On Nutrition by Ed Blonz


DEAR DR. BLONZ: I eat a vegetarian diet (no fish) and have been relying on flaxseed as my source for omega-3 fatty acids. I do this by sprinkling the flax on my cereal or using it in baking. My concern relates to whether there is something toxic in raw flaxseed. I read that one should avoid, or at least limit, flax intake until it has been heated. Does this mean I should stop eating it raw? Should I stick to pure flaxseed oil? -- S.F., Dallas

DEAR S.F.: Let's address your "toxic" concerns first. Flaxseed contains very small amounts of compounds that can produce cyanide, a metabolic poison. But the mere presence of these compounds does not make flaxseed dangerous. With cyanogenic (cyanide-producing) and other potentially dangerous compounds, it comes down to the dose, and it also depends on the nutritional status of the consumer.

Such compounds are widely distributed in nature. A book on my shelf since graduate school, titled "Toxicants Occurring Naturally in Foods," was published by the National Academy of Sciences in 1973. It is an academic text that is now available online (tinyurl.com/lqlrtgu). Cyanogenic compounds cause problems primarily in individuals who are malnourished, particularly those with an inadequate intake of protein. The cyanogenic compounds in flaxseed are a greater concern for livestock, where very large amounts are consumed. Heat, or processing, does cause a breakdown of these substances, thus reducing the risk, but it is questionable whether this is a valid food safety concern.

The fatty acids in flaxseed are highly unsaturated, more so than most other vegetable oils. This makes flaxseeds more susceptible to oxidation, a reaction that destroys the nutritive value of an oil and turns it rancid. This doesn't make it toxic, but rancid fats are not what you want in your food or in your body. Exposure to air (oxygen) and heat can speed up the oxidation process.

The intact flaxseed has a protective coat that keeps the oil safe inside. The seed coat is so strong that most intact flaxseeds tend to pass right through our digestive system. Inside the flaxseed are also a number of antioxidants, this being nature's way of helping assure the viability of the seeds once planted. The healthful components of the flaxseed become available to us once the seeds are cracked or ground, but this process also increases the susceptibility to oxidation. This is why ground flaxseed should be stored in airtight containers and kept in the refrigerator once opened.

If you were to take pure flaxseed oil, you would get its omega-3s, but not the fiber and phytochemicals naturally present in flax. You would avoid the cyanogenic compounds, but the risk of rancidity would remain. If you are interested in using flaxseed oil, consider a brand that contains all the beneficial compounds found in the intact seed, including the phytochemicals known as lignans. Flaxseed oils, particularly when purchased as liquids, need to be stored in the refrigerator or freezer.

Send questions to: "On Nutrition," Ed Blonz, c/o Universal Uclick, 1130 Walnut St., Kansas City, MO, 64106. Send email inquiries to questions@blonz.com. Due to the volume of mail, personal replies cannot be provided.