Omega 6: good or bad for health? Part 1

Omega 6 in vegetable oils stands accused of causing ill health by increasing the risk for cancer, macular degeneration, Parkinson’s disease, inflammation and heart disease. Sounds bad, but is any of it true?

What is omega 6?

All fats – animal or vegetable – are made up of building blocks called fatty acids which generally fall into three main groups called saturated, monounsaturated and polyunsaturated. There are two classes of polyunsaturated fatty acids – omega 6 and omega 3. The omega 3 in vegetable oils is called α-linolenic acid and the omega 6 is linoleic acid. The concern about omega 6 relates to linoleic acid which is the most common polyunsaturated fatty acid in the diet.

Do omega 6 fats cause cancer?

In the early 1980s it was thought that fat intake may affect the risk for cancer of the breast and bowel. This hypothesis was based on very basic evidence i.e. different cancer incidence in countries with differing fat intakes, but worth investigating nevertheless.

Several prospective cohort studies were commenced around this time. Data from seven of these studies were pooled in the definitive breast cancer study by Hunter and colleagues. Breast cancer risk was not associated with intake of total fat, animal fat, vegetable fat or polyunsaturated fat. Another pooled analysis of cohort studies published five years later confirmed that polyunsaturated fat was not associated with breast cancer. So omega 6 was off the hook.

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Fruits and vegetables don’t prevent cancer

Evidence that fruits and vegetables prevent cancer has fallen away over the last decade but our national nutrition authorities continue to perpetuate the myth. Why don’t they just review the science and give us the facts?

In 1981 the eminent epidemiologists Richard Doll and Richard Peto wrote a famous paper in which they argued that many cases of cancer were due to environmental factors and were therefore potentially preventable. It was suggested that poor diet was second only to smoking as a cause of cancer and could account for 35% of cases. Based on the evidence available at the time, Doll and Peto suggested that increased intakes of fruits and vegetables may prevent cancer. A generation of nutritionists and dietitians embraced the idea with a passion and started communicating the good news.

Better evidence now available

The early evidence suggesting protective effects of fruits and vegetables against cancer came primarily from case-control studies, with all their well-documented bias. Over the last two decades much better evidence in relation to fruits, vegetables and cancer has become available as large prospective studies and in some cases pooled analyses of these studies have been conducted. Most of the results are negative.

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