If you grew up in the 70s, you will recall the extreme emphasis on the low fat diet. Thank goodness that singular emphasis has gone by the wayside. Back then, the best science of the day indicated that saturated fat and cholesterol was the primary concern. Therefore, we were cautioned to reduce our consumption of things like beef, butter, and heavily marbleized meats from feedlot cows, which are full of saturated fat.
Here is a section from this websites page on Fats.
The Chemistry :
Different fats are different hydrocarbons. They have different levels of saturation, meaning a varying number of hydrogen atoms along their carbon chain structures. Fats can be classified as polyunsaturated, monounsaturated and saturated. More hydrogen atoms makes the chain more saturated, solid and more stable to temperature. Unsaturated fats are more liquid at a given temperature, and clog the arteries less.
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Back in the seventies, we were encouraged to substitute margarine for butter since it was made of less saturated fat, including some trans fats. Since then we have discovered the dangers of trans fats, which, have been prevalent in margarines. There is also concern that overly unsaturated so called polyunsaturated fats are easily oxidized , and then become unhealthy to consume.
More importantly, we have discovered the benefits of monounsaturates like olive oil, which reduce cardiac risk. Additionally we have come to understand the short chain fatty acids, like omega 3s which actually reduce risk of cardiac disease and also help with inflammatory disorders.
Finally we have come to understand that it is our carbohydrate intake that has contributed greatly to the problem of obesity, which leads to so many other problems including heart disease. In particular, our simple refined carbohydrate ( e.g. white flour products) and plain old sugar intake has been off the biological charts. These are almost always processed foods not found in nature and they enter our bodies very rapidly. We cannot metabolize them quickly enough and they are stored as fat. Additionally they can raise triglycerides, which in women, are more associated with heart disease than cholesterol.
Taking all of this into consideration, what are the current recommendations ? Roughly a quarter of our calories should be each from fat and protein, and half from carbs. But the devil's in the details. The fats should all be healthy ones like olive oil, avocado, and walnuts, and the carbs should be complex, unrefined, and nutritious. You probably think I mean whole wheat seedy bread. Actually I mean carbs even more complex than that: vegetables and fruits. Yes, they provide carbs too.
In summary, our current thinking on consuming fats, and avoiding becoming fat has changed quite a bit in the last 40 years. And quite frankly, we are eating quite a bit better because of it. Diet conscious folks used to run around with a diet coke, and slice of wheat toast and margarine for breakfast. These days, it would be more like an omelette with kale and fresh tomatoes, salsa, fresh fruit, green tea, and a small bowl of oat bran cereal with olive oil and sea salt. Bon appetite.