I've heard that people who eat a low-carb, high-fat diet don't gain weight. is this true?
It’s more accurate to say that weight gain is generally associated with our response to the foods we eat. Insulin is one of the most powerful responses to food, especially moderate and high glycemic foods. Up to 50% of these carbohydrates are turned to fat and are stored. The excess insulin response is associated with many signs and symptoms associated with lower quality of life, chronic diseases, and the worldwide overfat epidemic. This is a key focus in “The Big Book of Health and Fitness.”
We can certainly eat too many calories of anything and gain weight. This is true of fat, and even protein because some amino acids can convert to glucose and stimulate insulin. But overall, I don’t like counting calories.
Also, when talking about a “high” fat diet, it needs to be defined; some say a diet that’s 30% fat is considered “high.” I think this and even higher levels can be healthy for most people but only when carbohydrate intake is low or very low.
The issue of saturated fat is often confused. Some, but not all, saturated fatty acids (which make up saturated fat), can be harmful in higher amounts while other saturated fatty acids are very health-promoting (including as stearic acid from cocoa butter and lauric acid in coconut oil).
Your question about how we relate to fat is generally answered two ways: 1) moderation, and 2) balance. These issues are discussed in detail in many of my articles and books.