Enhancing plant seed oils for human nutrition.

The age of true designer plant oils has arrived. Using the tools of biotechnology, it is now possible to modify the fatty acid content of oilseed plants to change the relative abundance of individual fatty acids in seed oil for health purposes or to produce nutritional fatty acids not normally found in crop plants. Fats and oils are an essential part of the human diet, and as a species we consume a lot of them, on average 25 kg per person per year, mostly (80%) from plant sources (Subar et al., 1998; USDA, 2008). Since at least the 1930s, it has been realized that not all fats are created equal in terms of human nutrition. Studies in rats determined that certain polyunsaturated fatty acids (PUFAs) derived from plants, namely, linoleic and linolenic acids, were necessary to sustain life in animals (Burr et al., 1932). Other unsaturated fatty acids, such as oleic and eleostearic acids, could not substitute for these omega-6 and omega-3 PUFAs. It is now known that while animals, including humans, can make monounsaturated fatty acids from sugar, they do not have the necessary desaturases to convert oleic into linoleic and linolenic acids and must derive them from their diet (Wallis et al., 2002). These PUFAs, it turns out, are further converted in humans to arachidonic acid (ARA) and eicosapentaenoic acid (EPA), which are in turn precursors to important molecules that regulate the immune response and to docosahexaenoic acid (DHA), an important component of membranes, especially in the brain (Tocher et al., 1998). ARA (an omega-6 fatty acid) and EPA (an omega-3 fatty acid) play a yin-yang type of role in human metabolism; ARA is a precursor molecule for the proinflammatory molecules necessary for an immune response and EPA provides balance by being the precursor for antiinflammatory molecules. The human body is not very efficient at synthesizing very-longchain PUFAs and it has been postulated that this is because humans have evolved with diets abundant in fish and game meats, which are quite rich in these types of fats (Cordain et al., 2000, 2005). Modern grainfed meat and grain-rich diets are particularly abundant in omega-6 fatty acids, and it is thought that a deficiency of omega-3 fatty acids, especially the EPA and DHA found in fish oils, can be linked to many of the inflammatory diseases of the western diet, such as cardiovascular disease and arthritis (Hibbeln et al., 2006). DHA has been recognized as being vitally important for brain function, and a deficiency of this fatty acid has been linked to depression, cognitive disorders, and mental illness (Conklin et al., 2007). Over the past 50 years, various other lipid components of the foods we eat, such as cholesterol as well as saturated and trans fatty acids, have been correlated with an increased risk of cardiovascular disease (Van Horn et al., 2008). These correlations are usually confounded by other lifestyle aspects, such as smoking, and by genetic differences among individuals. Nevertheless, there is a general consensus that the healthiest oils are those rich in monounsaturated fatty acids, such as olive oil, and those rich in long-chain omega-3 fatty acids, such as fish oils (Hardin-Fanning, 2008). Olive oil, although stable, is too expensive for many food applications, and the need for heat-stable oils has been addressed in the past through the use of chemical hydrogenation of common vegetable oils such as soy (Glycine max) and canola (Brassica napus). The desire to replace hydrogenated oils in these food applications, and thus eliminate trans fatty acids from the diet, has led to the development of new vegetable oils that are inexpensive, stable during cooking, and rich in monounsaturated fatty acids. The increasing popularity of fish oils has placed a huge demand on natural ecosystems, and the current demand cannot be met in a sustainable way. The nutritional needs of humans for long-chain omega-3 fatty acids can also be addressed in a safe and sustainable manner by the genetic modification of common oilseed crops, such as soybean.