If that's the case, what's the difference between eating 100 calories of carbs versus 100 calories of protein or 100 calories of fat?
I guess the big difference is that 100 calories of fat is really bad for your heart, but 100 calories of good carbs (whole grains) would be really good for your heart and digestive system! I think the main point is that if you cut out something really bad for you, like a three Snickers a day habit, but eat a bunch of low fat snacks instead, you are not necessarily going to lose weight. Not if you eat the same amount of calories in say 100 calories packs of cookies instead of candy. Granted, it is better for your health wise to cut out fat and sodium, but to lose weight, you have to cut calories. Not too hard really if you choose to snack on healthy items, you usually get a lot more food for the same amount of calories from a healthy option as opposed to a fat filled one!
100 calories of fat may actually be a lot more heart healthy than 100 calories of carbs - nuts for example, good oils, avocados etc. and the most recent diet research strongly suggests that shifting carb calories to fat calories can be very beneficial for people who want to lose weight. I'm not talking low carb - about a 40% carb, 30% protein and 30% fat.
One problem people have with fat is portion control. The calories mount up really fast because fat is so calorie dense.
It all depends on the kind of fat or carbs you are talking about. Yes, 100 calories of healthy fats (containing large amounts of mono- and poly-unsaturated fatty acids) are better for your health than 100 calories worth of sugar. Some good fats can actually lower bad cholesterol. On the other hand, saturated fats and trans-fats are a lot worse than starch and fiber. Fiber is very important to keep the digestive tract working properly.
Low-carb and protein rich diets are not healthy in the long run. The digestion of proteins creates some toxic by-products and the kidneys have to work hard to eliminate them in form of uric acid.
Portion control is a big factor in any diet. Eating a wide variety of healthy foods and watching the total calorie intake is the best method of losing and/or maintaining weight.
They have different nutritional content.