Body Fat vs Dietary Fat on Keto: How Do You Burn Your Own Fat?

2017-07-25

Someone asked me how you get your body to run on its own fat instead of the fat you eat. This two-and-a-half-minute answer from July 2017 was never put on YouTube. It's short, and it holds up well. A month later it became a whole video on eating too much fat.

In this recording

[0:01] The question: how do you run on body fat rather than dietary fat? There's no real difference between the two. At first you break the cycle of glucose spikes and insulin spikes that drives cravings, by filling up on fat and protein.
[0:01] As you become fat adapted, ketone bodies are around, fat is broken down throughout the body, and your craving for that dietary fat goes down. What drives your hunger now is the carbohydrate and the insulin response to it.
[0:50] Eat fat and protein to stay full and that cycle goes away. You get less hungry in general. Protein needs stay steady for your size and muscle, so the drop in what you eat comes mostly out of fat, and your body makes up the gap from body fat.
[1:33] You still need a calorie deficit to lose weight, but you don't have to get there by forcing restriction. You let hunger fade and fall into a deficit naturally.
[1:33] A group slogan I liked: if you're already high fat, all you need is low carb. If you have plenty of body fat, you can rely on it once you're fat adapted.
[2:17] People who are already very lean may need more dietary fat because they're running low on stored fat. For most of us trying to lose weight, there's plenty of stored fat to support ketosis.

Drill-down: the deficit comes from hunger, not willpower

The most important sentence here is that you still need a calorie deficit, but you don't have to get it by restricting. That was my whole approach in 2017. Ten days later I made a video about it, and the line there was that a calorie deficit matters but forced calorie restriction isn't a good idea (video 8Jk5ShSUpFY).

The order matters. Early on you eat fat and protein to fullness, because the job in week one is getting past the cravings, not cutting calories. Once hunger drops, you eat less without fighting it, and protein stays put. What shrinks is the fat on your plate. That's why "eat fat to satiety" is good beginner advice and bad long-term advice. Once you're adapted, you stop adding fat you aren't hungry for.

The evidence pages support the "protein keeps you full" half of this. In a 12-week study, raising protein from 15% to 30% of energy, with carbohydrate held the same, cut how much people ate on their own by 441 kcal a day, and they lost 4.9 kg (Weigle 2005).

Where the literature adds a caveat

"There's no difference between dietary fat and body fat" is true in the sense I meant it: fat is fuel, and your body will use whichever is around. It doesn't mean eating less carbohydrate burns more body fat than eating less fat, calorie for calorie.

The evidence pages have a direct test of that. In a metabolic-ward study of 19 adults, cutting the same number of calories from carbohydrate led to 53 g of body-fat loss a day, while cutting them from fat led to 89 g a day (Hall 2015). It was a short ward study, not real life, but it fits what I said: the payoff of low carb is the hunger, not a fat-burning trick. If you eat more fat than you need, there's less room for your body fat to make up the gap.

One small fix: in the clip I said the liver produces the enzyme that breaks down fat. What the evidence pages describe is the liver turning fat into ketone bodies, regulated by insulin and glucagon (StatPearls, Ketogenesis). The main point, that lower insulin lets you use stored fat, stands.

Then vs now

By the end of 2019 I said the same thing more bluntly: you don't need any special amount of fat for ketosis, because your body has plenty unless you're underweight (live FPMJ7326). The lean-person exception in this 2017 clip survived intact.

If you take insulin or another medicine that lowers blood sugar, eating fewer carbs can change your dose. Talk to your doctor before you start.

Key points

Your body uses fat as fuel whether it came from your plate or from your fat stores.
Early on, fill up on fat and protein to break the craving cycle.
Once hunger fades, keep protein steady and let dietary fat drop. Body fat fills the gap.
You still need a deficit, but let it come from lower hunger, not forced restriction.
Very lean people are the exception and may need more dietary fat.