Fat Burning and Ketones During Fasting: UK 2026 | Complete Nutrition

Understanding Fasting

How your body burns fat and produces ketones during a fast

Glycogen stores drop through the first 12 to 24 hours of fasting. Fat mobilisation from adipose tissue rises as insulin falls. The liver converts some fatty acids into ketone bodies which start appearing in blood from 16 to 24 hours and reach nutritional ketosis levels by 48 to 72 hours in most people. This metabolic shift is the basis of fasting physiology. Standard 16:8 fasting produces meaningful fat oxidation but not deep ketosis.

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Updated:
May 2026

Written by:
Dominic Walton, MD

Reading time:
7 min

The full answer

The fuel switch in detail

The body burns a mix of fuels at all times. Fasting shifts the proportions toward fat and ketones. Four phases describe what happens.

1. Hours 0 to 8: glucose dominant, glycogen breakdown begins

After eating, glucose from food fuels most tissues for several hours. Insulin remains elevated and inhibits fat mobilisation. As blood glucose returns to baseline (typically 3 to 5 hours after a mixed meal) the liver starts breaking down its glycogen stores to maintain blood glucose for the brain and other obligate glucose users. Insulin falls. Fat oxidation begins to rise slowly. By 8 hours of fasting glycogen breakdown is the dominant glucose source.

2. Hours 8 to 16: glycogen depletion, fat oxidation rises

Liver glycogen capacity is around 100 to 120 grams. During fasting this depletes at roughly 5 to 7 grams per hour for glucose maintenance. By 12 to 16 hours of fasting most liver glycogen is gone. Muscle glycogen remains but is not released into blood. Insulin is low. Fat mobilisation from adipose tissue accelerates. Fatty acids become the main fuel for most tissues including muscle. The brain still needs glucose which is now produced by gluconeogenesis from amino acids and glycerol.

3. Hours 16 to 48: ketogenesis ramps up

As fat oxidation continues the liver receives more fatty acids than it can fully oxidise. Excess acetyl-CoA is converted into ketone bodies (beta-hydroxybutyrate, acetoacetate and acetone). Blood ketones rise from 0.1 to 0.3 mmol/L in fed state up to 0.5 to 1.5 mmol/L by 24 to 48 hours. The brain begins using ketones as a partial fuel reducing its glucose requirement. By 48 hours ketones may supply 30 to 40 percent of brain energy needs. This is the classic fasting metabolic state.

4. Hours 48 plus: nutritional ketosis stable

Beyond 48 hours blood ketones typically stabilise at 1 to 3 mmol/L in nutritional ketosis range. The brain may meet up to 60 to 70 percent of energy needs from ketones at extended fasts. Muscle protein breakdown for gluconeogenesis is suppressed to preserve lean mass. The metabolic state is sustainable for days to weeks with adequate water and electrolytes although other risks accumulate at extended durations. This is the physiology underlying medically supervised extended fasts.

Practical implications

What this means for your fasting practice

Five practical takeaways from the fuel switch physiology.

16:8 fasting produces meaningful fat oxidation in the final hours

The 16 hour fasting window does not reach deep ketosis but produces meaningful fat oxidation particularly in hours 12 to 16. Daily 16:8 practice produces sustained fat-burning emphasis even without nutritional ketosis levels. This is part of why 16:8 produces modest weight loss without requiring deep metabolic shifts.

Ketosis is not required for fat loss

Total energy balance drives fat loss. Ketosis is a marker of metabolic state not a direct driver of fat loss. People doing 16:8 without ketosis still lose fat when in caloric deficit. Pursuing ketosis through longer fasts is not necessary if your goal is weight loss. Some people pursue ketosis for other reasons (cognitive effects, epilepsy management, athletic adaptation) but for typical health goals ketosis is optional.

Low carb diets accelerate the fuel switch

People already on low carb or ketogenic diets reach fat oxidation and mild ketosis faster during fasts because they enter the fast with depleted glycogen. Standard mixed diet eaters need longer to reach equivalent states. Neither approach is superior for weight loss but the metabolic timeline differs.

Exercise during fasting accelerates depletion

Physical activity during the fasting window accelerates glycogen depletion and fat mobilisation. A 30 minute walk in the morning before breaking a 14 hour fast produces faster fuel switching than a sedentary morning. This is one reason fasted training is popular among some athletes although evidence on performance and adaptation benefits is mixed.

Ketone measurement is optional

Most people fasting for general health do not need to measure ketones. Blood ketone meters work well if you want objective data. Urine ketone strips are unreliable for assessing fasting state. The presence or absence of ketones does not directly predict whether you are getting the benefits you want from fasting.

Safety

When ketosis becomes a problem

Mild fasting ketosis is safe in healthy people. Diabetic ketoacidosis is a separate dangerous condition.

