the most usable forms of vitamin A, is found in animal foods, such as liver and eggs. Some plants contain orange pigment - provitamin A carotenoid - that the liver converts to retinol. Beta-carotene is one such carotenoid found in many foods (Phamacol Ther, 1997; 75: 185-97; Eur J Clin Nutr, 1996; 50 [Suppl 3]: S38-53). Beta-carotene has two roles in the body. It is converted into vitamin A if the body needs it. If not, beta-carotene acts as an antioxidant to protect cells from damage by harmful molecules called free radicals.
It's important to regularly eat foods that provide vitamin A or beta-carotene, even though your body can store vitamin A in the liver to help meet your needs when carotenoids or vitamin A is low (J Nutr, 1994; 124: 1461S-6S; Eur J Clin Nutr, 1996; 50 [Suppl 3]: S7-12).
Such foods include a considerable array of fruits and vegetables that are widely available, but which must be eaten fresh, such as carrot, mango, sweet potato, spinach, cantaloupe, sweet pepper, apricot, spinach, broccoli, oatmeal, tomato, peach, papaya, orange and asparagus (Br J Nutr, 1999; 82: 203-12; Am J Clin Nutr, 1999; 70: 1069-76; Cancer Epidemiol Biomarkers Prev, 1997; 6: 617-23; Gac Sanit, 1999; 13: 22-9; J Agric Food Chem, 1999; 47: 1576-81).
Beta-carotene is not easily assimilated by the body. However, one study has shown that grating carrots and pureeing papaya enhances the body's takeup of the beta-carotene contained in these foods (J Nutr, 2001; 131: 1497-502).
Another study suggests that a diet rich in vitamin C and beta-carotene can increase your lifespan. Those who eat two oranges and two carrots a day can reduce the risk of death in middle age by over 30 per cent.
The eating habits of 1556 male employees of a US company were tracked for 24 years. Those who ate appreciably more vitamin A, beta-carotene in particular, reduced their risk of developing cancer or heart disease (Am J Epidemiol, 1995; 142: 1269-78).
Indeed, provitamin A carotenoids, such as beta-carotene, have not been proven to have any specific adverse health effects. The conversion of carotenoids to vitamin A is decreased when body stores are full, which puts a natural limit on how much you can store. Although a too-high intake of carotenoids can turn the skin yellow, this is not considered dangerous to your health (Council for Responsible Nutrition, 1997: 26-7).
A number of studies suggests that diets rich in beta-carotene and vitamin A lead to a lower risk of many types of cancer (Int J Epidemiol, 1990; 19: S32-42; J Am Diet Assoc, 1996; 96: 693-702). Indeed, there is evidence that a high intake of green and yellow vegetables, or food sources of beta-carotene with or without vitamin A reduced the risk of lung cancer (Int J Cancer, 1997; [Suppl 10]: 22-9).
A trial testing the effectiveness of four nutrient combinations in preventing throat and stomach cancers in 30,000 men and women in China found that, after five years, there was a 13 per cent reduction in cancer deaths among those who'd taken a combination of beta-carotene, selenium and vitamin E (J Natl Cancer Inst, 1993; 85: 1483- 92).
However, vitamin A can be toxic. Symptoms can arise suddenly, such as after consuming very large amounts of vitamin A within a short time, and include dizziness, blurred vision and muscular incoordination (J Clin Pharmacol, 1997; 37: 551-8; Adv Exp Med Biol, 1994; 352: 187-200; Am J Clin Nutr, 1989; 49: 358-71). Vitamin A toxicity can also cause severe birth defects (J Am Diet Assoc, 2000; 100: 1068-70; Am J Clin Nutr, 2000; 71: 1325S-33S).
The recommended daily retinol equivalent (RE) intake is 800 mcg for women (2664 IU) and 1000 mcg for men (3330 IU) (Int J Vit Nutr Res, 1997; 67: 71-90). A recognised safe upper limit for vitamin A from diet and supplements is 1600-2000 mcgRE (5328-6660 IU) a day (Am J Clin Nutr, 1989; 49: 358-71; Int J Vit Nutr Res Suppl, 1989; 30: 42-55). Women of childbearing age should limit their total daily intake to no more than 1600 mcgRE (5328 IU) per day, although some nutritionists, like Dr Melvyn Werbach, suggest that as much as 25,000 IU (7508 mcgRE) can be taken safely.
