Even more worrying, studies on breastfeeding mothers who were eating diets high in hydrogenated seed oils showed that up to 17 per cent of the fat in their breast milk was trans fat, whereas it would normally be less than one per cent. When tested, the babies of those women had significantly lower visual-acuity scores than babies whose mothers had eaten less trans fats. Hydrogenated seed oils at the time were 25 to 50 per cent trans fats, so the 1990s research was a marketing nightmare for the seed-oil industry. Under pressure from the nutrition establishment, McDonald's in the US switched from frying in beef fat to seed oils in the early 1990s, but within a decade it was facing lawsuits because of the levels of trans fats in the seed oils. By 2006, KFC was in similar trouble over the levels of trans fats in its seed oil-filled fryers. Seeds of discontent

The seed-oil industry needed a solution and it needed one fast. The officially healthy vegetable oils, such as olive oil, are largely mono­unsaturated, which means they're thicker and don't need to be hydrogenated for most uses. Olive oil seemed the ideal alternative to seed oils, but supply would never be able to meet demand and the cost was also much higher. If food manufacturers had to pay that much for their oil, they might as well return to using animal fats. The obvious solution was to switch from cottonseed- and soy-based oils to an oil that was much higher in monounsaturated fats but was also cheap. Canola oil fitted the bill perfectly. Because canola oil has fewer polyunsaturated fats than soybean oil, it doesn't require as much hydrogenation and has about half the trans fats of soybean oil. So it was no surprise that consumption of canola oil exploded in the 21st century. In Australia today, canola represents about 45 per cent of vegetable oils consumed. From a minor crop in the late 1980s, canola is now our third-largest broad-acre crop - we supply 20 per cent of the world market. Over the past 15 years, Australian consumption of canola oil has increased 2.4 times. While canola oil is certainly healthier than peanut oil, soybean oil and cottonseed oil - all of which have high levels of omega-6s and next to no omega-3s (a much healthier fatty acid) - canola oil still contains twice as much omega-6 (20 per cent) as omega-3 (nine per cent). Only flaxseed oil, at 57 per cent, has a large omega-3 content. The research now consistently shows that there's something very, very wrong with a diet where most of the fat comes from seed oils. And trans fats are just the tip of the iceberg. The science suggests that polyunsaturated fats (even without hydrogenation), and in particular the omega-6 fats, could be strongly linked to many cancers and autoimmune diseases. One of the largest human trials examining the effects of replacing animal fats with seed oils was the Los Angeles Veterans Trial, completed in 1969. It was conducted with 846 Californian military veterans randomly assigned to two different hostel kitchens. One kitchen replaced all animal-fat products with a seed oil (corn oil) for the eight-year duration of the study. The other kitchen kept on serving a normal high-animal-fat diet.

The purpose of the trial was to determine whether there was any heart disease-related benefit to removing animal fats from the diet, and on that front it was an abject failure. The seed-oil group had a slightly lower average blood-cholesterol level, but heart disease-related events were not significantly different between the two groups. However, analysis of the published trial data revealed a different, and totally unexpected, result: there was a dramatic difference in cancer deaths between the two groups, with the incidence of fatal cancers in the seed-oil group nearly double that of the normal-diet group by the end of the eight-year trial. The kosher conundrum

Israel, completely by accident, has been conducting its own trials of a diet high in polyunsaturated fats. The requirement that food be kosher places significant restrictions on animal foods but not on most plant-based foods. As a result, the Israeli diet has the world's highest concentration of polyunsaturated fats; in 1996, about 12 per cent of the energy Israelis consumed came from polyunsaturated fats, compared to about five per cent for Australians at the time. Unfortunately, Israel also boasts among the world's highest rates of heart disease and some types of cancer. This is despite eating what nutrition authorities would once have regarded as the perfect diet: high in polyunsaturated vegetable fats. Often referred to as the "Israeli paradox", this is the flip side of the "French paradox" (the French have historically had very low rates of these diseases and consume a diet that is relatively high in saturated fats). But the harm doesn't stop there. In 1996, researchers from the Karolinska Institute in Sweden recruited 63,870 women aged between 40 and 76 and monitored their diet and the occurrence of breast cancer for an average of 4.2 years. The dietary questionnaires used in the study enabled the researchers to determine how much saturated, monounsaturated and polyunsaturated fat the women were eating. They found no association between a woman's total fat or saturated-fat intake and her risk of developing breast cancer. Higher consumption of monounsaturated fat reduced the risk of breast cancer by 20 per cent, but high consumption of polyunsaturated fat did exactly the opposite. Just as earlier studies on rats had predicted, those women consuming the most polyunsaturated fat were 20 per cent more likely to develop breast cancer than the women consuming the least. More recent research is starting to suggest that polyunsaturated fats, and in particular the omega-6 fats in seed oils, may also lie behind the accelerating incidence of diseases as diverse as macular degeneration, allergies and even asthma. We don't appear to need any more than three to six grams a day of polyunsaturated fat (and of that, only half should be omega-6). But now that seed oils are used in just about every commercially produced food, the amount of omega-6 oil we consume has exploded. In Australia, our average polyunsaturated fat intake is currently at least 22 grams a day, or 11 per cent of our total kilojoule intake - more than double what it was in 1996. That means our consumption of omega-6 fats has substantially increased.

Fried food is a significant source of omega-6 fats. When we deep-fry, say, a potato, it acquires about 20 per cent of its cooked weight from the fat it is boiled in. This means that food fried in an animal fat will have around two grams (or less) of polyunsaturated fat per 100 grams, but food fried in a seed oil will have almost seven times as much; we would obtain twice our minimum daily requirement from just one small serve of McDonald's fries. And the deep fryer isn't the only place you will find these unhealthy fats. A slice of toast spread with margarine will have 15 times as much omega-6 fat as a slice covered in the same amount of butter. Even the bread is a problem, because there are almost no supermarket breads which are made without seed oil, and some contain your entire maximum daily allowance of omega-6 in just two slices (without the spread). Unhealthy processes

As a general guide, seed oils are used to make baked food crispy and sauces gooey. Mixer sauces (you know, the sort of pre-made sauce that turns your chicken or mince into something flash) often include vegetable oils and should be avoided. You'll also find huge amounts of seed oil in mayonnaise, pesto, most salad dressings, soy milks and, bizarrely, liquid versions of breakfast cereals. Almost everything in the supermarket freezer section, except the snap-frozen vegetables, contains seed oils. If the food you are considering is not baked or fried and is neither crunchy nor saucy - which leaves, well, not much - then you are likely to be on to a winner. Of course, the easiest way to avoid all seed oils is to not eat processed food. But if you have to live in the real world and you'd prefer not to have to assemble everything you eat from the raw ingredients, then you'll need to get good at reading labels. If you eat this way, your life will be immeasurably better. You'll materially reduce your risks of developing macular degeneration, heart disease and cancer, and it is likely you will significantly reduce the risk of your children suffering from food allergies and other auto-immune diseases. The best solution of all is to keep seed oils out of your life. David Gillespie's book Toxic Oil (Viking, $30) is released this week. Like Good Weekend on Facebook to get regular updates on upcoming stories and events – www.facebook.com/GoodWeekendMagazine