Flaxseed provides a unique mix of fatty acids. It is low in saturates (less than 9% of total fatty acids) and contains the essential polyunsaturated fatty acids omega-3 α-linolenic acid (ALA; 18:3n-3) and omega-6 linoleic acid (LA; 18:2n-6). Essential fatty acids are required in the diet as they cannot be made by humans. Most notably, ALA comprises on average 53 to 57% of the fatty acids in flaxseed oil, making it the richest plant source (1).
Oils rich in omega-6 include soybean, corn, traditional sunflower, grapeseed and safflower. Fried and processed foods are also high in omega-6 fatty acids. The balance between omega-3 and omega-6 intakes is very important as this article describes.
α-linolenic intake is below recommendations
The Institute of Medicine recommended dietary intake of ALA is 1.6 g/day for men and 1.1 g/day for women, or 0.6-1.2% of energy intake, with a dietary omega-6 to omega-3 ratio of 5:1 to 10:1 (2). However, the consumption of omega-3 fatty acids is very inadequate in North America. Data from the National Health and Nutrition Examination Survey (NHANES) in the U.S. showed that 40.8% of adults aged ³19 years were not meeting the recommended dietary intake for ALA (3).
In the past, dietary intake of omega-6 and omega-3 fatty acids were approximately equal (that is about 1:1), but modern diets have significantly changed resulting in very high omega-6 to omega-3 ratios (16:1 or higher) (4).
Not only are people eating less omega-3, the overall availability of omega-6 LA in the food supply has increased from about 2.79% to 7.21% of energy from 1909 to 1999. The level of ALA in the food supply has only increased from 0.39% to 0.72% of energy due mostly to modifications in food processing. These changes resulted in the ratio of LA to ALA increasing from 6.4:1 in 1909 to 10.0:1 in 1999 (5). Consumption of soybean oil, which is comprised of approximately 54% LA, has increased from 0.009 kg per capita per year in 1909 to 11.64 kg per capita per year in 1999, a 1163-fold increase (5).
Metabolism of α-linolenic acid
ALA and LA are components of cellular membranes and act to increase membrane fluidity. ALA is converted to the long-chain omega-3 fatty acids, eicosapentaenoic acid (EPA) and docosapentaenoic acid (DPA), and to some extent to docosahexaenoic acid (DHA), fatty acids that naturally occur in fish and fish oil. Similarly, LA is converted to long-chain omega-6 fatty acids, in particular arachidonic acid, by the same series of desaturation and elongation enzymes that metabolize ALA (6).
The metabolism of omega-3 fatty acids depends on other nutrients, particularly omega-6 fatty acids due to the competition for the same enzymes (7). High levels of omega-6 fatty acids reduce the amount of omega-3 fatty acids incorporated into lipids that comprise human tissues as well as the conversion of ALA to the longer chain omega-3 fatty acids (7). Higher amounts of dietary ALA as well as decreased LA increase the conversion of ALA to EPA and DHA (7).
Eicosanoids, such as prostanoids and leukotrienes, are hormone-like substances that affect inflammation. They are produced from both ALA (less inflammatory) and LA (mostly pro-inflammatory). Arachidonic acid, derived from LA, is the starting point of the eicosanoid inflammatory cascade that affects a wide array of body functions (8). Chronic inflammation is linked with age-related diseases such as heart disease, obesity, diabetes and cancer. Diets that are high in LA and low in ALA skew eicosanoid production towards a more inflammatory profile. Increasing dietary ALA intake from flaxseed can help to guard against inflammation (9).
Maintaining a balance between omega-3 and omega-6 intake can lead to better overall health
A recent study determined the link between omega-3 and omega-6 intakes and mortality in adults in China and the US (10). The investigators tracked 14,117 participants in the China Health and Nutrition Survey (CHNS) and 36,032 participants in the NHANES up through 2011.
An omega-6 / omega-3 ratio of 6:10 was linked to reduced risk of death in both surveys. The authors concluded that “These findings suggest the importance of maintaining an omega-6 / omega-3 balanced diet for overall health promotion outcomes … “
Dietary flaxseed and flaxseed oil can help to improve LA to ALA ratios since it contains more than three times as much omega-3 as omega-6 fatty acids. Flaxseed enrichment in common foods will provide healthier fatty acid profiles.
References
- Choo WS. J Food Compost Anal 2007;20:202-211.
- Institute of Medicine. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Macronutrients). Washington, D.C.: National Academy Press, 2005.
- Papanikolaou Y, et al. Nutr J 2014;13:31.
- Liu J, et al. Nutrients 2014;6:5184-5223.
- Blasbalg TL, et al. Am J Clin Nutr 2011;93:950-962.
- Fleming JA, et al. Adv Nutr 2014;5:863s-876s.
- Brenna JT, et al. Prostaglandins Leukot Essent Fatty Acids 2009;80:85-91.
- Anand R, et al. Inflammation 2014;37:1297-1306.
- Caligiuri SP, et al. Exp Gerontol 2014;59:51-57.
- Zhuang P, et al. Clin Nutr. 2018; pii: S0261-5614(18)30082-7. doi: 10.1016/j.clnu.2018.02.019.