Fasting Ketones: How Quickly You Get Into Ketosis (With Real Data)
Hey folks! I’d like to share my actual data on how fast I got into ketosis during my recent extended fasts. Thought it might be interesting for anyone tracking fasting ketones or curious how long it takes to reach deep ketosis.
What Happens to Glucose and Ketones During Fasting
When you stop eating, your body gradually switches from using glucose (sugar) to using fat for energy. As fat is broken down, your liver produces ketones, which become the main fuel for your brain, muscles, and other organs.
Here’s the key relationship:
When glucose drops, ketones go up.
When glucose rises again (like after refeeding), ketones fall.
You can see this perfectly in my fasting data below.
Glucose levels drop during a 10-day water fast.
Ketone levels rise during a 10-day water fast.
During my 10-day water fast:
Glucose dropped from 108 mg/dL to the 60-70 range, hitting as low as 42 mg/dL at one point.
Ketones climbed from 0.4 mmol/L to 8.0 mmol/L by Day 10.
As soon as I started eating again, glucose spiked and ketones crashed.
That’s textbook fasting physiology in action.
How Fast I Got Into Ketosis During Different Fasts
Time to reach deep ketosis during three extended fasts: activity and glycogen levels make a big difference.
Here’s how my 7-day, 9-day, and 10-day fasts compared:
7-day fast (Nov 2024, blue line): hit 5.0 mmol/L at around 60 hours (2.5 days)
9-day fast (Feb 2025, green line): took 150 hours (6+ days) to reach 5.0
10-day fast (Aug/Sep 2025, red line): reached 5.0 mmol/L at 60 hours
A couple of interesting observations. The 9-day fast was tough and slow to reach ketosis. I probably started with full glycogen stores and wasn’t very active. The 10-day fast, on the other hand, was much easier. I worked out - cardio and strength training - early in the fast and got into deep ketosis quickly, basically skipping the usual “day 3 crash” that that happens when glycogen runs out and the body switches fully to ketones. So activity level before and during fasting seems to speed up the transition into ketosis.
Body Fat Loss and Ketone Levels
There’s a clear link between ketone levels and fat burning. The higher my ketones, the more body fat I lost:
7-day fast: lost 4.6 lb of fat
9-day fast: lost 3.8 lb of fat
10-day fast: lost 5.5 lb of fat
More activity + faster ketosis = more efficient fat loss.
Understanding Ketone Zones
To put the numbers in perspective, here’s how Keto-Mojo defines ketone ranges:
0.5–1.5 mmol/L: Nutritional ketosis (typical low-carb state)
1.5–3.0 mmol/L: Moderate or weight-loss zone
3.0–5.0 mmol/L: Optimal fasting or therapeutic ketosis
5.0–8.0 mmol/L: Deep fasting ketosis
Above 8.0 mmol/L: Very deep fasting or rare ketoacidosis (only if glucose is also very high)
All my extended fasts landed solidly in the deep fasting ketosis range.
Different ketone zones – from nutritional ketosis to deep fasting ketosis.
What About Ketoacidosis?
Ketoacidosis is a dangerous and completely different metabolic state. It happens mostly in people with type 1 diabetes (and occasionally type 2) when insulin is extremely low and glucose is extremely high – often above 240 mg/dL. In that state, the body keeps producing ketones uncontrollably while blood sugar skyrockets, making the blood too acidic. It’s a medical emergency.
In contrast, fasting ketosis is a controlled, physiological process. During fasting, insulin drops but never disappears completely, glucose stays low or moderate, and ketone production is self-regulating. My glucose was between 42 and 108 mg/dL, never even close to dangerous diabetic levels. That’s why even with ketones up to 8.0 mmol/L, I was perfectly fine and not in ketoacidosis.
In short, ketoacidosis is uncontrolled, dangerous, and high-glucose, and fasting ketosis is natural, safe, and low-glucose. That’s a big difference!
My Takeaways
Movement matters. Physical activity early in the fast accelerates the switch to fat metabolism.
Ketones mirror fat burning. When ketones rise, you’re tapping into fat stores efficiently.
Every fast is different. Starting glycogen, sleep, stress, and activity level all affect how quickly you enter ketosis.
Fasting isn’t starvation. As long as you have body fat, your body knows how to use it as clean fuel.
Deep fasting ketosis is not ketoacidosis – your body is still fully in control.
If you’re tracking your own fasting data, pay attention to both glucose and ketones – the inverse relationship tells you exactly where your body is in the fasting cycle.
Final Thoughts
Fasting ketones are more than just numbers – they’re a real-time indicator of how efficiently your metabolism switches from sugar to fat. Watching glucose fall and ketones rise during fasting gives a fascinating look into your body’s adaptability.