There's growing evidence that eating fat won't make you fat, but sugar will
"In the World Cup of food wars, sugar and fat are the playoff teams.
It makes intuitive sense that rich foods like creamy avocados or butter-drenched salmon would contribute to weight gain. Similarly, stacking up on sweet treats like sugary cereals and carb-heavy bagels can't be too great for your waistline, either."
"There's growing evidence that eating fat won't make you fat, but sugar will" source
"But scientists are increasingly examining what happens inside our bodies when we regularly indulge in large amounts of either sugar or fat on their own.
In many parts of the world, the two ingredients are rarely eaten alone. Take doughnuts as an example. When the fresh carb-laden dough is deep-fried in oil, you get a classic combination of sugar and fat with a rich flavor and powerful mouthfeel that's tough to pass up.
But an increasing body of evidence is beginning to suggest that when eaten in isolation, fat doesn't contribute to weight gain. On the other hand, dozens of studies indicate that sugar alone is significantly tied to packing on pounds.
Aaron Carroll, a professor of pediatrics at the Indiana University School of Medicine, put it this way in the recent book, "The Bad Food Bible: How and Why to Eat Sinfully."
"There is one thing we know about fats," he wrote. "Fat consumption does not cause weight gain. To the contrary, it might actually help us shed a few pounds."
That means that foods like buttery avocados, rich salmon, and savory nuts should probably have a place in your diet. If you banned them during the low-fat dieting craze of the 1990s, it's time to bring them back." source, read more