Nevertheless, vitamin A can prove invaluable during times of infection, when our needs begin to skyrocket, by helping the body regulate its immune system (Int J Vit Nutr Res, 1997; 67: 71-90; Nutr Rev, 1998; 56: S38-48). It may also help prevent bacteria and viruses from entering your body by bolstering the skin's defences (Proc Nutr Soc, 1998; 57: 159-65; Nutr Health, 1996; 10: 285-312; FASEB J, 1996; 10: 979-85).
As for how much vitamin A to take during an infection, according to John Stirling, technical director at BioCare, Birmingham, dosages vary wildly from country to country and between practitioners. In Germany, viral infections are treated with 10,000 IU, providing the patient isn't pregnant while, in the US and Australia, 25,000-50,000 IU are given. 'Generally, English practitioners tend to be more conservative, using 5000 IU as their baseline dosage, but going up to 10,000 IU,' he says.
When working in a clinic in Germany, Stirling often gave 500,000 IU for seven days (for the duration of the infection or until an adverse symptom appeared), but only with the patient under strict medical supervision and with close laboratory monitoring. 'Certainly we wouldn't have done this with anyone who was pregnant, or had a history of kidney stones, where vitamin A might increase the uptake of calcium,' he says.
Of our six road-test products, from The Nutri Centre as usual, three delivered vitamin A as retinol palmitate and three used natural beta-carotene. We used Worcestershire County's Scientific Services laboratory for the analyses.
Says County analyst Carol Stevens, 'The international standard units were discontinued [in 1954 for vitamin A and 1956 for provitamin A] when actual measurements of retinol itself could be made.' Says Stevens, 'UK law requires all vitamin A and beta-carotene declarations to be given as retinol equivalents or 'mcgRE'. This is to ensure that the consumer can make simple comparisons between products.' The Food Labelling Regulations (1996) require that vitamin A declarations use a conversion factor of six mcg of beta-carotene equals one mcgRE.
There's no doubt that the labelling on the products in our road test could have been clearer. Indeed, not a single one stated vitamin A amounts using the term 'mcgRE'. Unlike multivitamins, some single-vitamin products don't declare all contents, as required by regulations, and many still use international units (IU).
Although one of the preparations cost lb3.50, most of the six products we looked at cost between lb7.55 and a staggering lb18.95; daily-serving costs ranged from 0.02 p to
20 p per lb. Why there should be such a wide disparity among these products is not clear.
Points were awarded according to what we considered to be the best all-round products, based on the amount of vitamin A in mcgRE and other nutrients contained in the supplement, and overall value for money.
There are two final caveats. Most of these supplements are potent and need to be taken with caution. Furthermore, there are many inconsistencies associated with vitamin A sampling techniques which can influence laboratory results.
Vitamin A 7500iu
Manufacturer: Lamberts Healthcare
Price: lb3.50 for 100 capsules
At lb3.50, this was the cheapest of our six products. According to the label, each capsule contains 2252 mcg of vitamin A from retinol palmitate, halibut liver oil - a rich source of vitamin D - and beta-carotene. This is the only preparation containing both retinol palmitate and beta-carotene. Our lab found 4640 mcgRE (15,451 IU), more than twice what was declared on the label.
This product carries a warning that pregnant women shouldn't take vitamin A, one of only two of the six to do so.
Each capsule also contains sunflower-seed oil, a good plant source of vitamin A (Am J Clin Nutr, 2001; 74: 501-9).
This product costs a mere 0.04 p per day and offers 441,457 IU of vitamin A per lb.
Dry Vitamin A 2252 ug
Price: lb8.85 for 100 tablets
Each tablet has a vitamin A content of 5160 mcgRE (17,183 IU) from retinol palmitate, the largest amount among our six products and more than twice the stated amount. The label says this is suitable for vegans and is 'sugar, salt and starch free'.
