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.
When the early results of studies into fats and bowel cancer were published there were hints that animal fat or saturated fat may increase risk, but not polyunsaturated fats or omega 6. Soon it became clear that the problem wasn’t even animal fat; it was red meat. In their review Giovannucci and Goldin concluded:
Intake of red meat or beef has been related to colon cancer risk in most case-control and cohort studies, whereas dietary fat from sources other than red meat, including dairy, poultry, and vegetable oils, does not increase risk of colon cancer.
No smoking gun for omega 6 there.
In fact, there is simply negligible evidence that omega 6 increases cancer risk in humans. Today no credible scientific organisation anywhere in the world recommends restricting omega 6 intake to lower cancer risk.
Do omega 6 fats cause macular degeneration?
Over a decade ago a case-control study suggested that different types of fatty acids may be linked to the risk for age-related macular degeneration, a condition that can lead to loss of clear vision in older people. A small cohort study (261 subjects) then found that total fat and all major classes of fatty acids were linked to progression of the disease.
However, these findings were not confirmed by two larger cohort studies conducted in Australia. The Melbourne Collaborative Cohort Study and the Blue Mountains Eye Study found no associations between macular degeneration and intakes of total fat, total polyunsaturated fats or omega 6.
Do omega 6 fats cause Parkinson’s disease?
There is a hypothesis that eating lots of omega 6 leads to lots of these polyunsaturated fats in the membranes of brain cells, which in turn leads to oxidative stress, which may then lead to Parkinson’s disease. Okay, it’s a long shot but does the evidence support it?
The question was addressed by Harvard researchers using data from two large cohorts. This study found that replacing saturated fat with polyunsaturated fat was associated with lower risk of Parkinson’s disease. In the Rotterdam Study, intake of polyunsaturated was associated with lower risk for Parkinson’s disease. And that’s all the cohort data we have to go on. Neither of these studies supports the hypothesis that high omega 6 intake increases the risk for Parkinson’s disease.
Do omega 6 fats cause inflammation?
It is often said that too much omega 6 in the diet causes inflammation and therefore may worsen inflammatory conditions, such as rheumatoid arthritis, ulcerative colitis and asthma. However, epidemiological studies do not find higher omega 6 intakes to be associated with higher levels of inflammatory markers. In fact, studies in the United States and Japan find the opposite.
Let’s dig into this. It’s complicated so stay with me.
Inflammation is not a bad thing per se as it is part of the body’s response to infection or injury. But when it occurs in an uncontrolled way the body’s tissues may be damaged. At the cellular level inflammation is governed by long-chain omega 6 and omega 3 fatty acids in cell membranes. The more potent of these is the long-chain omega 6 which is called arachidonic acid (AA). AA can be made in the body from the common omega 6 in the diet, linoleic acid. So the argument is that too much omega 6 in the diet leads to too much AA in the cell membranes, which in turn leads to too much inflammation.
It makes sense, so why doesn’t it happen in real life?
Just because the body can turn omega 6 from the diet into AA doesn’t mean that it automatically happens. The rate of conversion of omega 6 to AA varies depending on the circumstances. If the amount of AA in cell membranes is too low (essential fatty acid deficiency), the rate of conversion of omega 6 to AA is very high. However, if AA in cell membranes is normal the rate of conversion of omega 6 to AA is tiny, about 0.2%. Once the desirable level of AA in cell membranes is achieved it is tightly controlled and stays constant even if omega 6 intake is increased to high levels. Rather than feeding the inflammation process most of the excess omega 6 is burned off for energy.
For a good review on omega 6 and inflammation take a look at the American Heart Association’s Science Advisory on omega 6.
None of these arguments against omega 6 stacks up which is why we hear most of them via the internet and social media rather than from expert scientific organisations.
In my next post I’ll consider omega 6 and coronary heart disease, a topic that is actively debated in the scientific literature.