According to the National Center for Complementary and Integrative Health, the flaxseed plant has been used since ancient times for its health benefits. In ancient Greece it was used as a laxative, while early Americans used the oil as a salve for cuts and burns. Today, flaxseed supplements are used to treat many conditions including heart disease, inflammation, diabetes, cancer, and high cholesterol. 
The health benefits of flaxseed come from two sources. The plant is high in fiber, which has health benefits, and it also has an omega-3 fatty acid called alpha-linoleic acid. You may know that omega-3 fatty acids from fish oil have been shown to have health benefits for heart disease and inflammation. Plant-based omega-3 fatty acids found in flaxseed and other plants are not powerful as the fish oil acids, and they have not been as well studied in humans. The active acids in fish oil are EPA and DHA. Only about one percent of plant acids are converted to EPA and DHA. 
The Research on Flaxseed Benefits
Human studies on flaxseed benefits for humans have been limited and have had mixed results. This is what we know so far:
- Heart Disease. Two studies found that eating a diet high in alpha-linoleic acid decreased the risk of heart attack, but it is not known if supplements would have the same effect.
- Cholesterol. Cholesterol studies have had mixed results. A 2009 review of studies found that people with high cholesterol could benefit from flaxseed. Other studies have not found that alpha-linoleic acid benefits cholesterol.
- High Blood Pressure. One study found that people who eat a diet rich in alpha-linoleic acid reduced their risk of high blood pressure.
- Cancer. Flaxseed may reduce estrogen levels, which may have a benefit for cancers that are triggered by estrogen like breast cancer and prostate cancer. So far, studies on cancer benefits have been mixed. In prostate cancer, the most recent studies suggest no effect. In one trial of 30 women with breast cancer, flaxseed supplements over one month decreased signs of cancer growth. So far, there is not enough evidence to say that flaxseed increases or decreases cancer risk.
- Fiber. A diet high in fiber provably decreases the risk of colorectal cancer, so fiber in flaxseed may have some benefit also. Flaxseed may also help constipation, in fact diarrhea is a side effects of flaxseed supplements.
- Weight Loss. Claims that flaxseed can help with weight loss are not supported. If there is any benefit it would be from the fiber content. There is good evidence that fiber added to your diet can help with weight loss because it is digested slowly, fills you up, and you may eat less calories. [1-4]
How to Take Flaxseed
Flaxseed can be added to your diet or taken as a supplement. Because supplements are not regulated, there is no standardized dose. An adequate dose to get benefits from flaxseed is 1-2 grams per day. (In the breast cancer study, a dose of 1.5 grams was used.) One tablespoon of ground seed has 2.2 grams of flaxseed. One tablespoon of flaxseed oil has 8.5 grams of flaxseed. 
Flaxseed oil and flour can be used for baking. Oil can be added to a salad or used for cooking. Flaxseed supplements are considered to be safe in recommended doses, but as with all supplements, it is best to check with your doctor first. [2,3]
Note that un-ripened or raw flaxseeds should not be eaten—they may have toxins. Flaxseed taken as fiber to treat constipation must be taken with lots of water, otherwise it gets too thick in your colon and can cause more constipation. Too much flaxseed may also cause diarrhea. Flaxseed has not been shown to be safe during pregnancy or breastfeeding, so check with your healthcare provider before taking any supplements. [2,3]
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- NIH, Flaxseed and Flaxseed Oil | NCCIH (nih.gov)
- Mount Sinai School of Medicine, Alpha-linolenic acid Information | Mount Sinai – New York
- University of Rochester Medical Center, Flaxseed – Health Encyclopedia – University of Rochester Medical Center
- American Institute for Cancer Research, Flaxseed: Full of Fiber and Phytochemicals – American Institute for Cancer Research (aicr.org)