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Protein Power:
Much Ado About a Gram or Two

Volume 5, Number 42: 16 October 2002

In a recent analysis of 159 peer-reviewed scientific journal articles published between 1983 and 2000, dealing with the effects of atmospheric CO2 enrichment on the reproductive growth characteristics of several domesticated and wild plants, Jablonski et al. (2002) calculated some interesting mean responses.  For increases in the air's CO2 concentration ranging from approximately 150 to 450 ppm (rough average of 300 ppm), they found that, across all species studied, the extra CO2 supplied to the plants resulted in more flowers (+19%), more fruits (+18%), more seeds (+16%), greater individual seed mass (+4%), greater total seed mass (+25%, equivalent to yield), and greater total mass (+31%).

Normally, these findings would have been heralded as great news; but there was another - seemingly negative - finding that assumed center stage in subsequent discussions of the paper: mean seed nitrogen concentration across all of the species studied decreased by 14%, which Jablonski et al. equated with a similar decrease in seed protein content.

The negative spin began with a report that was posted on the Ohio State Research News website on 2 October 2002.  Entitled "Increased CO2 Levels Are Mixed Blessing for Agriculture," it quoted one of the study's authors - Peter Curtis, a professor at Ohio State University - as saying that, "while crops may be more productive," when growing in air enriched with CO2, "the resulting produce will be of lower nutritional quality."

As early as the next day, Environment News Service was trumpeting "More Carbon Dioxide Could Reduce Crop Value," while on 7 October Reuters joined the act with "Global Warming Boosts Crops, Cuts Nutrients."  And as surely as night follows day, global-change websites quickly began to chat up the alleged bad news.

Well, as you can probably guess, we have a thing or two to say about the matter; and we begin with Curtis's statement that "while crops may be more productive, the resulting produce will be of lower nutritional quality" in a future high-CO2 world.  This claim is simply false; but it could be corrected by merely interchanging its two verbs, so that the statement reads "while crops will be more productive, the resulting produce may be of lower nutritional quality."  Why do we say that?  We say it because all of the food crops studied by Jablonski et al. (rice, soybean, barley, wheat and maize) were more productive when exposed to elevated concentrations of atmospheric CO2, while only two of them (barley and wheat) exhibited decreases in seed nitrogen content under such conditions.

The most-benefited of the studied crops, exhibiting a whopping 42% increase in seed number as a consequence of atmospheric CO2 enrichment, was rice, which according to Wittwer (1995) is "the basic food for more than half the world's population."  It is particularly noteworthy that this incredibly important crop did not exhibit any decrease in seed nitrogen content in CO2-enriched air, which is a most wonderful result.  Indeed, in the case of this basic food crop, upon which over half the human population of the planet depends for its very existence, it is as if the carbon dioxide we emit to the atmosphere as a consequence of our industrial activities comes back to us as unadulterated manna from heaven.

Likewise, there were no decreases in the seed nitrogen contents of any of the legumes studied by Jablonski et al.  This result, too, is a bounteous blessing; for Wittwer reports that legumes provide fully 20% of the protein that is ingested by humans throughout the world.  In addition, he notes that "the soybean alone provides about two thirds of the world's protein concentrate for livestock feeding, and is a valuable ingredient in formulated feeds for poultry and fish."  Hence, it can be appreciated that the ongoing rise in the air's CO2 content will not adversely affect protein contents in the legume-based sectors of the human and animal food industries either.

But what about the CO2-induced seed nitrogen content decreases reported by Jablonski et al. for barley and wheat?  We do not deny that such decreases have been observed.  However, it has been convincingly demonstrated by Rogers et al. (1996), Pleijel et al. (1999) and Kimball et al. (2001) that higher levels of nitrogen fertilizer application have the capacity to totally offset this negative impact of atmospheric CO2 enrichment, as we (Sherwood and Keith) discuss in some detail in our recent review of the subject (Idso and Idso, 2001), so that with proper crop husbandry there need be no reductions in either grain nitrogen or protein contents, or in the baking properties of flour derived from the grains of those crops.  Hence, as noted in our paper, "it would appear that given sufficient water and nitrogen, atmospheric CO2 enrichment can significantly increase grain yield without sacrificing grain protein concentration in the process."

Nevertheless, for the sake of argument, let us suppose there was a modest CO2-induced decrease in the protein concentration of wheat and similar grains.  Would it negatively impact the amount of protein we ingest?  To investigate this question, we decided to see how much protein one of us (Sherwood, who eats considerably less than Keith and Craig) consumes in a typical day.

For breakfast, Sherwood doesn't eat much, usually just a cup of granola cereal, which according to the label on the box contains 10 grams of protein.  He puts it in a big bowl, however, where he covers it with a full 10 ounces of milk, for an additional 10 g protein.

Halfway through the morning, as you can probably imagine, Sherwood's hunger gets the best of him.  His solution is to have a bowl of blueberries or raspberries, neither of which contains much protein (a single gram per cup); but he puts the berries over a piece and a half of short cake containing 2 g of protein and covers them with another 10 oz of milk, for another 10 g protein.

Lunch is simple too: a tuna fish sandwich (two slices of bread, 3 g protein each, with a single serving of tuna, 13 g protein, spread between them), a cup of baked beans (14 g protein), and 15 oz of milk (15 g protein), followed by a 1.2-oz fruit & nut bar (3 g protein) for dessert.

