Kristi Storoschuk, BSc (Hons.) is a science communicator with a research focus on ketogenic diets, metabolic therapies, and fasting for health optimization. She currently works alongside the world’s leading ketogenic researchers providing scientific education for the mainstream audience. Outside of her research, you will find her traveling the world, doing CrossFit, and optimizing her health through diet and lifestyle.

By Dom D’Agostino and Kristi Storoschuk

The short answer, as it so often is: “it depends on your goals.” 

In order to understand the value of measuring fasting ketone levels, we first need to cover a bit of human physiology.

Humans are fuelled by one of two primary fuel sources: glucose (carbohydrates) and fat. While we’re always burning a bit of both (unless you are following a ketogenic diet), glucose tends to dominate the fed state, and fat tends to dominate the fasted state. So, what if there was a way to measure when we start burning fat, to get some actual proof that our bodies are prioritizing fat for fuel? 

As you may know from the Fasting Zones feature in Zero Plus, fuel usage while fasting works in a progression: first, glucose available in circulation, then glucose stored as glycogen, then fat. As the body starts breaking down body fat, it’s also starting to make ketone bodies. 

Ketones are a way to deliver energy to the brain since the brain can’t use fat directly as fuel. They also lower how much glucose the body needs, sparing our muscles from being broken down to glucose when the body demands fuel. 

Tracking ketones can provide a peek into the inner workings of your metabolism, offering a glimpse into what fuel source the body is using for energy. This is a bit of a simplification, but generally speaking, in a fasted state, the more ketones you are producing, the more body fat you are burning. During a prolonged fast, blood ketones will eventually stabilize around 3-7 mmol/L (higher in obese individuals) after a week or so of fasting. 

Since we do not recommend anyone fast for longer than 7-days unless medically supervised, you can expect to see your ketones continue to rise while using Zero.

The message we want to get across is that if you are making ketones while fasting, you know you are burning body fat; you’ve made the switch from being a carb-burner to being a fat-burner. There’s nowhere else those ketones could be coming from! This means that measuring ketones during a fast can be indicative of fat loss. 

The same cannot always be said for a ketogenic diet (an alternative way to elevate ketones) as you are supplying fat from the diet rather than from your own “fuel reserves” of fat. So you can still be consuming more calories than you’re burning and the ketones could be coming from newly-ingested fat rather than stored fat. 

Many of the benefits of fasting ramp up once we’ve made this metabolic switch from glucose to ketones, and there are several reasons why we might care about ketone levels beyond weight loss, including improvements in overall metabolic health.

There are few different methods for measuring ketones and they mainly diverge based on which ketone body we are testing for. The body produces three primary ketone bodies when we are in ketosis: acetoacetate (AcAc), beta-hydroxybutyrate (BHB), and acetone.

If you are pricking your fingers to test blood ketones with a blood ketone meter, you are likely measuring BHB. This ketone body represents ~78% of the ketones we produce because it’s the most stable and easily transported in our blood since this is how ketones make it from our liver (site of origin) to the rest of our tissues. 

If you are using a breath ketone reader such as the Biosense breath acetone device*, you are likely measuring AcAc. While we are in ketosis, AcAc breaks down to acetone which can be detected in our breath. The more acetone in your breath, the deeper your state of ketosis. Those who practice prolonged fasting or follow a ketogenic diet may be familiar with the “fruity” breath that can occur with ketosis - a high level of acetone is responsible for this. 

You can also measure ketones (AcAc) in your urine using urine ketone strips, but the accuracy of this method varies. Whereas breath acetone correlates well with blood BHB levels, urine ketones can vary due to the influence of hydration status and keto-adaptation (the better you are at burning ketones, the less will be excreted as “waste” in your urine).

*Biosense is an FDA-registered patented tool and is the only breath ketone monitor backed by clinical data.

Blood and breath ketones are the two most accurate ways to measure ketones, but when it comes to fasting, there are definite advantages to measuring ketones in your breath. For one, there’s no blood involved - it’s far less invasive than pricking your finger throughout your fast. It might also provide a more accurate measure of fat burning -- there is research showing a relationship between breath acetone and fat loss.

During a prolonged fast, we are burning through BHB and the amount in our blood might seem lower than what’s actually being taken into our cells for fuel. Acetone on the other hand will rise in proportion to the amount of fat we are burning during a fast. 

A simple proof of concept for this is what happens to our ketone levels during fasted exercise. Typically, ketones will drop since you’re burning through them at a higher rate, but acetone will rise, representing increased fat-burning. If you were to measure blood ketones, it would look as if exercise kicked you out of ketosis, but in reality, exercise is supporting and expediting ketosis, which breath ketones reflect.

You can use ketone measurements to identify how long it takes you to make the transition into ketosis and maximize the benefits of your fast. Maybe you ate a high-carb meal the night before, and it’s taking longer to get into the Fat Burning Zone. Or maybe exercise causes ketones to rise sooner. 

Measuring ketones while fasting is immediate feedback that can help you learn which types of exercise accelerate your fasts, the impact of sleep, and how your diet leading into your fast influences ketone production. This way you can optimize your fasting regimen to suit your goals and correlate stages of your fast with subjective feelings of overall well-being. 

The Benefits of Measuring Ketones While Fasting (and How to Do It)

The Benefits of Measuring Ketones While Fasting (and How to Do It)

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