Why Are My Ketones Low During Fasted Exercise?

Julia Tulipan's Fasted Hike: What Her Ketone Data Showed

Julia Tulipan went on a strenuous mountain hike after fasting for about four and a half hours. Because she was fasted, moving for a long time, and climbing at a demanding intensity, she expected her ketone levels to rise clearly.

That is not what she saw during the hike.

While she was climbing, her ketones stayed low, around 0.3 to 0.4 mmol/L. For anyone who tracks ketones, that kind of reading can feel confusing. Fasting often increases ketone production. Long exercise can increase energy demand. So it is natural to expect the number to climb.

Julia had the same expectation. In the moment, the low reading was disappointing because it seemed to conflict with what she thought should be happening metabolically.

Then the pattern changed.

After she reached the hut at the top of the climb, sat down, and rested for about 10 minutes, her ketones rose to above 1.5 mmol/L. The shift was fast enough to change how she understood the earlier low reading. Instead of seeing the low number as a simple failure to make ketones, she began to think about the timing of ketone production, ketone use, and energy demand.

That is the useful lesson from this case. The number seen during exercise may not tell the full story of what is happening before, during, and after the activity.

Why Ketones Can Stay Low During Fasted Exercise

A ketone reading shows concentration at one point in time. It tells you how much ketone is present in the measured fluid at that moment. It does not directly show how much ketone your body is producing, how much your tissues are using, or how much fat you are oxidizing.

That distinction matters during exercise.

When you are at rest, a ketone reading may feel easier to interpret because energy demand is relatively stable. During a strenuous hike, workout, or long climb, the body is under changing demand. Muscles need fuel. Hormones shift. Circulation changes. The body may be drawing from several fuel sources at once, including glucose, fatty acids, and ketones.

Julia interpreted her pattern as a balance between ketone production and use during the climb. One possible explanation is that ketone production and ketone use remained relatively balanced while she was hiking. If ketones were being made and used at the same time, they may not have accumulated enough in the measured reading to create a high number.

This is different from saying that the low reading proves she was burning ketones efficiently. Continuous ketone monitors, blood meters, breath analyzers, and urine strips do not directly measure ketone production rate, ketone use rate, or fat oxidation rate. They measure signals that need interpretation.

The rise after rest may also fit this pattern. When Julia stopped hiking and sat down, her immediate energy demand dropped. Ketone production may not have fallen at the exact same speed. For a short period, ketones may have accumulated more visibly, which could explain why her reading rose above 1.5 mmol/L after she stopped moving.

The key point is not that everyone will see this exact pattern. The key point is that ketone readings are dynamic. During fasted exercise, a low number can have more than one explanation.

What Low Ketone Readings Do and Do Not Mean

A low ketone reading during exercise does not automatically mean keto is not working.

It also does not automatically prove that your body is using ketones efficiently.

Both interpretations are too simple. Ketone numbers need context. A reading during a workout can be influenced by the type of exercise, intensity, fasting duration, recent food intake, sleep, stress, hydration, adaptation level, and individual metabolic condition.

A higher ketone reading is not always better, either. Higher ketones may reflect more ketone availability, but they do not automatically mean better fat loss, better metabolic health, or better exercise performance. In some situations, a high reading may simply mean ketones are accumulating faster than they are being used. In other situations, it may reflect a normal fasting response.

That is why Julia's hike is useful as a teaching example, but not as a rule. Her experience shows that low ketones during fasted exercise can happen, and that the number can change quickly after rest. It does not prove that every person will see the same response, or that every low reading during exercise has the same meaning.

For most lifestyle and educational tracking, the better question is not only, "Is this number high or low?" A more useful question is, "What was happening when this number appeared, and what happened next?"

For people with diabetes, those taking glucose-lowering medication, pregnant or breastfeeding individuals, or anyone with a complex medical condition, ketone interpretation should be discussed with a qualified healthcare professional. Consumer ketone tracking is not a substitute for medical care.

How to Interpret Ketone Patterns Around Exercise

The most useful way to interpret ketones around exercise is to separate timing.

A reading before exercise tells you something different from a reading during exercise. A reading shortly after exercise may tell a different story again. Several hours later, the pattern may shift once more as recovery, food intake, hydration, and stress hormones change.

Instead of reacting to one unexpected number, compare patterns under similar conditions. Ask:

• Was the workout similar in type and intensity?
• Was the fasting window similar?
• Was the previous meal similar in carbohydrate and protein content?
• Was sleep, stress, or recovery noticeably different?
• Did the same kind of workout produce a similar ketone pattern more than once?

This approach is more useful than chasing one ideal number. If your ketones are lower than expected during a workout, that single result does not need to change your entire keto plan. It may simply be a data point that becomes meaningful only when you see what happened before and after.

Keeping notes can help. Record workout timing, approximate intensity, meals, fasting window, sleep, stress, and recovery. The notes do not need to be complicated. Even simple context can make ketone data easier to understand.

The goal is not to test constantly or make exercise feel like a lab project. The goal is to avoid overinterpreting one reading when the body is in motion.

What Continuous Data Revealed in Julia's Case

Julia's case is a good example of why continuous data can change interpretation.

If she had tested only once during the hike, she might have seen a reading around 0.3 to 0.4 mmol/L and concluded that she was not in the ketosis state she expected. That single reading would have captured one moment, but it would have missed what happened next.

Because Julia was using SIBIO CKM during the hike, she could see the broader timing of the pattern: low readings during the climb, followed by a rapid rise after she stopped and rested. The value was not only in seeing a number. It was in seeing the direction and timing of change.

That kind of trend view can be especially helpful when interpreting meals, fasting, exercise, sleep, or stress. A single spot check may answer, "What is my ketone level right now?" Continuous data can help answer a different question: "How is my ketone pattern changing over time?"

Still, the same caution applies. Continuous ketone monitoring does not directly measure ketone production rate, ketone use rate, or fat oxidation rate. It also does not diagnose medical conditions or replace clinical guidance. Its value in a lifestyle context is helping users see patterns that would otherwise be easy to miss.

In Julia's case, the fuller pattern made the low exercise reading less confusing. The number during the climb was only one part of the story.

Final Takeaway

A ketone reading is most useful when viewed in context. Julia's hike shows that the number seen during exercise may not tell the full story of what happens before, during, and after activity.

Low ketones during fasted exercise do not automatically mean keto is not working. They also do not automatically prove efficient ketone use. The more useful approach is to look at timing, conditions, and trends, then interpret the number as part of a larger metabolic pattern.