By mid-afternoon, although he's not really hungry, Sherwood has another fruit & nut bar (3 g protein) with 6 oz of milk (6 g protein), just to help keep him awake, as he works feverishly on the coming week's editorial.

Then comes supper.  Again, it's nothing fancy: a modest-sized hamburger patty (3 ounces of beef, computed to contain 22 g protein according to the latest release of the USDA Nutrient Database for Standard Reference) sandwiched between two pieces of bread (3 g protein each), served with another cup of baked beans (14 g protein) and two-thirds of a cup of potato salad (4 g protein), all washed down with another 15 oz of milk (15 g protein).

Later at night, just before bed, Sherwood sneaks out to the kitchen one last time for yet another fruit & nut bar (3 g protein, he really loves them!) and a final 6 oz of milk (6 g protein).

So let's see.  That comes to a grand total of 163 grams of protein for a typical day in the life of a rather modest eater, perhaps 15 grams of which may have been derived from wheat or similar grain products.  Assuming, for the sake of argument, that a doubling of the air's CO2 concentration induces a 20% reduction in the protein contents of those grain products, Sherwood's typical daily protein intake would decline from 163 g to 160 g in a doubled-CO2 atmosphere.

Is this difference a lot or a little?  Compared to Sherwood's daily intake of "normal-atmosphere protein," the decline is clearly but "a little," amounting to only 1.8% of his total daily protein intake under current conditions.  But what is the impact of this decline on his daily protein requirement?

According to the most recent pertinent publication of The National Academies Press - Dietary Reference Intakes for Energy, Carbohydrates, Fiber, Fat, Protein and Amino Acids (Macronutrients), which was prepared by the Food and Nutrition Board of the Institute of Medicine (2002) - the Recommended Dietary Allowance for both men and women is 0.80 g of protein per kg of body weight, which for Sherwood at 165 lb (75 kg) amounts to 60 g protein per day.  Hence, instead of having 2.72 times as much protein as he requires each day, Sherwood would have only 2.67 times as much protein as he requires in a doubled-CO2 world of the future, which is really no problem at all ... and could conceivably even be beneficial to him, as it is possible to eat too much protein and suffer a number of deleterious consequences.

Clearly, these several observations provide little support for Curtis' contentions - as expressed in the Ohio State Research News interview - that, in a high-CO2-world of the future: (1) "the quality of the food produced by the plant decreases, so you've got to eat more of it to get the same benefits," (2) "under the rising CO2 scenario, livestock - and humans - would have to increase their intake of plants to compensate for the loss," and (3) "a growing global population demands more food, but humans would have to eat more of the food to get the same nutritional benefits."

Quite to the contrary, on the basis of what we learn from the scientific literature, the great bulk of our food in a future high-CO2 world would not - or need not - be deficient in protein content (the "quality" or "nutrient" factor considered by Jablonski et al. and discussed by Curtis).  But even if some of it were (for we know that most of it would not be deficient), we clearly would not be required to eat greater quantities of the protein-deficient food, for we already have a significant overabundance of protein in our daily diets ... except, perhaps, for people living under the most dire of circumstances, as, for example, in parts of the world where civil unrest prevents normal agricultural activities and commerce.  But where this is the case, we are confident that people would not complain about having to eat the little extra that would be required to get their daily protein intake up to par.  In fact, they would likely jump at the chance to do so, simply because they would be hungry; and that chance - the opportunity to eat more - would be much greater in a CO2-enriched world of the future than in the one in which we currently live, as the extra CO2 would boost crop yields far above those of the present day.

Sherwood, Keith and Craig Idso

Idso, S.B. and Idso, K.E.  2001.  Effects of atmospheric CO2 enrichment on plant constituents related to animal and human health.  Environmental and Experimental Botany 45: 179-199.

Jablonski, L.M., Wang, X. and Curtis, P.S.  2002.  Plant reproduction under elevated CO2 conditions: a meta-analysis of reports on 79 crop and wild species.  New Phytologist 156: 9-26.

Kimball, B.A., Morris, C.F., Pinter Jr., P.J., Wall, G.W., Hunsaker, D.J., Adamsen, F.J., LaMorte, R.L., Leavitt, S.W., Thompson, T.L., Matthias, A.D. and Brooks, T.J.  2001.  Wheat grain quality as affected by elevated CO2, drought, and soil nitrogen.  New Phytologist, in press.

Pleijel, H., Mortensen, L., Fuhrer, J., Ojanpera, K. and Danielsson, H.  1999.  Grain protein accumulation in relation to grain yield of spring wheat (Triticum aestivum L.) grown in open-top chambers with different concentrations of ozone, carbon dioxide and water availability.  Agriculture, Ecosystems and Environment 72: 265-270.

Rogers, G.S., Milham, P.J., Gillings, M. and Conroy, J.P.  1996.  Sink strength may be the key to growth and nitrogen responses in N-deficient wheat at elevated CO2Australian Journal of Plant Physiology 23: 253-264.

Wittwer, S.H.  1995.  Food, Climate, and Carbon Dioxide: The Global Environment and World Food Production. CRC Press, Boca Raton, FL.