Vegetable cellulose and vegetable magnesium stearate are used as binding agents. Each tablet also contains 240 mg of di-basic calcium phosphate, a source of calcium in nutritional supplements, but it is also an excipient - a substance that helps to carry the tablet's contents into the body correctly. The label also warns that pregnant women shouldn't be taking vitamin A.
These tablets cost only 0.09 p a day and provide 194,158 IU of vitamin A per lb.
Beta Carotene 15 mg
Price: lb8.75 for 60 softgels
Solgar maintains that its offerings are free of corn, yeast, wheat, soy and dairy products, and don't use preservatives, artificial flavours or colours. At lb8.75, this was the third most expensive product in our test.
This contains 15 mg of 'natural form oceanic beta carotene' - from Dunaliella salina, a single-celled saltwater algae that is also an antioxidant and a rich source of beta-carotene, as well as other carotenoids such as alpha-carotene, cryptoxanthin, zeaxanthin, lutein and lycopene (J Nutr Sci Vitaminol, 1994; 40: 421-30). As per the food-labelling regulations, Solgar has also included a listing of these other carotenoids on the label, in descending order by weight.
The beta-carotene content of each softgel is 23,560 mcg (3927 mcgRE or 13,077 IU), more than the stated content of 2500 mcgRE. It also contains 49 mg of safflower oil and 500 mcg of alpha-carotene.
A daily serving costs 15 p. With this product, Solgar offers 89,671 IU of vitamin A per lb.
Price: lb7.55 for 15 mL
The only liquid-based supplement of our selection, this contains 395 drops (one drop per serving). According to the label, this little plastic bottle contains 2500 IU of vitamin A per drop from retinol palmitate (751 mcgRE). In fact, this delivers 2664 IU (800 mcgRE), the least in our road test.
At lb7.55, this was the fourth most costly product in our test. A daily serving works out to be the cheapest at 0.02 p, yet the whole bottle works out to 139,375 IU per lb, third in terms of value for money.
The low dose of vitamin A per drop may be because this product is aimed at children, the only one of our sample to do so.
Natural Beta Carotene 25,000 IU Vit A Activity
Manufacturer: Nature's Plus
Price: lb18.95 for 120 softgels
At lb18.95, this product is the most expensive of the six. The label claims a beta-carotene content of 25,000 IU per serving which, according to our calculations, works out to 7508 mcgRE in each softgel. Judging from the County lab results, however, this refers to provitamin A. The true vitamin A content, according to the lab report, is only 2770 mcgRE (or 9224 IU), considerably less than suggested by the label and placing it fourth in terms of vitamin A content.
The ingredients include a trace amount (1 IU) of vitamin E in the form of d-alpha-tocopherol (an antioxidant which stops the vitamin A from going off) as well as soy oil and the saltwater algae D. salina.
Unlike Solgar, Nature's Plus hasn't provided a complete listing of these other ingredients, in descending order by weight, as required by the Food Labelling Regulations (1996).
A daily serving costs 16 p, and you only get 58,411 IU of vitamin A for your lb.
One-A-Day Beta Carotene
Price: lb5.90 for 30 vegetable capsules
Although the label says each capsule contains 15 mg of beta-carotene, the lab found only 10 mg, equivalent to 1667 mcgRE (5551 IU), putting this product in last place.
There is no listing of where the beta-carotene comes from, but we are told that each capsule contains lecithin, vitamin E (as d-alpha-tocopherol) and cellulose.
However, in all fairness, BioCare tells us that it uses a particular type of processing that is known to skew lab analyses.
At lb5.90, this is among the cheaper purchases, but a daily serving works out to 20 p. Also, this product gives you only 28,225 IU of vitamin A per lb, the least in our survey.
vitamin A supplements, beta-carotene, retinol palmitate, carotenoids, Vitamin A 7500 IU, Dry Vitamin A 2252 UG, Beta Carotene 15 mg, Micellized VitaSorb-A, Natural Beta Carotene 25,000 Vit A Activity, One-A-Day Beta Carotene
Road testThe best vitamin A supplement The beta-carotene controversyOver the past decade, a number of studies have cast doubt on beta-carotene efficacy.
Researchers from the Dartmouth Medical School, New Hampshire, could find no strong connection between beta-carotene and a lower risk of heart disease after studying the reports of 1118 men (JAMA, March 6, 1996). Similarly, the National Public Health Institute in Helsinki could find no correlation between vitamin E and beta-carotene and a reduced risk of angina (JAMA, March 6, 1996).
