Testing Caloric Restriction and Ketone Levels with Cronometer & BIOSENSE

This is a guest blog post by Trey Suntrup, Head of Product for Readout Health.

Fad diets have been around for decades with some prove more beneficial than others. But what is the goal? For most people the goal is losing weight and living a healthier lifestyle.

Over the past few years, the keto diet has become popular due to its ability to burn fat fast. The science behind the keto diet is simple. After every meal, ingested carbs turn into glucose, which the body will use to produce energy. But, when the concentration of available glucose drops below a certain level, the primary fuel source switches from glucose to fat. This phenomenon is ketosis. The goal of the keto diet is to reach this state of ketosis.

But is the keto diet the only way to increase your ketone levels and burn fat? NO! Many other diets can put your body into a fat-burning state. Studies have shown that even simple caloric restriction leads to fat loss (1). Early breath acetone studies discovered that ketone levels can rise without carb restriction (2). Caloric restriction (or it’s more extreme cousin: fasting) is something that anyone can do. Cutting back in the amount of food and making healthy choices throughout the day can put your body into ketosis.

Testing Caloric Restriction and Ketone Levels

With this in mind, I decided to give caloric restriction a shot. I swapped my normal diet for meal replacements provided by an obesity management company. These meal replacements added up to exactly 800 calories per day with a macronutrient split of 30% carbs, 55% protein, and 15% fat. This was a dramatic reduction from my normal caloric intake of 3,000-3,500 per day.

I tracked my macronutrients in my Cronometer app and my breath ketone readings using the BIOSENSE breath ketone meter. Both of these products are industry leaders in data accuracy. Tracking my macros in the Cronometer app proved to be especially helpful because it put all my ketone readings into context. I was able to make genuine conclusions about how this new diet was affecting me.

For my experiment, my goal was to see how my body reacted to calorie restriction and if I could reach ketosis without cutting carbs.

Day 1

I started with my usual 3,000 calorie diet and tracked my macros. I wanted to find out what my baseline ketone levels looked like to see how my body reacted to the rapid change in diet. My ketone levels stayed in the region of 1’s and 2’s all day. To note: ketone levels of 0-2 ACEs (which are approximately equal to 0.1-0.2mM in blood ketones) will almost always appear regardless of your diet.  Even though my body was producing a few ketones in the background, I wanted to increase these levels further. Everybody has a different threshold, but on average, we switch to a fat burning state at around 5 ACEs.

Day 2

I had my first 800 calorie day, and man it was tough. I felt like I had less energy, and I didn’t feel the best. My ketone levels were beginning to rise into the 2 and 3 range, but not quite into ketosis. At this point, my body was still burning the stored glycogen in my muscles and had not started to tap into my fat stores. This explains why my ketones did not shoot up immediately. This 1-3 day transition period is typical for any diet or lifestyle change. The exact length of the transition period depends on how extreme the change is.

Day 3

The next day (Day 3), I woke up with a reading of 5 ACEs, which is the approximate threshold for ketosis. The very low-calorie diet (VLCD) I was following put me into ketosis after 24 hours.

I had a protein shake at breakfast which pushed me back down to an ACE level of 3 almost immediately. Protein can convert into glucose in the liver and blood, causing ketone levels to drop. Your body always wants to burn glucose, so as soon as that protein turned to glucose, my ketone levels went down.

I was able to recover my ketones throughout the day and began feeling better as my body entered ketosis. I went to bed with my body in ketosis at an ACE level of 5 and excitement to keep going.

Day 4

On Day 4, my body was in full ketosis. I was measuring breath ketone numbers of 7’s and 8’s, and I spent almost the whole day in ketosis. I was beginning to feel more energetic and less hungry. My ketone metabolism had kicked in and my body was now getting more used to an alternative fuel source. I was training my body not to want my usual carb and sugar loaded meals.

I ended the experiment at about 6:30 pm with a large, carb-loaded dinner. After dinner, my ketones dropped down to 3 ACE, kicking me out of ketosis. This shows how the body immediately wants to switch to burning glucose instead of fat every chance it gets; making fat loss difficult.

Wrapping up the Experiment

In the beginning of the experiment, I wanted to see if I could reach ketosis without following a strict low-carb diet. My results spoke for themselves as I was able to reach ketosis within 24 hours after cutting my calories. To put this into perspective, a 2 year-long study cut participants caloric intake ~20% for 6 months (as opposed to my 80% decrease in 3 days). On average, these patients experienced a 10% weight loss, most of which was fat. (3).

Before my experiment, I thought ketones equated to the keto diet, but in fact that is all wrong. Your body produces ketones when it burns fat instead of glucose. The state of ketosis can be entered from many angles.

Putting Data in Perspective

Below is a snapshot of my Cronometer app. Here I tracked my ketones and weight side by side (a feature available to Gold users). This feature allowed me to see direct results from my caloric restriction, and gave context to my ketone readings.

On Day 3, my weight dropped from roughly 234 lbs to 230 lbs. This was most likely due to water weight and loss of glycogen. But, by Day 4 extra weight loss was very likely due to fat loss. We can also see that my ketones continued to rise as my calories stayed at 800. We can see a correlation of ketone levels rising as caloric intake decreases. This experiment showed that caloric intake does affect ketone levels.

Before this experiment, I had never given any thought to tracking my ketones. I thought ketones were only important when following a strict keto diet. After this week, I understand that tracking ketone levels allows me to test any diet or lifestyle changes I want to make.

With Cronometer and my BIOSENSE device, I am able to examine my current diet and determine how my body is reacting. If you are struggling to find the right diet or lifestyle, tracking your ketones is a good first step. By tracking feedback, you can discover which foods raise or lower your ketone levels. This makes it easy for anyone to try a few things out and find the best possible fit for their body and lifestyle.



  • Ravussin, Eric et al. “A 2-Year Randomized Controlled Trial of Human Caloric Restriction: Feasibility and Effects on Predictors of Health Span and Longevity.” The journals of gerontology. Series A, Biological sciences and medical sciences 70,9 (2015): 1097-104. DOI:10.1093/gerona/glv057
  • Anderson, Joseph C. “Measuring breath acetone for monitoring fat loss: Review.” Obesity (Silver Spring, Md.) 23,12 (2015): 2327-34. DOI:10.1002/oby.21242
  • Dorling, J.L., Das, S.K., Racette, S.B. et al. Changes in body weight adherence, and appetite during 2 years of caloric restriction: the CALERIE 2 randomized clinical trial. Eur J Clin Nutr (2020). https://doi.org/10.1038/s41430-020-0593-8

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