Fats and oils

The current Food Pyramid lumps all fats and oils together at the top of the pyramid. However, there are four different sorts of fats: saturated (such as animal fat, butter, palm oil and coconut oil), trans fats (found in commercial margarines and many processed foods), monounsaturated (such as olive and canola oil) and polyunsaturated (such as sunflower oils).

In addition, canola, flaxseed, walnut and fish oils are rich in omega 3 fatty acids, and some seed and grain oils are good sources of omega 6.

Saturated and trans fats behave differently in your body from unsaturated fats. They increase the amount of "bad" cholesterol in your bloodstream, and are known as "bad fats".

Unsaturates, in contrast, actually reduce the amount of "bad" cholesterol. Along with omega 3 and 6 fats, they are called "good fats" (see The science of food).

What's at issue is whether you can have too much of a good thing. Most nutritionists say it's important to cut back on saturated and trans fats and on your overall level of fat consumption.

But the Harvard team has proposed that good fats belong at the bottom of the pyramid. They say liquid oils like olive, canola, soy, corn, sunflower and peanut should be eaten daily. This is a radical departure from the established view, because it doesn't say we should eat less fat overall - in fact, it puts greater emphasis on oils than vegetables.

To support their argument, they point to the diets of people in Japan, Finland and Crete. The traditional Japanese diet contains only 10 percent fat, and the rate of heart disease among men is around five percent.

In Eastern Finland there is about 38 percent fat in the traditional diet. It's mainly saturated (from dairy products) and it leads to a whopping 30 percent rate of heart disease among men. But in Crete, where they eat around 40 percent fat, mainly from olive oil, the heart disease rate among men is an astonishingly low two percent.

However, it's important to remember that the traditional Mediterranean diet includes a lot of fruit, vegetables and fish, and that diet isn't the only issue. Other lifestyle factors, like smoking and exercise, also play important roles.

All the New Zealand nutritionists we spoke to disputed the Harvard view that people should eat large amounts of unsaturated fats.

Suzi Penny, of the Institute of Food Nutrition and Human Health at Massey University, says putting "good fats" on the bottom layer of the pyramid is likely to significantly increase the problems of obesity. These include a greater risk of type 2 diabetes, strokes and other illnesses, as well as heart disease.

"All fats, whether they are monos, polys or omegas, provide the same amount of energy, which is double that of carbohydrates. Dipping bread in olive oil instead of butter is great, but in large amounts and often, it means weight gain for those who may not need the extra kilojoules."

Willett says the correlation of high intakes of fat and high rates of heart disease is limited to saturated fat. He adds: "No study has demonstrated long-term health benefits that can be directly attributed to a low-fat diet."

Who is right? In our view, the evidence for eating lots more unsaturated fats is inconclusive. But the critical message is that saturated fats and trans fats are bad. Cut these back in your diet, and you'll be taking an important step towards healthy eating.