Fed versus fasted: how hormones switch your cells between burning and building
September 29, 2026 · 20 min read
A biochemist, an obesity researcher and a longevity geneticist describe the same hormonal switch, with insulin, leptin and related signals telling cells whether to store, build or burn fuel. Understanding it clarifies why fasting, weight regain and metabolic disease are so tightly linked.
The rules, numbers and steps worth keeping in front of you.
- Jared Rutter: insulin and GLP-1 are the fed signal; glucagon is the fasting signal that tells fat cells to release fuel.
- Rutter: food can be converted to energy or to biomass, and pyruvate is the fork in the road.
- Rutter: the heart gets an estimated 70 to 80 percent of its energy from fat; neurons rely mostly on glucose.
- Rutter: high blood sugar harms over years, but blood sugar that drops too low can kill within minutes.
- Zachary Knight: leptin rises in direct proportion to body fat and tells the brain how full the reserves are.
- Knight: each kilogram lost cuts daily energy expenditure by about 30 kilocalories.
- Knight: every two pounds or so lost raises hunger by about 100 calories a day.
- Knight: most obese people have high leptin and are resistant to it, like insulin resistance in type 2 diabetes.
- Knight: 25 to 33 percent of weight lost is typically muscle unless you eat enough protein and do resistance training.
- David Sinclair: low insulin switches on the sirtuin longevity genes; being fed all day keeps them off.
- Sinclair: build up fasting gradually; the first two to three weeks are hard.
- Sinclair: leucine, growth hormone and testosterone may trade long-term health for short-term gains.
- Diabetes, a history of eating disorders, or blood sugar medication: talk to a doctor before fasting or a new diet.
- Metformin is a prescription drug for use only under medical supervision.
This is the short form. The full insight has every expert's reasoning, their own words and the sources.