Investing in Nutrient Dense Foods: Vitamin A
Some years back I read with fascination the book Nutrition and Physical Degeneration. Originally published in the 1940s, the book followed dentist Weston A. Price to distant lands as he searched for the dietary secrets that gave some isolated, traditional societies their comparatively healthy teeth and robust health. One takeaway from the book for me was how much more nutrient dense the diets of some traditional peoples were, and how far from this ideal the eating patterns of many in developed countries have become.
I touched on the health consequences of relying on poor quality foods in Treating Food as an Investment, and have long been an advocate of eating nutrient dense foods. The challenge, of course, is convincing people that nutrient dense foods are affordable, as most people think in terms of cost per pound rather than cost per calorie, an issue I noted in my previous post Making Good Food Affordable. I readily acknowledge that there's more to the idea of nutrient density than calorie content, so this post will be the first in a series that looks at other nutritional elements of food, beginning with Vitamin A.
What is commonly known as 'Vitamin A' is not really a single chemical compound, but rather a suite of fat soluble compounds that include retinol, retinal, retinoic acid and retinyl esters . These compounds play important roles in immune function, reproduction, cell growth, cell differentiation and cellular communication, and play a pivotal role in vision by contributing to the structure of the protein rhodopsin, which absorbs light in our eyes. Pre-formed Vitamin A comes exclusively from animal foods, including dairy products, meat and organs, although our bodies can convert some caroteniod pigments from plants, among them β-carotene, into the vitamin at low efficiencies. Since retinol is one of the primary forms of Vitamin A, the nutrient is typically measured in micrograms of retinol activity equivalents (μg RAE). The US Recommended Daily Allowance of this vitamin varies according to age and sex, with women who are pregnant or breast feeding requiring more while children require less. I've presented the cost of meeting a daily allowance of 900 μg RAE from several foods in the graph below, with the Vitamin A content of foods taken from the US Department of Agriculture's National Nutrient Database and food prices noted at my local farmers' markets and grocery cooperative . The prices are for pasture raised livestock and certified organic fruits and vegetables.
Liver is by far the cheapest means of meeting a daily allowance of 900 μg RAE of Vitamin A, costing the consumer only eight cents per day. Eggs offer a modestly priced source of Vitamin A, but beyond this the cost of meeting a daily allowance rises rapidly. Sweet potato, kale and spinach are also fairly cost effective sources, although these are plant foods that provide only precursor carotenoid pigments and as any researcher in the field of carotenoid bioavailability and conversion will attest our understanding of how these pigments are converted into Vitamin A remains in its infancy. I recorded data on Vitamin A content for plant foods as presented by the US Department of Agriculture, but I'm not confident that it accurately represents how much actual Vitamin A the average person can get from the plant foods listed in the graph above.
Our body's capacity to turn carotenoid precursors into Vitamin A depends on a range of factors, and in the best of circumstances this conversion happens at low efficiency. Among the many factors are the species of the carotenoid pigment, the matrix in which the pigment exists in the plant, how much of it we eat in a meal, nutrient status of the consumer and the consumer's genetics [3, 4]. To make one unit of retinol, which we could easily absorb and utilize from animal foods like liver, most people need to eat 12 units of β-carotene or 24 units of α-carotene or β-cryptoxanthin, although the absorption and conversion of these latter caroteniod pigments can vary tremendously.
Vitamin A was one of three compounds that Weston A. Price identified as hugely important for the health of the traditional peoples he studied, and I suspect it's a vitamin that many in modern societies face deficiencies in. Most of the societies he studied had 'sacred foods' that were used to nourish growing children and expectant mothers, and these foods were commonly of animal origin and almost universally were prime sources of Vitamin A. The next of Price's "Big Three" is Vitamin D, and I'll focus on this important vitamin in my next post.
- Vitamin A: Fact Sheet for Health Professionals. United States Department of Health and Human Services, US National Institutes of Health.
- National Nutrient Database for Standard Reference. United States Department of Agriculture, Agricultural Research Service.
- J. Castenmiller & C. West (1998) Bioavailability and bioconversion of carotenoids. Annual Reviews of Nutrition, Vol. 18, Pgs. 19-38.
- K-J. Yeum & R. Russell (2002) Carotenoid bioavailability and bioconversion. Annual Reviews of Nutrition, Vol. 22, Pgs. 483-5
Teaser photo credita: Wikipedia/Wulgulmerang/CC BY-SA 3.0
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