To most people, N3 and N6 sound like roads, and triglycerides could be paint stripper. In fact, they're circulating in your blood and they will help determine whether or not you die of a heart attack, especially if you're a woman. Every time you choose a packet of low-fat spread you're making an N6 decision and every time you turn your nose up at mackerel or salmon, your N3s are taking a battering.
In the New England Journal Of Medicine, there is a huge argument going on: to reduce all fat in the diet, regardless of which type, and the other is to leave fat intakes where they are and replace bad fats with good fats. But how do you replace bad with good when there are so many different kinds of fats hidden in foods?
Our inability to get a grip on our fat intake is a major reason why the proportion of overweight Irish adults is spiralling and today half of all Irish men and women are overweight. The alarming increase in obesity has been measured in the UK, where 8 per cent of women in the mid-1980s were classified as obese compared with 15 per cent today. Despite a lack of data, experts believe the situation here is similar.
Rates of car use and hours spent watching TV - used to assess physical inactivity - are increasing quickly and many studies show that the hours spent watching TV by children is closely associated with obesity levels, says Dr Mary Flynn, lecturer in nutrition at the Dublin Institute of Technology (DIT).
But another crucial factor is fat - all kinds of fatty acids in a bewildering array of forms: monounsaturated, saturated, polyunsaturated, polyunsaturated N6 versus N3, transunsaturated and hydrogenated vegetable, marine and animal fats, which can be found in dairy spreads, biscuits, cakes, confectionery, butter, vegetable oils and processed foods.
And often the label is completely confusing because these fat-laden foods contain mixtures of different fats, making it impossible to understand, much less measure, what you are eating.
Fat makes you fat because it is energy-dense, providing 9 calories per gram of energy - no matter what kind of fat it is, olive oil or butter for instance - compared with 3.75 calories per gram for carbohydrates such as grains, pasta and sugar. Fat may have a lot of calories, but it hardly makes a dent on appetite because it does not make you feel full as quickly as carbohydrates do, so that you can easily consume more calories than you need in a few mouthfuls - what nutritionists call "passive over-consumption".
It's easy to overeat without realising that you're doing it when every delicious mouthful is more than twice as rich in calories as a mouthful of carbohydrate. Fat also contributes to obesity because the body stores fat calories more efficiently as body fat. The body metabolises carbohydrates less efficiently, so that more energy is burned in the process of converting excess carbohydrate calories into body fat stores. There is one school of thought that if you overeat on carbohydrate - as in bread with jam but no butter - you are less likely to store as much body fat as if you eat the same amount of extra calories in the form of cream, oil or high-fat biscuits.
About 40 per cent of calories in the Irish diet come from fat. Theoretically, anyone can become healthier and, hopefully, lose weight by taking more exercise and lowering the proportion of fat in the diet to 30-35 per cent of calories. Needless to say, it's easier said than done.
In Dr Flynn's study on women and nutrition, 96 per cent of those questioned knew that to achieve a healthier diet they needed to reduce fat intake, but on further examination the women hadn't a clue how to achieve this . . . More than half considered their diet to be mostly healthy, and a smaller proportion believed their diet to be downright unhealthy. Yet when their diets were analysed, no difference emerged. Both the "healthy" and "unhealthy" groups were eating the same amount and the same type of fat - it was merely a question of perception.
Many of us, on being told by the "experts" to cut fat intake down to 30-35 per cent, shrug our shoulders and ask, what's the point? How can you possibly know what the proportion of fat in your diet really is?
"It's very hard even for a nutritionist to calculate it. If you asked me that question, I'd have to think quite hard and make calculations in my head. The average adult can't do that," says Dr Flynn.
Add to this the controversy over whether the proportion of fat in the diet is as significant as we have been led to believe, and you might give up altogether. The debate over whether a low-fat diet is good for you at all has exercised the comment pages of the New England Journal Of Medicine for some time now. On one side is the argument that you should reduce all fats in the diet, regardless of what they are. On the other side is the argument that good fats - such as monounsaturated fats and fish oils - are beneficial and do not need to be limited.
