How much fat-free food can you eat and not gain weight?
None. Not unless you burn it off, anyway. Whatever you're eating, the formula stays the same: Consume more calories than you burn and you will gain weight.
Labeling products "fat-free" has been a great marketing concept. We've heard so much about fat being bad, and being fat being bad, that anything without fat sounds good.
A gram of fat contains 9 calories, while a gram of protein or carbohydrate contains only 4. So if you compare equal quantities of two foods, one containing fat and the other fat-free, the fat-free food will have fewer calories. But it still has calories.
Also, fat-free products are often filled with sugar to replace the flavor that is lost when the fat is removed. And as if that's not bad enough, without any fat to slow it down the sugar is absorbed into your blood faster. As explained in How Blood Sugar Levels Affect Weight Loss, this can have a significant effect on weight loss.
The problem with the way the public has accepted products labeled fat-free is that too often they treat them as if they are calorie-free. How many times have you heard someone say "it's fat-free" to justify eating all they want?
And then they become fat.