Fasting, deliberately abstaining from food for a defined period, changes how the body handles sugar and fat in ways that have attracted significant research attention. Understanding what actually happens metabolically during a fast clarifies why fasting protocols have the effects they do, and where the evidence supports their benefits.

The Fed vs. Fasted Metabolic State

Fed state (0-4 hours after eating): Blood glucose is elevated. Insulin is high. The liver stores glucose as glycogen. Fat cells store fatty acids. The brain uses glucose as its primary fuel.

Post-absorptive state (4-12 hours): Blood glucose normalizes. Insulin falls. The liver begins releasing glycogen to maintain blood glucose. Fat cells begin releasing fatty acids.

Early fasting (12-24 hours): Liver glycogen depletes. Gluconeogenesis (making glucose from amino acids and glycerol) picks up. Fatty acid oxidation increases. The liver begins making ketone bodies (beta-hydroxybutyrate, acetoacetate) from fatty acids.

Extended fasting (24-72 hours): Ketone production increases substantially. The brain, which normally can only use glucose, begins using ketone bodies for 30-40% of its fuel. Glycerol from fat breakdown provides glucose for glucose-dependent tissues (red blood cells, some kidney cells).

The transition from glucose-based to fat/ketone-based metabolism is called the “metabolic switch.”

What Fasting Does to Insulin

Insulin is the key hormone that determines whether the body is in storage mode or breakdown mode. When insulin is low, as it is during fasting, the body shifts from fat storage to fat breakdown.

For people with insulin resistance, extended periods of low insulin (from fasting or very low carbohydrate eating) reduce the sustained hyperinsulinemia that blocks fat oxidation and drives fat accumulation. Multiple fasting intervention trials show improvement in insulin sensitivity, even before significant weight loss occurs.

Intermittent Fasting Protocols

16:8 (time-restricted eating): Fast for 16 hours, eat within an 8-hour window. Common implementation: skip breakfast, eat noon-8pm. No caloric restriction required; simply compressing the eating window.

5:2: Eat normally 5 days; restrict to 500-600 kcal for 2 days.

OMAD (one meal a day): A 23:1 window. Extreme time restriction.

Alternate day fasting: Alternate between normal eating days and fasting/very low calorie days.

What the Research Shows

A 2020 NEJM review by de Cabo and Mattson summarized the evidence: intermittent fasting trials in humans show consistent improvements in metabolic markers, reduced fasting insulin, lower triglycerides, reduced blood pressure, reduction in inflammatory markers. Weight loss is similar to continuous caloric restriction at equivalent caloric deficits.

A key question is whether the metabolic benefits of IF go beyond calorie restriction. A 2022 NEJM trial by Lowe et al. compared 16:8 time-restricted eating to calorie restriction in 90 obese adults and found equivalent weight loss and metabolic improvement, suggesting the metabolic switch itself may not confer additional benefit beyond the calorie reduction that most IF protocols produce.

However, a 2019 trial by Sutton et al. in Cell Metabolism tested early time-restricted eating (5-hour eating window, 6am-1pm) in men with prediabetes at exactly the same calorie intake as a control group with a 12-hour window. The early TRE group had significant improvements in insulin sensitivity and blood pressure despite zero weight loss, suggesting circadian timing of eating (aligning food intake with daytime cortisol peaks and insulin sensitivity) confers some benefit independent of calories.

Blood Glucose During Fasting

After 12-16 hours of fasting, blood glucose stabilizes in the 70-90 mg/dL range in healthy adults. Ketone levels rise to 0.5-1.0 mmol/L by 24 hours and 1-5 mmol/L by 48-72 hours.

For people with type 1 or type 2 diabetes, fasting requires careful management. Prolonged fasting with insufficient carbohydrate can cause dangerous hypoglycemia in those on insulin or sulfonylureas. Fasting should not be undertaken without medical supervision in these populations.

Myths About Fasting and Sugar

Myth: You must eat every few hours to maintain blood glucose. In healthy adults, gluconeogenesis maintains blood glucose perfectly during fasting. The perceived need to eat constantly is a conditioned response, not a metabolic requirement.

Myth: Coffee breaks a fast. Black coffee (no milk, no sugar) contains essentially no calories and does not meaningfully raise insulin. It doesn’t interrupt the fasted metabolic state.

Myth: Fasting causes muscle loss. Short-term fasting (under 48 hours) does not produce significant muscle protein breakdown. Growth hormone rises during fasting, which is protein-sparing. Longer fasts do eventually break down muscle if fat stores are depleted.

Sources:

  • de Cabo R & Mattson MP. “Effects of intermittent fasting on health, aging, and disease.” NEJM 2019
  • Sutton EF et al. “Early time-restricted feeding improves insulin sensitivity, blood pressure, and oxidative stress even without weight loss in men with prediabetes.” Cell Metabolism 2018
  • Lowe DA et al. “Effects of time-restricted eating on weight loss and other metabolic parameters.” JAMA Internal Medicine 2020