Type 1 diabetes. Diabetic ketoacidosis (DKA) is a medical emergency with blood ketones over 3 mmol/L plus high glucose and acidosis. Fasting ketosis can shade into DKA in type 1 diabetics without insulin. Specialist supervision required.
Insulin-treated type 2 diabetes. Similar DKA risk exists particularly with SGLT2 inhibitor medications. Specialist supervision required.
History of eating disorders. Pursuit of ketosis can become obsessive. Avoid fasting and ketone measurement.
Pregnancy or breastfeeding. Ketosis is not appropriate in pregnancy as ketones cross the placenta with unknown long-term effects.
Children and adolescents. Ketosis is appropriate only in specific medical contexts (epilepsy) under specialist supervision.

Ketogenic diets are increasingly used as adjunct therapy for refractory epilepsy and being studied for several other conditions. Always under medical supervision. Self-prescribed ketosis chasing is not appropriate medical practice.

For the wider picture on fasting from the gentlest protocols to extended fasts plus the science behind hunger, metabolism and refeeding, our Understanding Fasting hub brings every guide together in one place.

Part of the hub

Back to the Fasting Hub

This article sits inside our complete knowledge base on fasting covering protocols, physiology, safety and practical guidance. Head back to the hub for the full index.

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Keep reading

More fasting physiology

Several related pages cover the rest of fasting metabolism. Our piece on how the body responds to fasting covers the broader physiological picture. Insulin levels and fasting covers the hormonal driver of the fuel switch. And what happens to blood sugar during fasting covers the glucose side of the equation.

Frequently asked

Fat burning and ketones questions

When does fat burning start during a fast?

Fat oxidation rises gradually from around 6 to 8 hours after the last meal as insulin falls. Significant fat mobilisation typically begins between 12 and 16 hours of fasting once liver glycogen is largely depleted. The rate of fat burning continues to rise for the first 24 to 36 hours and then plateaus. The acute switch is gradual not sudden. Standard 16:8 fasting produces meaningful fat oxidation in the final hours of the fasting window.

How long until ketones show up?

Ketone production starts modestly around 16 to 24 hours of fasting in most people. Measurable blood ketones (beta-hydroxybutyrate over 0.5 mmol/L) typically appear by 24 to 48 hours. Nutritional ketosis levels (1 to 3 mmol/L) require 48 to 72 hours of fasting in most people. The timing varies based on activity, prior diet (low carb diets accelerate ketosis), individual metabolism and body composition. People doing daily 16:8 do not reach nutritional ketosis during the fasting window.

Does fasting burn more fat than just eating less?

Acutely yes. Sustainably no. Fat oxidation rates during the fasting window are higher than during the fed state. Total fat loss over time depends on total calorie deficit not the timing of fat oxidation. The 2020 Lowe trial found similar fat loss between 16:8 fasting and calorie restriction at matched deficits. The acute fat burning during fasts is real but does not translate into more total fat loss compared to other approaches with matched deficits.

What is the fuel switch?

The metabolic shift from primarily burning glucose to primarily burning fat and ketones during fasting. The body normally uses glucose from food and from glycogen stores. As fasting progresses glycogen depletes and the body increasingly mobilises fat from adipose tissue. The liver converts some fatty acids into ketone bodies which can fuel the brain and other tissues. This shift is the basis of fasting metabolism and is often described as metabolic flexibility.

Can I be in ketosis with 16:8 fasting?

Usually not nutritional ketosis. The 16 hour fasting window is not long enough to reach blood ketones above 0.5 mmol/L for most people. Some people on low carb diets may show mild ketosis during 16:8 fasting because they enter the fast already with low glycogen. People on standard carbohydrate-containing diets typically do not produce significant ketones during 16:8 fasting. Nutritional ketosis requires either prolonged fasting (over 48 hours) or sustained ketogenic dieting.

Why do some people produce more ketones than others?

Individual variation in ketone production reflects several factors. Prior diet matters: low carb adapted individuals reach ketosis faster. Physical activity raises ketone production during fasting. Body composition matters: leaner people often produce ketones faster. Sex differences exist: men typically produce slightly higher ketone levels than women during equivalent fasts. Age affects the response: older adults often have slower ketone production. Genetic variation in metabolic enzymes also plays a role.

Does measuring ketones tell you anything useful?

It confirms metabolic state but does not directly predict outcomes. Blood ketone meters and breath acetone meters give objective measurement. Urine ketone strips are unreliable. Ketones above 0.5 mmol/L confirm meaningful fasting metabolism. Higher ketones do not equal more fat loss or better health outcomes. Some people use ketone measurement to track their fasting state for personal interest. Routine monitoring is not necessary for standard intermittent fasting.