More controversially, two studies have suggested that large doses of beta-carotene might actually cause cancer. The 1994 Alpha-Tocopherol Beta-Carotene Cancer Prevention Study (ATBC) involved Finnish men who were heavy smokers and alcohol drinkers. The volunteers were either given 20 mg of synthetic beta-carotene, vitamin E, a combination of the two, or a placebo. The rather unexpected outcome suggested that there was an 18 per cent increase in lung cancer rates in the beta-carotene-only group.
In another study a few years later, based on 29,000 men, the incidence of lung cancer was greater in a group of smokers who took a daily supplement of beta-carotene (J Natl Cancer Inst, 1996; 88: 1560-70). The Carotene and Retinol Efficacy Trial (CARET), a lung cancer chemoprevention trial that provided randomised subjects with supplements of synthetic beta-carotene and retinol (preformed vitamin A), was stopped after researchers discovered that the subjects receiving the beta-carotene had a 46 per cent higher risk of dying from lung cancer (Cancer Epidemiol Biomarkers Prev, 1998; 88: 1560-70).
At around the same time, however, the 12-year US Physicians' Health Study (PHS) - which included over 2000 smokers and, on average, used twice as much beta-carotene for twice as long as the ATBC and CARET studies - compared the effects of taking 50 mg of synthetic beta-carotene every other day with a placebo in over 22,000 men and found no adverse side-effects, not even in the 11 per cent of volunteers who were smokers (N Engl J Med, 1996; 334: 1145-9).
In a letter sent out to customers in August 2000, Solgar's UK technical director, Stephen Terrass, says he's not surprised by the PHS findings: 'Other than the contradictory findings of the other studies, the most important fact that has not been given its due scrutiny is that the beta-carotene used in these studies was 'synthetic', not 'natural'. Dietary intake of various other carotenoids - such as lycopene and cryptoxanthin - is far more strongly associated with a lower cancer risk than dietary beta-carotene intake.
'With this in mind, high doses of synthetic beta-carotene seem to inhibit the uptake of other dietary carotenoids. If this blocking of absorption was occurring over the seven-year duration of these studies, this could help to explain a statistical increase in lung cancer rates - or as the PHS trial and others might suggest, this could help explain why the beta-carotene did not reduce lung cancer rates. Many experts are convinced that the ATBC results were severely impacted by the volunteers' alcohol intake.'
According to Terrass, alcohol causes severe oxidative damage to synthetic beta-carotene, so removing its protective properties. 'Research has shown that this damage could be prevented by protecting beta-carotene with other antioxidants,' he says, underscoring the point by observing that there was no increase in cancer rates in the ATBC volunteers who used a combination of synthetic beta-carotene and vitamin E.
'Had these studies employed natural beta-carotene supplements, rather than synthetic ones, it is very likely that the results of all three studies would have been very different - not necessarily because of the beta-carotene itself, but rather because of the synergistic actions of the mixture of protective carotenoids,' says Terrass. In conclusion, he says, it seems only prudent to recommend that smokers avoid high doses of synthetic beta-carotene, especially smokers who drink alcohol.
Lamb's liver 18,000 mcg* Green leafy vegetables, especially
Calf's liver 14,000 mcg kale (1000 mcg) and spinach (810 mcg)
Cod liver oil 18,000 mcg Broccoli 400 mcg
Butter 820 mcg Endive 333 mcg
Cheese, especially Cheddar and Carrot 2000 mcg
Parmesan 350 mcg Sweet potato (orange) 700 mcg
Single cream 220 mcg Squash 160-200 mcg
Eggs 140 mcg Sea vegetables 3900 mcg
Pumpkin 240 mcg Mango 300 mcg
Cantaloupe 300 mcg Apricot 250 mcg (dried 600 mcg)
*approximate mcg of retinol equivalent (RE) per 100 g of food
See also the US Department of Agriculture Nutrient Data Laboratory's homepage at www.nal.usda.gov/fnic/foodcomp, which lists the carotenoid content of different foods and is regularly updated.