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How to Know When Flax Is Rancid


Story at-a-glance -

  • Flaxseeds contain many valuable nutrients, such as lignans, plant-based omega-3 fats and fiber, that stave off many serious health problems, such as heart disease, type 2 diabetes and cancer
  • You can detect whether or not the flaxseeds you’re buying are fresh by examining their appearance, fragrance and taste
  • When you consume flaxseeds, your gut bacteria convert the “plant” lignans to “human” lignans, such as enterodiol and enterolactone, which have weak estrogenic activity.

By Dr. Mercola

Flax is a food that many may say wasn't on their shopping list 10 years ago, but because of the healthy benefits it offers, this small oilseed has escalated in the American consciousness.

Flaxseeds are much more common in other parts of the world. Cultivated in Babylon as early as 3,000 B.C., Linum usitatissimum (flax) was used to weave strong linen cloth and was ground to add to breads, muffins and cookies.

Containing 42.16 grams of fat, 28.8 grams of carbohydrates and 18.29 grams of protein for every 100 grams in weight, today flaxseeds are acknowledged as a bona fide superfood, primarily due to three key elements:

  • Fiber
  • Omega-3 fats
  • Lignans

These and other compounds found in flaxseed are associated with benefits throughout your whole body. Flaxseeds contain more polyphenols than vegetables like olives and fruits like blueberries,2 relating to improved digestion and a lower risk of serious diseases such as type 2 diabetes,3 heart disease and cancer.

Being either brown or yellow, flaxseeds are available whole, ground or made into flaxseed oil, not to be confused with "linseed" oil which, though made from flaxseed, is reserved for industrial purposes.

How to Store Flax for Maximum Freshness

When you buy flaxseeds already ground, it's easy to toss a little into soups, on steamed veggies or into smoothies. However, ground flaxseeds have a shorter shelf life than the whole variety. According to the George Mateljan Foundation, ground flaxseeds:

"Even when carefully packaged in a gas-flushed, light-protective pouch and refrigerated after opening, typically last about [six to] 16 weeks.

Whole flaxseeds, on the other hand, will typically last for [six to] 12 months when stored in an airtight container in a dark, cool dry spot. If directly refrigerated, they may last for [one to two] years."4

You can often find either type of flaxseeds in bulk bins at large grocery stores, but they should be covered and sold on a regular basis to ensure freshness. Once brought home, they should be kept refrigerated. If any moisture is present, it can cause rapid oxidation and rancidity.

Because flaxseeds are highly perishable and turn rancid rapidly, you should buy organic whole seeds and grind them yourself just prior to use (avoid pre-ground versions). Flaxseed oil is even more susceptible to going bad than ground flaxseed, so I don't typically recommend using it.

If you choose to use flaxseed oil, however, it should always be purchased in a bottle that light can't penetrate and must be refrigerated.

In addition, while you can add flaxseed oil to foods such as vegetables or soups after those foods are cooked, heating flaxseed oil is not recommended because of its delicate, easily oxidized nature.

Past 225 degrees Fahrenheit, flaxseed oil begins breaking down, destroying the omega-3s along with the oil's fragrance, flavor and even appearance.

Here is my strategy for optimizing. I have one tablespoon of flaxseeds a day that I put in water that I will use with my smoothie and let them soak for around 12 hours before grinding them in my Nutrabullet smoothie.

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How to Know Whether Flaxseed Oil Is Fresh or Rancid

Luckily, your nose can often tell you the status of your flax. SF Gate explains:

"With a crisp, mild nuttiness redolent of raw sunflower or sesame seeds, fresh flaxseed oil gives an impression of lightness when inhaled. Although you might not detect the hints of sunflower and sesame unless you have a sensitive palate, a clean nuttiness always stands out in the aroma."5

A good way to test the freshness of flaxseed oil is to pour a few tablespoons into a wineglass, which will usually have a narrower opening than a regular glass, making the aroma more distinct. Place your nose right over the opening and inhale deeply. Rancid flaxseed oil is said to emit a slightly bitter, "off" odor, similar to old, cooked oil.

Flavor is as dependable as smell in this instance. If you taste flaxseed oil and it's "strong," bitter or burnt-tasting, it's probably rancid. Naturally, you want a clean, light fragrance as well as taste, denoting freshness. If either taste or smell is in any way unpleasant, trust your instincts and give it a thumbs-down.

Fresh flaxseed oil is clear, golden yellow, with no cloudiness, although it may also contain tiny bits of ground flaxseed, denoting valuable lignan compounds.

While the flaxseed oil is still in the wine glass, hold it up to the light and tilt it sideways. A hazy "texture" in a dark-yellow hue rather than clear translucence is not a sign of freshness.

Health Benefits of Fiber and Omega-3s in Flaxseed

Flaxseed has been categorized as being a "superfood" for several reasons, but fiber is among the top:


Flaxseed contains 95 percent fiber, both soluble,,and insoluble, like lignin6 Insoluble fiber is sometimes called "roughage" and is found in nuts, seeds, veggies and fruits, particularly skins and stalks such as celery stalks and apple skins.

