A flurry of studies in recent years has reported that organic fruits and vegetables are, on average, more nutritious than conventional foods, while another flurry has reported little or no nutritional difference. While most review articles and meta-analysis conclude that Vitamin C, phenolic acids, antioxidant, and some mineral levels are typically higher in organically grown produce, the levels of some other nutrients are often higher in conventional food (e.g., protein, Vitamin A). Opinions vary widely on whether the typically small incremental differences in nutrients between organic and conventional foods are biologically meaningful.
A team of scientists in Brazil set out to first, quantify the nutritional differences in organic versus conventionally grown tomatoes, and second, to assess the roll of plant stress in driving differences in nutrient levels across production systems. Their findings are provocative and appeared February 20, 2013 in an open-access online paper at PlosOne.
The differences in nutrient content were sizable – there was a 2.4 higher level of phenolic acids and 55% more Vitamin C in the organic tomatoes. Their findings led the authors to ask “Why do stressed tomatoes contain higher levels of health-promoting secondary plant metabolites?”
In plants, stress triggers oxidation, and oxidative stress controls the synthesis of plant secondary metabolites. These defensive compounds are produced in an attempt to counteract or deal with the underlying, stress-inducing condition. High quality wine gains its intense and unique flavors from the defensive compounds produced by grape plants struggling to thrive in stressful conditions. This is why premium wines are always produced from low-yielding vineyards, where plants have been purposefully and systematically stressed, whereas most jug wines come from vineyards that push yields as high as possible, producing lots of mass but relatively little flavor.
It turns out a little stress does essentially the same thing for tomatoes, by increasing the production of a range of phenolic acids and antioxidants.
While the PlosOne paper provides powerful new evidence supporting the role stress can, and sometimes does play in determining nutrient concentrations, stress is not the only factor driving observed differences in nutrient content. A phenomenon called “nutrient dilution” is often as or even more important. In short, “nutrient dilution” occurs when a plant grows and sets fruit in the presence of ample to excessive levels of nutrients and water, without any other significant yield-limiting stressors. What is behind nutrient dilution, why does it occur?
The number of cells is essentially set when a very small apple, grape, or carrot begins to gain size and mature. If the tree, vine, or plant is drawing up ample-to-excessive nutrients and water from the soil, it has to use the nutrients for something, and typically, the easiest thing for a plant to do with excess nutrients is to convert them to carbohydrates. This process increases the sugar and moisture content of harvested produce, and it also leads to larger cells, more space between cells, and larger fruit at harvest.
Larger cells, coupled with more space between cells, produce fruit in which the levels of phenolic acids, minerals, and vitamin-based nutrients are diluted by the presence of extra weight composed mostly of sugars, starches, and moisture.
Fruit size also plays an important role in nutrient density. The organic tomatoes produced by the Brazilian team weighed 75 grams when ripe, much less than the average weight of the conventional tomatoes (125 grams). The size of a tomato governs the fruit’s surface area – in large fruit, the tissues right under the skin account for a lower percentage of the overall weight of the tomato than in smaller fruit. This is not just about the aesthetic appeal of the tomato. Nutrient concentrations are much higher in the surface layer of fruit, and for good reason. The sun, most insects and plant diseases, and physical damage all threaten the integrity of fruit first and foremost on its surface, so this is where plants deploy their defensive compounds.
I agree with the Brazilian scientists that, in general, plants on organic farms must contend with both a greater diversity of stresses, as well as periodic, more serious stress triggered by insects or plant diseases that are either not controlled by pesticides, or are controlled to a lesser extent by organically approved, but less effective pesticides. But on well-managed and stable organic farms, pest levels are often actually lower than on nearby conventional farms, and soil nutrient levels can be managed in ways that fully meet plant needs. Yet under these conditions, nutrient levels in harvested organic produce may still be higher than in nearby, conventionally grown produce. Why? Perhaps the pursuit of higher yields and faster growth on conventional farms opens the door for nutrient dilution, and Mother Nature does the rest.
So, a little stress can be a good thing, and too much of good things like nitrogen and water can become a bad thing for farmers wanting to produce nutrient-dense food. For consumers hoping to choose organic (or conventional) fruit with relatively high nutrient levels, look for fruit that is small to moderate in size. And don’t forget the icing on this cake – smaller, more nutrient dense fruits and vegetables are almost always tastier.