Most of the research (by Ansel Keys) has been debunked. This is the research that the US govt founded their 4 food groups recommendation on. In short, Ansel tried to do a multivariate study without a computer (he did this in the 60's), and he did not follow through on the analysis correctly. Therefore, he made a lot of incomplete assumptions. Therefore, most of what the govt has published in regards to diet is flawed. Also, the marketing of various special interest groups has served to perpetuate this knowledge, when sound and clear research disputes it.
In short, fat should be a major part of your diet. Carbs should not. Do the research. Don't listen to 'recommendations' or believe biased, non-scientific studies. If you look at the science illuminated by $300 million worth of government money, then you will find the truth. There is a ton of information out there. The government basically disproved most of Key's research, but they have not significantly altered their diet recommendations. There is too much money tied up in agricultural subsidies and special interestes for the politicians to let it go. They live off of that money.
I'll start with a simple, great research piece on high fructose corn syrup. This is a scientific, fact-based look at this sweetener. It also has lots of good information about high carbs and tryglicerides (sp?) that is not common knowledge. Fruit juices, sodas, and other products with high corn syrup components should be cut out of the diet. Fructose is metabolised like ethanol, and you get 8 of the 12 harmful effects from drinking too much fructose as you do drinking alcohol. Sucrose (table sugar) is much more easily handled by your liver than fructose.
http://www.youtube.com/watch?v=dBnniua6-oM
Also do a search on google for fat studies. You will find that eating fat satiates the body much faster than other types of foods. It REDUCES your total caloric intake. Both fats and vegetables are important staples. Carbs are not and should be limited. Some carbs inhibit leptin, which is your signal to your brain to stop eating.
In addition, there are good parts to LDL (the 'bad' cholesterol). Therefore, just looking at LDL levels is not an accurate predictor of cardiovascular disease. There are some LDL particles that are very healthy for you. In addition to healthy HDL.
In addition, your brain and nervous system survives on fat and cholesterol. So they are not bad for you by themselves unless you just ate nothing but fat all day. There are also studies that show countries with high intake of fat and low intake of carbs have lower levels of heart attack, Type II diabetes, and obesity. This is also true in the United States. Before Keys research, America had LOWER levels of cardiovascular disease, Type II diabetes, and especially obesity. The high carb, low fat diet is the main reason obesity has shot up in both the US and abroad in countries that have adopted the Western Diet.
In addition, the true scientific research shows that your cholesterol should be between 160 and 240. 200 was a number used in a research study as a SELECTOR of the population, but had nothing to do with a scientific study of cholesterol levels. The researchers just wanted to have enough subjects, and choosing a lower level would not have gotten enough for the study. Below 160, neurological problems start to manifest. In fact, there is very strong research that indicates cholesterol below 160 kills more people than cholesterol above 200. 240 is the cutoff for heart disease, where the odds go way up. So don't kill yourself if you are at 200 or 210. You could reduce it some, but the better move would be to exercise and cut carbs.
Also there is strong research in humans and animals that show polyunsaturated fats (vegetable oils, canola oil (which is actually rapeseed oil - a mustard plant), safflower, etc..) are not healthy. They do not raise LDL, but they do nothing for HDL. They have longer chain fatty acids that are harder for your body to break down. They also go rancid more easily when cooked, and in the body. This is a carcinogenic effect, so you could be raising your chances at having cancer.
Warm oils, such as coconut oil, do not go rancid when heated. They store much longer. Coconut oil will store for 5 years in your pantry. They have medium chain fatty acids, which are burned more easily in your body. And the saturated fat content makes you feel satiated much faster, thereby reducing your overall intake of food. Animal studies show that the ag industry stopped using coconut oil in their livestock because they got THINNER and LEANER. Ag industry uses corn and other grains because the animals will eat more and will get fatter. Humans do the same thing in response to vegetable oils and grains.
Lastly, not all fruits are good for you. Many fruits (bananas, grapes, others) are very high in sugar. The berries (strawberry, blackberry, blueberry) are low in sugar. This will help you regulate your sugar intake and insulin response and keep you from Type II diabetes. In addition, both corn and potatoes are very high in starch, which turns to sugar in the body. You get the same sugar content from a potato as you do from a soda pop. Don't think that you can eat any fruit or vegetable you want. That is simply not true. You have to know which fruits and vegetables are healthy for you.
I could go on and on, but trust me the scientific research is there. What you may think is the truth is probably just good advertising by those who stand to make money off of grains and sugars.