Warming World, Waning Nutrients

You'd have to be living on another planet (and that's precisely the issue, we don't have a planet B) to not be aware of the escalating climate crisis. But there’s one aspect that’s received less attention — the insidious depletion of essential nutrients in our staple crops.

Our good friend and renowned authority on dietary assessment, Professor Rosalind Gibson, has long warned about this overlooked aspect of climate change — citing reductions in important nutrients such as zinc in staple crops around the world. A recent article in the American Journal of Clinical Nutrition again piqued our interest in this topic. The paper brings to light the alarming shifts in the essential fatty acid composition of soybean oil — a cornerstone in global nutrition.

Previous studies have delved into the paradox of rising atmospheric carbon dioxide levels, which, while boosting crop yields of staples like rice, potatoes, and cassava through heightened carbohydrate synthesis, simultaneously dilute other vital nutrients. Termed 'hidden hunger,' this phenomenon underscores an inherent decline in nutritional quality. Now, the authors of this latest study have broadened this concept to encompass essential fatty acids.

Let's begin by briefly exploring the significance of soybeans as a staple in human nutrition. Its global production is eclipsed only by palm oil and accounts for nearly one-third of all plant-based oils consumed worldwide — a staggering 15 times more than olive oil.

Unlike palm oil, which is rich in saturated fat, soybean oil boasts a wealth of unsaturated fats including oleic acid (OA) and the 2 essential fatty acids — linoleic acid (LA) and alpha-linolenic acid (ALA). As a reminder, essential fatty acids are the ones our body can’t make so we need to obtain them from our diets. They play vital roles in maintaining overall health, supporting brain function, maintaining healthy blood vessel function, and regulating various physiological processes.

The authors conducted a comprehensive review of original research correlating soybean fatty acid composition with rising temperatures. To be included, studies needed to include detailed analytical methods for fatty acid quantification and data on daytime temperature during seed filling — a critical phase when seeds are enriched with nutrients and moisture. Only conventional soybeans were included, not varieties such as high-oleic soybeans.

Using publicly accessible data from NASS Quick Stats Database and meteorological records across US counties known for soybean production, the authors developed a model examining fatty composition against daytime high temperatures. In contrast to prior research, the authors opted for a sigmoidal function instead of a linear model. This 'S'-shaped curve is used to model relationships where there is an initial gradual response, followed by a rapid increase, and ultimately reaching a saturation or stabilisation phase. This choice enhances the model's capacity to accurately represent complex biological processes.

So, what did the authors unearth? Spanning over half a century (1957-2013) and incorporating 16 diverse studies, their findings painted an unsettling picture: as temperatures rose from 15°C to 40°C LA plummeted between 30% -55%, while ALA saw reductions ranging from 3.5% -14%.

Geographic location played its part too; cooler climes up North predicted higher LA (48.1%-55.4%) and ALA (3.88%-5.83%) compared to their southern counterparts.

Yearly fluctuations were evident; specific years like 2017 and 2019 witnessed increased concentrations of LA and ALA due to cooler August temperatures particularly noted in northern some plains regions.

The mechanisms behind this climate-induced nutritional shift are linked to fatty acid desaturase enzymes called FAD2 and FAD3. These enzymes convert certain fatty acids in soybeans to LA and ALA. The higher the FAD activity, the higher the concentration of these essential fatty acids. However, when the temperature rises during seed development, FAD activity in soybeans drops. The most critical factor for fatty acid makeup is the high daytime temperature during the pod-filling stage, when about 79% of soybean seed lipids are formed. Different soybean varieties, depending on their genetic sensitivity to light and temperature, reach this stage at different times in the growing season. If temperatures increase during this time, the FAD enzymes become less active.

This has implications for human nutrition. Previous work using NHANES and What We Eat In America data showed that replacing traditional vegetable oils with high-oleic varieties (which have lower LA and ALA content) would reduce the intake of these essential fatty acids. For example, a 25% substitution would mean that some age groups would not meet the adequate daily intake for ALA, and a 50% substitution would make most groups fall short. The 25% substitution level corresponds to an oil containing around 5.68% ALA, while the 50% substitution level corresponds to 4.55%. The increased temperatures observed in the present study suggest that the ALA concentration was lower than 5.68% for most regions, and lower than 4.55% in the southern regions. This means that hotter temperatures may compromise the intake of this vital fatty acid.

This research is valuable for bridging the gap between nutrition and agricultural modelling. In agriculture, models typically employ functions to forecast crop yield by considering variables such as nitrogen levels, water availability, temperature, and photoperiod. This research goes beyond yield, considering nutrient levels. Also, previous models for predicting fatty acids relied on limited data, but this study uses a larger dataset. Unlike previous linear models, the authors use a more realistic logistic curve that better represents biological processes (that ‘S’ shaped curve). This approach allows for predictions even in extreme temperatures projected by climate studies.

So, what’s the bottom line here? The findings of this study reveal a disturbing trend: as climate change heats up the planet, the essential fatty acids in soybean oil decline. This could have serious consequences for the health of millions of people who rely on this oil as a major source of unsaturated fats. The authors warn that this hidden cost of climate change could exacerbate the global burden of chronic diseases and malnutrition. More research and collaboration between nutrition and agriculture experts are needed to develop strategies to mitigate this problem and ensure food security and quality in a changing climate. Moreover, this issue extends beyond essential fatty acids, as highlighted earlier; declines in crucial nutrients like zinc and iron could have profound implications for populations worldwide.

So, considering we don't have a planet B, this serves as yet another compelling reason (not that we needed any more) for all of us to advocate for action on climate change.