Modifying Macronutrient Compositions
Home _____ Benefits _____ Drawbacks _____ References
Enhancing Vitamins/Minerals ___ Reducing Undesirable Components ___ Modifying Macronutrient Compositions
Macronutrients are substances that provide the body with energy - carbohydrates, proteins, and fats - which are vital components of the human diet. Genetic modification has been used to alter the levels of macronutrients produced by several crops to enhance their nutritional value.
Carbohydrates
Proteins
Fats/Oils
Fats/Oils:
High Oleic Vegetable Oils
High Omega-3 Vegetable Oils
High Lauric Acid Canola OilEveryone has probably heard talk of saturated, monounsaturated, polyunsaturated, omega-3, omega-6, and trans- and cis- fats. These terms all refer to the organization of the carbon-carbon bonds within the fatty acid. Fatty acids are long hydrocarbon chains containing, as the name suggests, only hydrogen and carbon atoms. [They are called 'acids' because there is a carboxylic acid functional group at one end of the hydrocarbon chain.] A fully saturated fatty acid will have all single bonds between the carbon atoms, resulting in a fatty acid that is fully saturated with hydrogen. Monounsaturation and polyunsaturation refers to one or more double bonds between carbon atoms within the fatty acid hydrocarbon chain. Trans- and cis- refers to how carbon and hydrogen atoms are oriented around the double bond; in trans- unsaturated fatty acids, the carbon atoms are on opposite sides of the double bond, whereas in cis-unsaturated fatty acids, the carbon atoms are on the same side of the double bond. (see below)
Image courtesy of Stephen Abedon
http://www.mansfield.ohio-state.edu/~sabedon/biol1030.htmOmega-3 and Omega-6 fatty acids are those in which a double bond is found at 3rd or 6th carbon from the methyl end of the chain (see below). Omega-3 fatty acids are essential for proper growth and development, and have also been associated with the prevention and treatment of heart disease, arthritis, inflammatory and autoimmune diseases, and cancer. (Simopoulos, 1991 and Ursin, 2003)
Image courtesy of The Linus Pauling Institute Micronutrient Information Center
http://lpi.oregonstate.edu/infocenter/othernuts/omega3fa/index.htmlExtensive research has been dedicated to deciphering which types of fatty acids are best for human health. Currently, organizations including the U.S. Department of Agriculture and the American Heart Association recommend that polyunsaturated and monounsaturated fats are preferred over saturated fats, and that cis- unsaturated fats are preferred over trans-, because trans-unsaturated fats closely resemble saturated fatty acids and tend to raise cholesterol. Hydrogenation is a process used to stabilize the fats by reducing double bonds. Hydrogenated or partially hydrogenated fats are unfavorable, because hydrogenation often results in the formation of trans- fatty acids. (www.americanheart.org)
Six major oil crops account for 84% of the world's vegetable oil production: soybean, palm, rapeseed (~canola), sunflower, cotton seed, and groundnut. About 85-90% of this oil production is used for food applications (the other 10-15% is used for industrial products). (Murphy, 1999 and Topfer et al., 1995) The vegetable oils listed above mainly consist of different compositions of five fatty acids - palmitic, stearic, oleic, linoleic, and alpha-linolenic acids. (Topfer et al., 1995) Palmitic (16:0) and stearic (18:0) are saturated fatty acids, oleic (18:1) is a monounsaturated fatty acid, and linoleic (18:2) and alpha-linolenic (18:3) are polyunsaturated fatty acids. Also, as can be seen above, linoleic and alpha-linolenic are also omega-6 and omega-3 fatty acids, respectively. (Topfer et al., 1995) The numbers in parentheses above describe the fatty acid: the first number refers to the number of carbons, and the second number refers to how many double bonds the acid has (the degree of unsaturation).
Researchers have isolated the genes responsible for the synthesis of these fatty acids, and through genetic modification, have been able to alter the fatty acid composition of the world's major oil crops. Plant oils are synthesized with a biosynthetic pathway in all plant cells.
This pathway consists of 3 major steps:
1) Within the stroma of their plastids, the plants assemble fatty acid chains with 2-carbon building blocks known as acetyl-CoA. Therefore, the plant can only produce fatty acids with an even number of carbon atoms.
