June  2002    Vol. 1   No. 6 <<<HOME

Health Myths Dispelled:

“Good” Omega-3 Fish Fats Make Cancer Worse

Many popular books and authorities on cancer recommend the replacement of saturated and omega-6 fats (the kinds of fats that are found predominately in corn and safflower oil) with omega-3 fats (the kind found in cold-water marine fish and flaxseed).1,2  There is evidence to support their recommendations;2 however, I believe these authorities are overlooking some important research that says this is a very dangerous practice.3,4

In a recent study using rats, investigators found that three weeks after transplantation of colon cancer cells into their livers that those on fish oil (omega-3) had 10 times more metastasis (cancer cells that spread) in numbers of tumors, and 1000 times more metastasis in volume of cells, than those on a low-fat diet.  Using safflower oils (omega-6), there were 4 times more metastasis in number of tumors, and 500-times metastasis in volume, than those rats fed a low-fat diet.  Similar cancer promoting effects of these “good” omega-3 fats have been seen in animal studies with lung cancer cell lines.5,6

There have been several proposed mechanisms for this increase caused by omega-3 and omega-6 vegetable oils, including the production of free radicals that damage the cells, suppression of the immune system, and changes in small hormones that promote tumor growth, known as prostaglandins.  One interesting proposal for this rapid spread of cancer is the reduction of the effectiveness of a matrix (covering) that is formed around the tumor by the body in an effort to contain its growth and expansion (metastasis).  Fish oil (omega-3 fats) inhibits the formation of this matrix and stimulates the growth of metastasis directly.4

“Good” omega-3 fats in the form of fish oils and flaxseed oils are recommended to cancer patients to help counteract their weight loss (cachexia), and to inhibit tumor growth.  However, these studies question the wisdom of that recommendation.  Doctors are conditioned by experience to fear weight loss in their cancer patients, because the final days of a cancer patient’s life are usually punctuated by profound weight loss due to their loss of appetite from their illness.  They think that as long as their patients are not too thin then all is OK.

For two reasons this can be faulty thinking:  First, as discussed above, prevention of weight loss by feeding patients “good” omega 3, and even omega 6, fats may actually hasten their death by increasing tumor spread and growth.  Second, my cancer patients lose weight by eating a low-fat diet, and as a result, become healthier.  The animal studies discussed above, as well as many others in animals and people (See the McDougall Program for Women book), show low-fat diets inhibit the growth of cancer and prolong a patient’s life.  Many of my patients have been told by their well-meaning doctors that they can change their diet, but don’t lose weight.  (This warning is made to feed the doctor’s biases associated with weight loss in the terminal days of life.)

But with a healthy diet most overweight cancer patients do lose weight, becoming healthier.  This is good – but most doctors fail to see this benefit.  Furthermore, I believe with a healthier diet cancer patients live longer, and without a doubt, they live better.

The Delany Clause (sometimes referred to as the Delany Amendment) of the 1958 Food, Drug, and Cosmetic Act prohibits the use of any food additive that is found "to induce cancer in man or animal."  If this standard were applied to fat added to our foods, and especially vegetable fat, then our overall health, and our risk of death from cancer, would be substantially improved.


1.  The Breast Cancer Prevention Diet by Dr. Bob Arnot

2.  http://www.dietsearch.com/article/fatcancer.htm

3.  Griffini P.  Dietary omega-3 polyunsaturated fatty acids promote colon carcinoma metastasis in rat liver. Cancer Res. 1998 Aug 1;58(15):3312-9.

4.  Klieveri L. Promotion of colon cancer metastases in rat liver by fish oil diet is not due to reduced stroma formation. Clin Exp Metastasis. 2000;18(5):371-7.

5.  Young MR.  Effects of fish oil and corn oil diets on prostaglandin-dependent and myelopoiesis-associated immune suppressor mechanisms of mice bearing metastatic Lewis lung carcinoma tumors.  Cancer Res. 1989 Apr 15;49(8):1931-6.

6.  Coulombe J.  Influence of lipid diets on the number of metastases and ganglioside content of H59 variant tumors.  Clin Exp Metastasis. 1997 Jul;15(4):410-7.


2002 John McDougall All Rights Reserved

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