For more than 25 years, researchers have been aware of the declining nutrient density in our crops. Many believe that the industrialization of agriculture is to blame, due to chemical fertilization techniques and declining soil quality. While these factors certainly contribute to the decline of our foods’ nutrient value, Irakli Loladze, a mathematician with a keen interest in biology, discovered a more ominous source of nutrient depletion: rising carbon dioxide (CO2) levels in the atmosphere.
Photosynthesize Me: C3, C4, and CAM
Some background on how plants use carbon dioxide will be helpful to understand why rising CO2 levels matter so much. All plants “fix” carbon dioxide from the atmosphere for fuel, using one of three photosynthesis methods, dubbed “C3,” “C4,” and “CAM.”
C3 photosynthesis transfers the carbon atom from carbon dioxide to a three-carbon molecule, which the plant then uses to produce sugars and starches. Of the three methods of photosynthesis, C3 is considered the least efficient, because in hot, dry conditions, the enzyme that “grabs” carbon dioxide from the air sometimes binds oxygen molecules instead. The plant then has to use extra energy — and release one of its hard-earned carbon atoms — to correct the mistake. About 85 percent of plant species are C3 plants. Rice and potatoes, as well as barley and wheat (the most studied of the cereal crops), are C3 plants, as are all woody trees, including nut and fruit trees.
Plants that use C4 photosynthesis have adapted to hotter, drier climates and have a more complicated process for carbon fixation that reduces the risk of fixing oxygen molecules. They use an enzyme with no affinity for oxygen to fix atmospheric carbon initially, and move the resulting four-carbon molecule into another cell, where the same enzyme used in C3 plants transforms it into sugars and starches. In hot, dry conditions, C4 photosynthesis is much more efficient than C3, but only 3 percent of plant species use this method of photosynthesis. Corn is the most commonly cultivated C4 crop; amaranth, millet, and sorghum are other commonly cultivated C4 crops.
CAM photosynthesis is mainly used by plants adapted to extremely hot, dry environments, such as pineapples, cacti, and other succulents. They alternate between fixing CO2 at night, and converting it to sugars and starches during the day. Very few CAM plants are used as crops, so we won’t go into further detail about this process.
Designing CO2 Studies
Researchers interested in the impact of rising CO2 levels can pump CO2 into a controlled greenhouse lab and study the results, but lab environments don’t accurately represent the complexity of open spaces. Weather patterns, interspecies competition, and other factors may all be of interest in studying the effects of CO2 levels on plants. Free air carbon dioxide enrichment (FACE) is a method developed to address this issue. Researchers can raise the concentration of CO2 in a specified open area and measure plants’ responses. It’s the current gold standard for measuring CO2’s effects on plants, and FACE studies can also include plants that can’t be grown in small spaces, such as trees. However, FACE experiments are extremely costly and have their own limitations. Most FACE experiments are conducted in temperate climates, and because of the high cost of CO2, don’t keep CO2 levels elevated at night. We don’t yet have much information on how agriculture in tropical and arid climates may be affected by elevated CO2 levels, nor on the possible effects of constantly high CO2 levels.
A Glimpse of the Future
In one recent study, scientists used FACE technology to create an environment that modeled Earth’s projected CO2 levels in 50 years. Earth’s CO2 levels are currently increasing at 3 parts per million (ppm) per year, so the FACE environment in this experiment was 150 ppm higher in CO2 than current atmospheric CO2 levels.
The study, which focused on protein levels in plant tissue, concluded that C3 plants respond to higher CO2 levels by producing more starch- and sugar-rich carbohydrates, which dilute the concentration of proteins and minerals in the plants’ tissues. Protein levels in the study’s fruit declined by 23 percent, vegetables by 17 percent, and rice and wheat by about 8 percent of their current protein content. Researchers predict grain crops might lose up to 15 percent of their current protein content under certain elevated CO2 levels.
Irakli Loladze’s study on mineral and trace-element composition of plants revealed that of the 25 minerals measured, only one, manganese, remained unaffected by elevated CO2 levels. Zinc, iron, calcium, magnesium, and potassium are among the major nutrients reported by several researchers as being impacted by rising CO2 levels. Loladze reports that the concentration of all 25 minerals studied decreases by 8 percent on average, but some minerals decrease by nearly 16 percent.
