We need fats in our diet to survive, but how much is an area of debate and how much of which types of fat is an even bigger debate. There’s a lot of evidence that the inclusion of oily fish - or even fish oil supplements - will have beneficial effects in respect of disease prevention. But how true is this and do we need fish oils in our diet?
There are two completely essential fatty acids (EFAs) that humans require from food: linoleic acid (LA – an omega-6 fatty acid) and alpha-linolenic acid (ALA – an omega-3). Although not essential, there are four other fatty acids that may be beneficial by reducing the requirement for LA and ALA: arachidonic acid (AA), gamma-linolenic acid (GLNA), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). If you’re not including much of these four in your diet, then you’ll need to compensate by consuming more LA and ALA.
EPA and DHA are the ones we’re going to focus on in this discussion. These are both, like ALA, omega-3 fatty acids and both can only be obtained from oily fish, marine algae or supplements based on either of the aforementioned. EPA and DHA are not essential in our diet as they can be synthesised by enzymes from ALA.
The amounts of EPA and DHA in the blood are reflective of both biological processes and dietary intake and insufficient levels of them in our blood has been shown, through numerous studies, to give an increased risk of inflammation. It is important to note though, that a number of the studies have been done on fish oil intake rather than specifically EPA and DHA intakes, and the actual amounts of these two fatty acids in fish oils can vary.
Although ALA is considered the only essential omega-3 fatty acid because it cannot be synthesised by humans, evidence that human conversion of ALA to EPA and, particularly more so, DHA is relatively inefficient. Thus EPA and DHA may be considered conditionally essential nutrients and should dietary intake of both be poor, you can only be assured of sufficient blood levels of either if there is a sufficient ALA intake. In short: if you’re not consuming EPA and DHA, then you need to have a considerably higher ALA intake.
There is much discussion and conflicting information regarding how much dietary ALA can be converted into EPA and DHA. The process is via enzymes which facilitate the conversion. However, for optimal conversion these enzymes need to be acting efficiently and unfortunately, many aspects of modern eating habits interfere with the conversion to the nutritional disadvantage of people such eating habits.
Factors that affect the conversion of ALA to EPA and DHA include:
Although there are mixed reviews on the level of conversion, it’s well accepted that there is a higher conversion in women than in men. This appears to be related to the effects of the hormone oestrogen which may potentiate conversion rates[3, 4]. Two studies with the same lead author and published at the same time looked at conversion rates in men and women. One looked at ALA metabolism in males and indicated that approximately 8% of dietary ALA is converted to EPA and 4% is converted to DHA. Another looked at women: approximately 21% of dietary ALA is converted to EPA and 9% is converted to DHA.
Diets with a high saturated fat intake have been shown to have lower conversion rates[7, 8]; it’s not clear why this is as there are different enzymes involved in the metabolism of saturated fats and omega-3s. Diets high in omega-6 fats have also been shown to have lower conversion rates[7, 8]. This is much easier to explain: LA, the other completely essential fatty acid, competes with ALA for the same enzymes in its conversion to the conditionally essential fatty acid AA[7, 8, 9]. Therefore a diet rich in omega-6 fatty acids, increases the demand for ALA in the absence of EPA and DHA.
It’s also been indicated that the less EPA and DHA you consume, the higher the conversion will be as the demand is higher[9-11].
In the absence of any dietary EPA and DHA the conversion of dietary ALA to EPA and DHA varies considerably. Some papers have cited that human conversion of ALA into EPA ranges from 8% to 20% and conversion of ALA to DHA ranges from 1% to 9%. One study indicated that ALA conversion is about 6% for EPA and 3.8% for DHA. The factors mentioned above are the reason for these huge variations of rates.
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