It might not be fat or sugar that is leading to the massive explosion in rates of obesity but our exposure to ultra-processed foods that is driving first world weight gain. Expose volunteers to ultra-processed vs unprocessed foods and they consume about 500 calories more of the former making them put on weight.
Scientists suggest this might be because these foods trigger scrambled brain messages leading to a lack of satiety awareness making us eat more than we need to. Natural sweeteners like honey and maple syrup turn on pleasurable sensations and the preparation to eat the calories associated with this experience whereas sugar substitutes simulate this pleasure but when we eat less this circuit is not fully realised and we are compelled to keep on eating. This might explain why a brownie made with sugar alternatives might not be as satisfying as the real deal.
Ultra-processed foods that contain combinations of fat and sugar not found in the natural world stoke cravings rather than fulfillment making us eat more.