The three nutrition experts begin their discussion of this important subject by stating that "diet plays a vital role in the production of the antioxidant defense system by providing essential nutrient antioxidants such as vitamin E, C, and ß-carotene, other antioxidant plant phenols including flavanoids, and essential minerals that form important antioxidant enzymes."  In addition, they note that "epidemiological data generally indicate a benefit of consuming diets that are higher in antioxidant nutrients, specifically diets high in fruits and vegetables."

But isn't it much easier to obtain these antioxidants by simply popping a pill or two (or even three or four) into one's mouth?  It certainly is, and millions of people do it daily.  But is this approach as effective as obtaining needed antioxidants via the food one eats?  Probably not.

Willcox et al. write that in many studies of antioxidant health benefits "it is not clear whether the benefit is derived from the specific nutrients under study or another food component having health benefits yet to be discovered," or that perhaps "there is a particular combination of antioxidant nutrients that provide protection."  While some epidemiological studies, in their words, "appear to demonstrate clear associations, direct tests of the relationships with clinical trials have not yielded similar results."  In fact, they say "the most convincing evidence of antioxidant effect on cancer prevention involves feeding fruits and vegetables rather than individual antioxidants."

Our reason for reporting these observations is that atmospheric CO2 enrichment has been documented to enhance the concentrations of a number of antioxidants in many of the foods we eat; but we have always wondered about the significance of those findings if one can readily obtain the same antioxidants from an assortment of pills.  Now we are beginning to understand why the natural food pathway may be preferred.  Hence, we briefly recount below some of the things we have learned about the effects of atmospheric CO2 enrichment on the antioxidant concentrations of various foods.

(1) Barbale (1970) and Madsen (1971, 1975) found that a tripling of the atmosphere's CO2 concentration produced modest increases in the vitamin C concentration of tomato fruit, while Kimball and Mitchell (1981) found that atmospheric CO2 enrichment enhanced the vitamin A content of tomatoes,
(2) Tajiri (1985) found that a mere one-hour-per-day doubling of the atmospheric CO2 concentration actually doubled the vitamin C content of bean sprouts,
(3) Idso et al. (2002) found that a 75% increase in the air's CO2 content led to increases of 5 to 15% in the vitamin C concentration of the juice of the fruit of sour orange trees,
(4) Wang et al. (2003) found that approximately the same increase in the air's CO2 content led to a mean increase of 55% in the concentrations of nine different flavonoids found in strawberries, while twice as much extra CO2 led to a mean flavonid increase of 112%,
(5) Caldwell et al. (2005) found that a similar (~75%) increase in the air's CO2 content increased the total isoflavone content of soybean seeds by 8% when the air temperature during seed fill was 18°C, by 104% when the air temperature during seed fill was 23°C, by 101% when the air temperature was 28°C, and by 186% and 38%, respectively, when a drought-stress treatment was added to the latter two temperature treatments, and
(6) Ali et al. (2005) found that atmospheric CO2 concentrations of 10,000 ppm, 25,000 ppm and 50,000 ppm increased total flavonoid concentrations of ginseng roots by 228%, 383% and 232%, respectively, total phenolic concentrations by 58%, 153% and 105%, cysteine contents by 27%, 65% and 100%, and non-protein thiol contents by 12%, 43% and 62%, all of which substances are potent antioxidents.

In light of these several observations, we are confident that the historical increase in the air's CO2 content has played a prominent role in enhancing many aspects of human health over the course of the Industrial Revolution, and that its continued upward trend will continue to provide more of the same benefits, not the least of which is increased longevity, as Willcox et al. report that one of the foremost theories of aging suggests that "age related changes are manifestations of the body's inability to cope with oxidative stress that occurs throughout the lifespan."  Hence, we end with the thought that perhaps the best, if not only, way to obtain these benefits is to directly consume foods that are known to contain high levels of antioxidants, i.e., fruits and vegetables, as with each passing year these foods will likely contain ever greater concentrations of these important substances, thanks to the ongoing rise in the air's CO2 content.

Sherwood, Keith and Craig Idso

References
Ali, M.B., Hahn, E.J. and Paek, K.-Y.  2005.  CO2-induced total phenolics in suspension cultures of Panax ginseng C.A. Mayer roots: role of antioxidants and enzymes.  Plant Physiology and Biochemistry 43: 449-457.

Barbale, D.  1970.  The influence of the carbon dioxide on the yield and quality of cucumber and tomato in the covered areas.  Augsne un Raza (Riga) 16: 66-73.

Caldwell, C.R., Britz, S.J. and Mirecki, R.M.  2005.  Effect of temperature, elevated carbon dioxide, and drought during seed development on the isoflavone content of dwarf soybean [Glycine max (L.) Merrill] grown in controlled environments.  Journal of Agricultural and Food Chemistry 53: 1125-1129.

Idso, S.B., Kimball, B.A., Shaw, P.E., Widmer, W., Vanderslice, J.T., Higgs, D.J., Montanari, A. and Clark, W.D.  2002.  The effect of elevated atmospheric CO2 on the vitamin C concentration of (sour) orange juice.  Agriculture, Ecosystems and Environment 90: 1-7.

Kimball, B.A., Mitchell, S.T.  1981.  Effects of CO2 enrichment, ventilation, and nutrient concentration on the flavor and vitamin C content of tomato fruit.  HortScience 16: 665-666.

Madsen, E.  1971.  The influence of CO2-concentration on the content of ascorbic acid in tomato leaves.  Ugeskr. Agron. 116: 592-594.

Madsen, E.  1975.  Effect of CO2 environment on growth, development, fruit production and fruit quality of tomato from a physiological viewpoint.  In: P. Chouard, N. de Bilderling (Eds.), Phytotronics in Agricultural and Horticultural Research.  Bordas, Paris, pp. 318-330.

Tajiri, T.  1985.  Improvement of bean sprouts production by intermittent treatment with carbon dioxide.  Nippon Shokuhin Kogyo Gakkaishi 32(3): 159-169.

Wang, S.Y., Bunce, J.A. and Maas, J.L.  2003.  Elevated carbon dioxide increases contents of antioxidant compounds in field-grown strawberries.  Journal of Agricultural and Food Chemistry 51: 4315-4320.

Willcox, J.K., Ash, S.L. and Catignani, G.L.  2004.  Antioxidants and prevention of chronic disease.  Critical Reviews in Food Science and Nutrition 44: 275-295.