One of the best advantages of fiber in flaxseeds is how its consumption relates to weight loss.7

Fiber contributes bulk to your stool, easing food through your colon and helping to prevent both constipation and colon cancer.8

When flaxseeds are subject to moisture such as what's in your stomach, it turns into a thick, gel-like substance. As a soluble fiber in your digestive system, it helps soften your stool for easier removal from your body and, as it travels through your colon, it tends to "clean" the cell walls.9

Flaxseed is noted for lowering blood sugar levels and protecting against heart disease, as well as enhancing your gut flora,10 which in turn positively affects your immune system, lowers inflammation, helps with weight management and even helps elevate your mood.11

Omega-3 fats

Flaxseeds are the second-richest source of the omega-3 fat alpha-linolenic acid (ALA). Only chia seeds contain more.12 As an essential fatty acid, which your body can't produce, ALA must come from an outside source; namely the food you eat. The oil contains the most, followed by the ground flaxseed variety.

One of the best things about consuming flax is that it may help improve your balance of omega-3 to omega-6 fats, which can significantly lower your risk of chronic diseases.13 Decreased risk of heart disease due to lowered inflammation was found in one study.14

While omega-3 fats have been shown to be very beneficial for your heart health, this is typically in relation to the omega-3 fats eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA).

Your body can convert ALA into DHA/EPA, but it does so at a very low ratio, and only when sufficient enzymes (that many people are deficient in) are present.

This is why consuming animal-based omega-3s in addition to plant-based omega-3s is very important. That being said, the ALA omega-3s in flaxseeds have been linked to a number of health benefits, and including both plant- and animal-based omega-3s in your diet is important.

The Importance of Lignans in Flaxseeds in Estrogen

Derived from plants, lignans are powerful antioxidant polyphenols found in flaxseeds, as well as in fruits, veggies, edible seeds, nuts and whole grains. However, there are 75 to 800 times more lignans in flaxseeds than any other food.15 Flaxseed oildoes not typically contain lignans.

Lignans have numerous uses in your body to stave off disease. They're similar to the female sex hormone estrogen and function as phytoestrogens.16 When you consume flaxseeds, your gut bacteria convert the "plant" lignans to "human" lignans, such as enterodiol and enterolactone, which have weak estrogenic activity.

One study examined lignans in relation to breast cancer risk and determined that in postmenopausal women, high lignan intake was linked to a significantly reduced risk of breast cancer.17 The impact of lignans on breast cancer may be expressed several ways, especially in regard to prevention.18

Shown to have a direct link, lignans can help lower estrogen production. In fact, the more estrogen exposure women have, the higher their risk of breast cancer. Lignans can help lengthen your menstrual cycle, releasing estradiol, "the strongest and most abundant type of natural estrogen and the one most associated with an increased risk of breast cancer."19

"The more menstrual periods a woman has over her lifetime, the more estradiol she produces, and the higher her risk of breast cancer. If a woman's period, for example, is lengthened from 28 days to over 32 days, the number of periods she will have over her lifetime will be lower and so will her risk of breast cancer," Women Living Naturally reported.20

Additionally, lignans create "good" estrogen, which is protective. In fact, estrogen is metabolized in the liver and broken down into three different estrone metabolotes: 2-OH, 4-OH and 16-alpha OH:

"The 2-OH estrone is considered a 'good' kind of estrogen because it does not stimulate the growth of breast cancer cells, whereas the 4-OH and 16-alpha OH estrone do — that's why they are considered 'bad' types of estrogen. Lignans influence how estrogen breaks down and help to create more of the 'good' kind of estrogen and reduce the 'bad.'"21

Additional Advantages From Flaxseeds

Besides the lignans, omega-3s and fiber content in flaxseeds, there are a plethora of other powerful nutrients, including vitamins and minerals listed by Authority Nutrition:22

  • Vitamin B1, aka thiamine, needed for optimal metabolism and nerve function
  • Copper, crucial for healthy growth, development and various related functions, and helpful for relieving oxidative stress23
  • Molybdenum, an essential trace mineral in flaxseeds, as well as other seeds and pulses, a deficiency of which is associated with esophageal and stomach cancers and other diseases
  • Magnesium, found in other foods such as grains, nuts, green leafy vegetables and other seeds, is an important nutrient in flaxseeds due to its role in metabolism, DNA replication and repair and cell proliferation; a deficiency can be traced to numerous diseases of the heart, thyroid and stomach, as well as cancer24
  • Phosphorus, for strengthening your bones and helping to maintain healthy bodily tissue

As the most important source of any lignan precursor, such as secoisolariciresinol diglucoside (SDG), flaxseeds help protect against lupus nephritis, kidney disease and even mental stress, one study reports.25 In addition, they can help inhibit: "Atherosclerosis, hemopoietic, liver necrosis and urinary disorders due to its various biological properties, including anti-inflammatory, antioxidant, antimutagenic, antimicrobial, antiobesity, antihypolipidemic and neuroprotective effects."26

Bone health and a lower risk of osteoporosis may come from consuming lignans,27 and eating flaxseed can reduce high blood pressure, as well as the headaches hypertension can cause.28

Flaxseeds contain high amounts of protein, but because they lack the essential amino acid lysine, they shouldn't be considered a protein source by themselves.29 However, perhaps to make up for it, flaxseeds contain good amounts of arginine and glutamine,30 recognized for their role in preventing heart disease31 and supporting your immune system.