Somewhere in the middle of all this, there has to be an ounce of good sense. Dr Flynn believes that we should try to limit all fats, especially saturated and transunsaturated fats, in order to prevent obesity and heart disease. But if you go too far and go on an extremely low-fat diet and cut out the beneficial fats - such as monounsaturates and fish oils - you could be doing yourself more harm than good.
"A low-fat diet can have a very negative effect, especially for women, because it can reduce the level in their blood of HDL (high density lipoprotein) cholesterol - which is good cholesterol and protects against heart disease," says Dr Helen Roche of Trinity College Dublin's Unit of Nutrition and Dietetics, who is researching the role of fat in women's diets.
"Low-fat diets can also increase plasma triglycerides, which put adults, especially women, at increased risk of coronary heart disease.
"A very low-fat diet with no butter on bread and no fat in cooking, is going to an extreme. It can be just as bad as a high-fat diet, especially for women. What we need to try to do is to promote a healthy balance of different fats," says Dr Roche.
N3 and N6 are types of polyunsaturated fatty acids (PUFAs), which can both be beneficial for the heart, but getting the right balance is essential. N6 PUFAs are contained in sunflower, corn and soybean oils and in partially hydrogenated vegetable oils made from these, while N3 is a beneficial fatty acid found in oily fish, including fresh and smoked mackerel, salmon, herring and tuna (fresh, not tinned).
It is emerging that although there are benefits against heart disease in N6 PUFAs, there is a limit to how much is beneficial. And there is concern that a high intake of N6, coupled with a low intake of N3, could have negative effects.
N3 makes the blood less likely to clot and helps lower triglycerides - blood fats which put women at higher risk of heart disease. And both men and women should eat oily fish twice a week to get enough N3 to prevent a heart attack.
We also need to reduce saturated fats. While most people know that butter and beef contain saturated fats, few know that most of the saturated fat in the Irish diet comes from biscuits, cakes and confectionery in the form of hydrogenated vegetable fats.
Hydrogenated fats are problematic - and have a worse effect on blood fat levels and heart disease than saturated fats, because they contain transunsaturated fatty acids (TFAs).
While the point is controversial, it would appear that TFAs raise the bad cholesterol (low density lipoprotein, or LDL) and decrease good cholesterol (high-density lipoprotein, or HDL), a net effect which makes them worse than saturated fats. While the latter increase the bad type of cholesterol, they also have a tendency to increase the good type, says Marie Cantwell, of DIT, who is investigating TFA intakes in Irish adults.
TFAs are contained in any food with the words "hydrogenated fat" on the packet. Hydrogenation is a process which makes vegetable oils spreadable in the manufacture of margarine spreads and which hardens oils to make them useful in the manufacturing of cakes and biscuits.
It's also important to eat more monounsaturated fats, which are found in olive oil, meat, chicken milk and dairy products - including butter, which might sound surprising. "You can't knock any food. All foods have bad bits and good bits in them. Steak can be great too - as long as you trim off the fat and don't eat it drenched in garlic butter," says Dr Roche.
If you are overweight enough for your doctor to be concerned, then the best approach is to get the help of a clinical nutritionist or dietician who is a member of the Irish Nutrition and Dietetics Institute. If you're just a few pounds overweight and want to lose the fat around your middle, try taking more exercise and cutting back on biscuits, cakes, confectionery and fried foods, leaving them as occasional treats. "To tell yourself that you can't have chocolate, for example, is unrealistic because chocolate-lovers cannot stick to that," says Dr Flynn. "When dieters break an overly rigid regime, they can tend to go to the other extreme and binge. The cycle of dietary restraint and bingeing is exactly the type of abusive relationship with food which characterises, at the severe end of the scale, eating disorders, and at the moderate end is typical of overweight people, who often don't enjoy food at any level."
The message seems to be: eat a little of what you love; cut down on fats but don't overdo it; when you do eat fat eat a variety of types of fats, including oily fish; and don't believe any advertising which implies that a particular type of fat will give you an edge on good health. A nutritious diet is all about balance and moderation and there is no such thing as a "good" or a "bad" food.