I believe the insulin-centric explanation (I'm a low carb eater and this is the perspective I tend to see) would be something like:
- Low carb and high fat: The fat goes to your fat cells (that is, whatever fat you don't burn for fuel immediately after eating), but since your insulin is low the fat is readily burned off relatively soon.
- Low fat and high carb: The carbs cause an insulin spike, and insulin inhibits fat burning, BUT since you haven't stuffed much fat into your fat cells, there isn't much to burn off when your insulin eventually comes back down. You need to be relatively insulin sensitive, so that your insulin level does come back down. Even if you're insulin resistant, this diet might still work if you eat only a tiny amount of fat.
- High fat and high carb: Fat goes to your fat cells and insulin levels are high, which inhibits fat burning, so you gain fat faster than you burn it.
(Edits: I thought Markdown lists would work but they didn't.)
- The fat you eat does not have to be stored as fat, the raised insulin levels are necessary for the body to store energy as fat.
"Low fat and high carb: The carbs cause an insulin spike, and insulin inhibits fat burning, BUT since you haven't stuffed much fat into your fat cells, there isn't much to burn off when your insulin eventually comes back down. You need to be relatively insulin sensitive, so that your insulin level does come back down. Even if you're insulin resistant, this diet might still work if you eat only a tiny amount of fat."
- Carbs can be converted to fat via lipogenesis.
stored in muscles as glycogen - however, the muscle stores can only store up to 1600 calories so they may be full. Also, once sugar gets into a muscle it can't get out - it can only be used by that particular muscle.
used by the brain - however the brain doesn't need more than 500 kcal per day. (and can live with as little as 120 kcal / 30g of glucose provided that the rest of the energy is supplied by ketones)
So... exercise doesn't help lose weight, but it sure helps prevent weight gain when eating carbs. With the amount of carbs we eat nowadays and the low amounts of physical activity, its no wonder at all we store a lot of fat.
I remember seing a book about that diet 20 years ago by a singer:
And, the example meal plan on the same first result page is for a 1200 calorie diet! That's a massive caloric deficit for any adult; of course you'll lose weight if you follow it!
I was specifically talking about a study on sugar and fat and how they might trigger cravings and overconsumption, and possibly also cause metabolic effects that can cause weight gain and other problems, and not at all about alkaline/acidic food combining theories.
I think there's a lot of superstition in general, but one could certainly have independently noticed that eating sugar or fat alone (say butter or honey) is somewhat self-regulating. You get sick of it pretty quick. But, put them together in a pie or pastry or donut or something and suddenly it gets really easy to eat hundreds of calories worth of sugar and fat in a few quick bites.
So, a keto diet is fine. Eating bread every now and then is fine. But, something like a fast food meal consumed with a sugary soda may be particularly unhealthy and may cause you to eat more and crave more similar foods.
I don't know all the mechanisms at play, and I don't think even experts have a really good understanding of how our very complex digestive and metabolic systems (and the bacterial colonies that live in our guts) interact. It's clear that caloric reduction works for weight loss, even if you're eating mostly carbs or mostly fats. Some people report better management of cravings and hunger on different kinds of diets, and the research indicates that a low-fat or low-carb diet results in weight loss at about the same rate (contrary to popular belief about low-carb diets lately).
Source: https://www.washingtonpost.com/lifestyle/wellness/low-carb-v...
But, really, if you run a caloric deficit, you'll lose weight. Cutting arbitrary foods can do it for some folks. Some folks like keto or other relatively extreme restrictive diets. Some folks do intermittent fasting (I've been intermittent fasting for several months and have lost a little over 20 pounds). No matter how you do it, if you cut 500 calories off of your diet, you'll lose weight at a healthy clip.