Food insecurity already affects approximately 815 million people worldwide, and micronutrient deficiencies affect approximately 2 billion people. The World Health Organization reports that protein deficiency, which stunts growth, limits immunity, and increases the risk of disease and death, already affects more than a third of the world’s children. Zinc and iron deficiencies are also serious and common in developing nations. Losing nutrients from an already scant supply of food represents a serious risk to malnourished and food-insecure populations.
Well-fed populations are also at risk of an increased incidence of chronic diseases, including diabetes, heart disease, and obesity, that are linked to an excess of starches and sugars. In 2007, the Centers for Disease Control and Prevention reported that nearly half of all American adults were diabetic or prediabetic, meaning they may develop diabetes within five years if they maintained their lifestyles. Nutritional shifts will likely lead to yet more chronic disease in nations, such as ours, where many diseases are rooted in low-fiber diets that are high in starches and sugars, and lifestyles with little physical activity and high stress.
Plant carbohydrates come in two forms: structural carbohydrates, such as cellulose, pectin, and insoluble fiber, and nonstructural carbohydrates, such as starches and sugars. Nonstructural carbohydrates are much easier for our bodies to digest than structural carbs, and they contribute to the caloric and nutritional impact of the diet. Elevated CO2 drives an increase in carbohydrate production in plants, and affects starch and sugar concentration more strongly, increasing the total concentration by 10 to 45 percent. Wheat responds to a 200 ppm increase in ambient CO2 with a 7 to 8 percent increase in starch concentration, according to one FACE study. That’s about 4 grams (or 1 teaspoon) more sugar per 3.53 ounces of wheat.
For the sake of comparison, current atmospheric CO2 levels are between 400 and 410 ppm, an increase of about 120 ppm over the past century. We can reasonably assume that modern CO2 levels have led to about 1/2 a teaspoon more sugar per 3.53 ounces of rice or wheat compared with those grown in the 19th century, when CO2 levels averaged about 280 ppm. Combined with a cultural narrative touting a low-fat, grain-based diet as the standard of health, it’s not surprising that the incidence of diabetes is so much higher now than it was even 40 years ago.
Planning for the Future
To meet the nutritional challenges of a more starch-heavy diet, researchers will begin breeding cultivars of C3 crops that retain more of their nutritional content under higher carbon dioxide levels. The development of such crops is only the first step of a viable solution, however; such cultivars must also be economical and palatable.
Another partial solution is growing C4 crops, such as corn, millet, amaranth, and sorghum, which exhibit less of a shift in protein and carbohydrate levels under increased CO2, in areas where such crops can be cultivated. Current research shows statistically insignificant protein depletion in C4 crops at elevated CO2 levels, but Loladze believes this may be the result of small sample sizes and insufficient study.
Because zinc, iron, and other minerals that are most abundant in animal proteins are becoming more dilute in plant foods, individuals who consume little or no meat may need to use multi-mineral supplements to offset their increased risk of nutrient deficiency. Unfortunately, access to supplements is highly dependent on income, and income inequality remains a serious global issue.
Beans and other legumes are less affected by elevated CO2 levels than other C3 crops, because they can respond with greater nitrogen fixation to maintain their carbon-to-nitrogen ratios. The simplest partial solution to the loss of vegetable-sourced protein and minerals may be cultivating more leguminous crops.
The best, but toughest, solution, of course, is to minimize the increase in CO2 levels altogether. Each individual can reduce their carbon footprint in small but meaningful ways. We can choose to walk, bike, or take public transportation whenever possible. We can grow our own food, and purchase locally produced goods that haven’t been wrapped in plastic and shipped long-distance. In addition to the dozens — if not hundreds — of small choices we can make every day to slow the rate of atmospheric CO2 increase, we can push for government and industry policy changes to help provide future generations with an environment conducive to growing nutrient-dense, traditional food crops.
Aimee Gallo is a nutrition coach and personal trainer whose passion for food as medicine began as a child. She delves into current research regularly, and you can find her online at Vibrance Nutrition.