Ask Dr. T: How to tell if I’m in sugar or fat burning mode?

Ask Dr. T


“How do I know if I’m still in fat burning mode, or if I’ve shifted back to sugar burning mode?”

This was the excellent question posed to me earlier this week by a patient in my clinic. After reading the book, she’d adopted an ancestral diet, and was eating around 20 grams of carbohydrate per day. Ultimately, she planned to increase her dietary carbs a little bit, but wanted to make sure she remained in the ever-desirable land of adipocentric metabolism, aka “fat burning mode.”

The short answer to her question: if you start noticing you can no longer go longer periods of time between meals, especially if you experience that ravenous “I have to eat now” hunger you used to experience, you’ll know you’re back to being a sugar burner. You’ve found the tipping point for your daily carbohydrate allotment.

To explain this answer, I’ll first provide a little context.

A Multi-Fuel Stove

We humans have two primary sources of fuel we can burn to meet our body’s energy needs: fat and sugar (i.e. carbohydrates). Like a car that can run on either gas or electricity, we too have more than one way we can power our machine.

And ultimately, there are two places we can get that fat and sugar from:

1. The fat and sugar in our diet.

2. The fat and sugar stored in our bodies. 

In other words, we can use the food we just ate – either the fat or sugar in it – as a source of fuel. Or we can used stored forms of fat and sugar in the body.

Every second, gazillions of chemical reactions are taking place in the cells of our body – reactions that require energy. Without a steady supply of fuel to power those reactions, we’re toast. So making sure we have enough energy at any given moment is the highest kind of biological priority.

After a meal, things are easy. We burn through the energy we just ate. What we don’t need immediately is stored for later, mostly as fat (regardless of whether that energy is in the form of dietary fat or carbohydrate).

After we burn through the energy from that last meal, the energy supply starts to dip. At this point, our body has two options for continuing to maintain an adequate energy supply.

1. Eat more food.

2. Mobilize stored energy for food.

Now, in humans, virtually all of our stored energy is in the form of fat. Yes, there’s a little bit of stored sugar in the liver and muscles, but not much. Not even enough to power our bodies for a day. Even those with a healthy amount of body fat have enough stored energy to power their bodies for over a month.

But there’s a catch. To use that stored fat, we’ve got to get it out of the adipose tissue and into the bloodstream. These things don’t just happen automatically. In fact, lots of stuff has to occur in the body to make that happen.

The technical term for this is metabolic flexibility, defined as your ability to switch back and forth between sugar and fat for fuel based on availability and need. Being metabolically flexible means you do this well.

It appears that one of the many unfortunate consequences of the standard western diet high in refined carbohydrates and plant toxins is that we become metabolically inflexible. Part of this means we become poor at accessing stored body fat, even when doing so would be the most sensible thing for our body to do. In other words, when we burn through the energy from our last meal and are looking for the next source, option two from above isn’t available. The brain (the hypothalamus specifically) senses an impending energy crisis, and, despite having enough stored energy to fuel your body for at least a month, thinks you must eat now. The result: ravenous hunger.

This is what happens when you place a brain adapted to a hunter gatherer diet in the modern world.

On the contrary, one of the consequences of an ancestral diet is that it promotes metabolic flexibility. When you eat the way your brain expects you to eat (i.e. an ancestral diet), you start becoming quite good at accessing that stored body fat. And so as time passes after a meal, instead of becoming ravenously hungry, you just tap into those energy stores and keep right on going. This is the reason why obesity is not an issue in indigenous hunter gatherer societies, and is also the reason why eating the way your brain expects you to eat is the best way to lose stored body fat.

Who needs lunch?

The Body Knows

Many who eat this way find eating two, or even one, meal a day easy to do. Something that would’ve seemed preposterous on the standard western diet. The three meals a day norm is an artifact of modern life and culture, not a biological imperative.

There are scientific ways of precisely measuring metabolic flexibility, the most common of which is the respiratory quotient, which can give a close approximation of how much fat and sugar you’re burning at any given time. This would be one way of telling whether you’ve shifted out of fat burning mode.

But for our purposes here, that’s not really necessary. Fortunately for us, our body is capable of giving us the signal we need, provided we listen.

So if you find yourself in a similar position as my patient who posed this question, of wanting to try to increase dietary carbohydrate without sacrificing the many health benefits of being an efficient fat burner, then listen close to those signals. If you’re dialing up your daily carbs and suddenly find yourself scouring the pantry for something to “get your sugar up,” you’ve probably found your limit.