After suffering intense headaches, vomiting, and having their skin peel off in sheets, it wasn’t long before Arctic explorers began to have doubts about a dietary mainstay: polar bear. A laboratory analysis ensued, confirming these suspicions and specifically implicating the liver of the polar bear as the source of the explorers’ condition. Liver contains high levels of vitamin A, and because polar bears subsist largely on seals, including seal liver, their own livers are loaded with vitamin A. It turns out that a single ounce of polar bear liver - the size of a golf ball - contains more than five times the vitamin A dose considered toxic for humans. For this reason, the latest government guidelines encourage you to be moderate with your polar bear liver intake.
Jokes aside, there is a growing concern that Americans are consuming an excessive of vitamin A from animal products, supplements, and fortified foods combined. Certainly this excess is not causing your skin to fall away, but it may be increasing your risk for osteoporosis. Take a closer look at your diet to find out if you’re getting too much.
Benefits of Vitamin A
Vitamin A is an essential fat-soluble nutrient that plays a critical role in healthy vision, growth, bone development, reproduction, immune function, and healthy epithelial tissues (read as cancer prevention). There is epidemiological evidence to suggest that a higher dietary intake of vitamin A lowers breast cancer risk among pre-menopausal women with a positive family history of breast cancer. Another large study found that vitamin A intake reduces the risk of cataracts.
Animal Vitamin A (Retinol) versus Plant Vitamin A
There are two main sources of vitamin A: animal sources which contain preformed vitamin A as retinol and plant sources containing carotenoids, like beta-carotene, that the body converts to retinol.
Â· Retinol - primary sources are certain animal products, fortified margarine, fortified low-fat dairy products, other fortified foods, and supplements. 66% of all vitamin A intake in the American diet comes from retinol or preformed vitamin A added to foods (mostly as retinyl palmitate). Vitamin A is also found in supplements (as retinyl palmitate and retinyl acetate). Tolerable upper limit: 10,000 IU daily (see more below).
Â· Carotenoids - primary sources are green and yellow vegetables, especially carrots. 33% of all vitamin A intake in the American diet comes from the carotenoids in foods. Mixed carotenoids (a blend of different carotenoids) can be taken as a supplement whereas multiple vitamins only contain one carotenoid: beta-carotene. Tolerable upper limit: none.
Risks of Not Getting Enough Vitamin A
Worldwide, up to half a million malnourished children go blind each year because of vitamin A deficiency. In the U.S., deficiency is more likely to be found in those eating a severely restricted and unhealthy diet or among alcoholics. Alcohol accelerates the breakdown of retinol through enzymatic activity and interferes with the conversion of carotenoids to retinol (Note: Heavy alcohol drinkers - 2+ drinks daily - are advised to take a multiple vitamin containing preformed vitamin A as retinyl acetate or retinyl palmitate).
Risks of Consuming Too Much Vitamin A
The primary risks of too much vitamin A (either acute or chronic excess) are birth defects, liver abnormalities, central nervous system disorders, and lower bone mineral density that might increase osteoporosis risk. Since the American diet contains many fortified foods, the relationship between vitamin A and decreased bone density has become a greater concern.
Both human and animal studies show that a higher vitamin A intake interferes with bone metabolism. Specifically, excess vitamin A suppresses osteoblast (bone-building) activity, stimulates osteoclast (bone breakdown) formation, and interferes with vitamin D’s role in calcium absorption and regulation.
In one study, women consuming more than 6,660 IU daily of vitamin A as retinol in food or supplements (the daily recommendation for adult women is 2,310 IU) were found to have twice the hip fracture risk compared to those consuming 1,700 IU daily or less. Men with the highest blood levels of retinol are seven times more likely to fracture a hip than men with lower levels (study details). There is evidence that older individuals have higher blood levels of vitamin A because of a reduced ability to metabolize vitamin A as efficiently as one ages.
The chart below shows the most concentrated vitamin A sources, both animal and plant. As you can see, fortification (see blue section) makes it easy to surpass the 6,660 IU level, especially if you regularly consume liver, cod liver oil, margarine, butter, and cheese.
Note: All vitamin A toxicity issues pertain to retinol intake (in foods and supplements containing retinol, retinyl palmitate, or retinyl acetate) and NOT supplemental or food-based carotenoid intake (like beta-carotene).
Animal Sources of Vitamin A
Polar bear liver
Cod liver oil
Braunschweiger (liver sausage)
Beef stew, canned
Malted drink mix, w/ whole milk 1 cup
Centrum Multi, Original & Silver
Slim Fast 1 can
Kellogg’s Product 19
Kellogg’s Nutri-Grain Cereal Bar
Fortified breakfast cereals
Kellogg’s Raisin Bran
Skim milk, added vitamin A
Margarine, added vitamin A
Plant Sources of Vitamin A
Turnip greens, cooked
Red peppers, cooked
Lettuce, Green Leaf
Green peas, cooked
Butternut squash, cooked 1/2 cup
Source: USDA National Nutrient Database for Standard Reference, Release 18
Â· If you’re taking a multivitamin, look for one that contains no more than 3,000 IU of vitamin A in the active form (indicated on the label as retinol, retinyl acetate, or retinyl palmitate). Many manufacturers are reducing levels or replacing with beta-carotene. There are no toxicity issues with beta-carotene or other carotenoids.
Â· Limit liver intake. If you eat liver, make sure it’s from an animal that was fed a vegetarian diet (no added by-products or medications). The liver is a detoxifying organ that can store pollutants.
Â· If you take cod liver oil, limit to a teaspoon daily and pay close attention to the other vitamin A sources in your diet
Â· Avoid/minimize fortified foods (breakfast cereals, cereal bars, shake mixes, meal replacements, energy bars, and margarine). Instead, choose foods that are fortified by nature, i.e. whole grains, beans, nuts, seeds, fruits, vegetables, and quality animal products.
Â· Instead of taking a carotenoid supplement (like mixed carotenoids), make carotenoid-rich plant foods, including dark green leafy vegetables, carrots, sweet potatoes, and winter squash a regular part of your diet. Try Casey’s Butternut Squash Soup and Popeye’s Collards this week.
More About Winter Squash and How to Eat It
Vitamin A & Hip Fracture Risk