Be in ketosis!

How to determine the state of ketosis with a continuous glucose monitor, without blood sampling

The essence of nutritional ketosis is simple: when the body does not get enough glucose, the liver produces ketone bodies, mainly beta-hydroxybutyrate (BHB), from fatty acids.

The question is not how it works, but how to know if this state already/still exists.

 

Accurate measurement and its practical limitations

The accurate determination of ketone levels is still only possible from capillary blood.

Zones used in clinical practice:

• 0.0 – 0.5 mmol/L: baseline ketone level, no ketosis

• 0.5 – 1.5 mmol/L: slow fat burning, beginning of nutritional ketosis

• 1.5 – 3.0 mmol/L: moderate, optimal fat burning

• 3.0 – 5.0 mmol/L: rapid fat burning

• Above 5.0 mmol/L: starvation ketosis, above 10 mmol/L risk of ketoacidosis 

Blood BHB measurement is accurate but cumbersome. It requires finger pricking, test strips, and multiple daily measurements, so few people do it regularly in everyday life.

Urine acetone strips indicate with a delay of 12-24 hours, and breath acetone measurement shows individual variation.

In contrast, continuous glucose monitors (CGM) are now widely available, provide minute-by-minute data, and are non-invasive.

 

Two related curves

The physiology of water fasting provides a well-reproducible pattern.

The first correlation is the increase in ketone levels over time:

• 24 hours fasting: ∼1.0 mmol/L – fat burning initiated

• 36 hours fasting: ∼2.0 mmol/L – optimal fat burning

• 48 hours fasting: ∼3.0 mmol/L – rapid fat burning

 

The second correlation is the mirror movement of blood sugar and ketones over 72 hours. As blood sugar decreases, ketones increase, as both are regulated by the same hormonal environment – low insulin, elevated glucagon.

 

Measured blood glucose and ketone average values during water fasting

This inverse proportionality makes estimation possible.

How can ketosis be estimated from CGM data?

The table does not claim that glucose equals ketones. It shows what glucose pattern is associated with what ketone zone.

The estimation is based on three parameters:

1. Average fasting glucose

If the average fasting glucose is between 4.0–5.5 mmol/L for 3 consecutive days, and there are no post-meal peaks above 5.5 mmol/L, this most likely corresponds to an optimal ketone zone of 1.5–3.0 mmol/L.

2. Glucose variability

In ketosis, daily variability is low. A CV (coefficient of variation) below 15% and nocturnal fluctuations below 0.3 mmol/L indicate the onset of nutritional ketosis.

3. The 24–48 hour rule

If the average falls below 4.5 mmol/L after 24 hours, fat burning has begun. If it stabilizes below 4.2 mmol/L after 36 hours, we can speak of an optimal zone; if it remains below 3.9 mmol/L after 48 hours without symptoms, we are in rapid fat burning.

In practice, therefore, laboratory accuracy is not needed, but rather zone classification. To answer whether the body is in ketosis, the combined evaluation of CGM trends and the table above is sufficient.

 

When is the estimation not applicable?

The glucose-ketone relationship cannot be evaluated in cases of type 1 diabetes, SGLT2 inhibitor treatment, pregnancy, or extremely high protein intake (>2.5 g/kg body weight). In such cases, only blood BHB measurement is authoritative.

 

Summary

Accurate ketone levels can only be determined from blood, but the presence of ketosis can be estimated from continuous glucose monitor data, if the 72-hour inverse dynamic of blood glucose and ketones is known.

Glucose levels corresponding to 1.0 mmol/L at 24 hours, 2.0 mmol/L at 36 hours, and 3.0 mmol/L at 48 hours provide a guide as to whether the body is in the baseline, slow, optimal, or rapid fat burning zone.

Staying in ketosis is greatly facilitated by the use of allulose.

May the Heavens and ketosis be with you!

 

 

This article is for educational purposes only and does not constitute medical advice. Consultation is recommended before extended fasting or dietary changes.