Myth: Canola oil and margarines cause macular degeneration.
Fact: Good fats – including canola oil and trans-free unsaturated spreads are probably protective against AMD.
One of worst things about food myths is the – albeit unintentional – harm they can cause. Such as was the case a few years back in Australia when an ophthalmologists thought he’d have a go at nutrition and started giving advice completely opposite to the prevailing dietary guidelines by recommending unhealthy fats like butter and warning against vegetable fats like canola oil and margarine spreads in an attempt to reduce the risk of age-related macular degeneration (AMD).
He got it wrong.
Macular degeneration causes damage to the retina and is the most common cause of blindness in developed countries. The cause is not fully understood but risk factors have been identified: the strongest factors are smoking and advancing age. Overweight, high blood pressure and high blood cholesterol are also risk factors, which is ironic because the advice the eye-doctor gave increases blood cholesterol levels. National dietary guidelines recommend we replace saturated fats with unsaturated fats found in vegetable oils and trans-free spreads, fish, nuts and seeds in order to reduce the risk of cardiovascular disease – still our biggest killer.
The first mistake the well-meaning ophthalmologist made was devising public health advice based on only a couple of studies; second was not understanding the types of studies they relied on are not designed to prove causation; third was ignoring the fact his advice conflicted with well established evidence-based advice for primary prevention of cardiovascular disease; fourth was starting a campaign to convince others including the media; and lastly he failed to properly translate the theory from the US studies into relevant food advice in the Australian food context.
The population studies he relied on can only indicate an association that needs further investigation, and are ranked as low-level evidence. Having a proposed mechanism also bolsters a case for causation but there was none. He transposed the American food supply on to Australia’s and got the food advice wrong (for example Australian margarines are very low in trans fats). He basically pitted eye health against heart health and challenged us to choose but there’s not much point having good vision if you’re dead from a heart attack! It also wasted the valuable time of organisations like the Heart Foundation (Australia), the CSIRO and the Dietitians Association of Australia and more sensible eye specialists who had to control the damage with media communications refuting his ill-conceived advice.
Since this fiasco – which sent conflicting messages and confused people – a study conducted in Australia, the Blue Mountains Eye Study (BMES), did not find any link between vegetable oils or margarine spreads and macular degeneration. In fact they found a significantly lower risk of developing macular degeneration in those consuming higher amounts of both long and short chain omega-3 fats, such as those naturally present in fish, canola oil, linseeds, some nuts and green leafy vegetables. The BMES also found a lower risk of AMD in those eating higher amounts of zinc, and lutein and zeaxanthin: the yellow/orange coloured pigments in vegetables, fruits and eggs (and the greatest benefit was seen among participants who regularly consumed a combination of the protective nutrients as part of a low GI diet). A systematic review and meta-analysis of lutein and zeanxanthin and AMD also found a protective effect of consuming higher amounts. The benefits of other dietary antioxidants are not clear, however a Cochrane review of the evidence does not support taking dietary supplements to ward of AMD. Lowering the GI of the diet appears to be protective with several other studies showing associations between a high GI diet and AMD. What we really need is high level studies such as randomised controlled trials to find out the best diet for preventing AMD and be prudent in the mean time.
So what does the current evidence suggest we do to reduce our risk of AMD?
- Don’t smoke
- Maintain a healthy weight
- Eat fish and seafood to obtain omega-3 fats
- Eat plenty of different coloured vegetables and fruits for their beneficial phytochemical antioxidants
- Make the switch to low GI carbs (the smart carbs)
- Replace saturated fats from animal sources such as butter, whole dairy foods and fatty meat with unsaturated fats from vegetable oils and trans-free spreads, nuts and seeds
- Minimise trans fats in partially hydrogenated oils by limiting commercially produced pastries, cakes and deep fried fast foods – be aware sources of trans fats vary by country