2) Once the desired length is achieved, desaturase enzymes (bound to the membrane of the chloroplasts and endoplasmic reticulum) selectively form double bonds within the fatty acids to form monounsaturated or polyunsaturated fatty acids. The major fatty acids of plants have a chain length of 16-18 carbons and contain from one to three cis- double bonds. (Ohlrogge and Browse, 1995)
3)The fatty acids are attached to a tryglycerol molecule to form the hydrophobic portion of glycerolipids, which are components of all plant cellular membranes. (Diehl, 1998 and Ohlrogge and Browse, 1995)
This biosynthetic pathway can be genetically modified at any one of these steps by inserting new genes or by silencing or over-expressing genes already present. Such modifications offer a plethora of possibilities.While polyunsaturated fatty acids are nutritionally valuable (as stated above), they are relatively unstable and breakdown under extreme conditions (i.e. high heat), making them unsuitable for many applications. (CSIRO, 2001) In order to remedy this problem of instability, hydrogenation has been used to reduce the fatty acids' double bonds, making them more stable. However, though hydrogenation may make the fat more stable, it also has the tendency to produce undesirable trans-fatty acids. (Liu et al., 2002)
In order to reduce the need for hydrogenation, researchers have prevented the formation of the unstable polyunsaturated fatty acids by silencing the genes within the plant that facilitate the conversion from monounsaturated oleic acid to polyunsaturated fatty acids. The genes are silenced using post-transcriptional gene silencing (PTGS). To learn more about PTGS, read this. By silencing select genes researchers have developed high-oleic oils from soybean (product already commercialized), palm, and cottonseed oil plants. (Monsanto, 2004)
Since oleic acid is an unsaturated fatty acid, High Oleic Vegetable Oil provides a more heart healthy fatty acid composition, reduces the need for hydrogenation, and eliminates trans- fatty acids. (Liu et al., 2002; CSIRO, 2001; and Monsanto, 2004)
Omega-3 polyunsaturated fatty acids are the hype right now in the world of nutrition. The ratio of omega-6 to omega-3 fatty acid consumption is higher than recommended in the U.S., due to the high linoleic (omega-6) content in soybean oil, the most widely consumed vegetable oil in the U.S. diet. (Ursin, 2003) Humans, and other mammals, are unable to synthesize polyunsaturated fatty acids (PUFA), so we must ingest them through our diets to reap their nutritional benefits. The three omega-3 fats most commonly discussed are alpha-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) all depicted in the diagram above. Even though the American Heart Association recommends that Americans consume 1.3-2.7 g/day of total omega-3 fats, actually consumption is as low as one-tenth of these recommended levels. (Ursin, 2003) Fatty fish - including swordfish, tuna, salmon, mackerel, and shark - are most often recommended as sources of omega-3 fatty acids, because they provide EPA and DHA, the most effective omega-3 fats for decreasing cardiovascular disease risk and improving overall health. (Huth et al., 2001 and Ursin, 2003) But due to the limited supply of farmed fish and the growing concern over toxic methyl mercury, fish do not provide an adequate and sustainable omega-3 source. (Ursin, 2003 and Abbai et al. 2001)
However, humans are able to convert ingested ALA into more desirable EPA. (Crawford et al., 2000) Flax and perilla contain high levels of ALA, but because of their low crop-performance and low yields, these crops are unsuitable for high ALA production. (Anai et al., 2003) Soybean and canola oils are the primary sources of ALA in the U.S. diet, and genetic modification is being used to engineer plants that can generate greater yields of ALA. For example, Anai et al. introduced a soybean-derived omega-3 desaturase gene into easily grown rice plants (Oryza sativa), drastically increase their ALA content from 2.2% up to almost 40%. (Anai et al., 2003)
One problem in increasing the consumption of ALA is that the bioconversion of dietary ALA to EPA in humans can be extremely inefficient, as low as 0.2%. (Ursin, 2003) One way to circumvent this problem is by eliminating the rate-determining (or the slowest) step in the conversion from ALA to EPA, by instead consuming Stearidonic Acid (SDA), an intermediate in the conversion from ALA to EPA. Researchers have found that by dietary SDA is converted to EPA three to four times more efficiently than ALA. (Ursin, 2003) Desaturase genes have been isolated from higher plants and fungi that naturally produce stearidonic and have been inserted into canola plants that provide a good source of SDA as the precursor for EPA. (Ursin, 2003)
Such GM plants may provide a land-based sustainable source of beneficial omega-3 fatty acids. These products would particularly benefit vegetarians or vegans who choose not to eat fish, and pregnant women who are pressured to consume omega-3 fats for their child's proper development but are warned about the toxicity in most fatty fish. (Ursin, 2003)
While this product has no obvious nutritious benefits, as lauric acid (12:0) is a saturated fatty acid, it can serve as a substitute for tropical oils (palm and coconut), and it has improved functional properties for application in fillings, margarines/spreads, shortenings, and commercial cooking oils. (Canola Oil, 2002 and Monsanto, 2004) Other products are also being produced to increase shelf-life of oils and enhance their cooking/baking ability.
