Jason Fung - Role of Insulin in Weight Loss | Jason Fung

Short Summary

Main recommendation: To lose weight, don't rely on calorie counting alone; also lower insulin levels through intermittent fasting or low-carbohydrate eating so the body can access stored body fat. The speaker argues the hormonal (insulin) model explains why calorie restriction often fails, since reduced calorie intake is frequently matched by a falling metabolic rate.

Key Points

• The calorie model treats food only as energy, while the hormonal model holds that foods also carry hormonal "instructions" that determine whether calories are burned or stored.
• Insulin rises when you eat, especially carbohydrates and, to a lesser extent, protein; it promotes storing calories as glycogen or body fat and blocks lipolysis, so stored fat cannot be used for energy while insulin is high.
• Dr. Fung says the energy balance equation (body fat = calories in minus calories out) is true, but it is a balance with three variables, so cutting calories in can reduce either body fat or calories out (metabolic rate).
• Dr. Fung cites historical and modern studies, including Ancel Keys' 1944 starvation study, Bomb's 1971 work, and Leibel's 1995 NEJM study, which he says show metabolic rate falling by roughly 10–40% under calorie restriction.
• Dr. Fung cites a 1991 meta-analysis of 29 studies that found a 10–25% drop in metabolic rate, calling this a near-universal response to calorie restriction.

Details Worth Noting

• Concrete instructions: Lower insulin levels by reducing how often you eat (periodic fasting) so insulin falls and stored body fat becomes accessible, while also cutting intake. Intermittent fasting and low-carbohydrate diets are the two methods the speaker names. Avoid eating constantly on high-carbohydrate foods, since high insulin blocks fat use even at a lower calorie intake.
• Worked example from the speaker: Worked example from Dr. Fung: at 2,000 calories/day maintenance, cutting to 1,500 while eating frequently with high-carb foods leaves insulin high, so he says the body reduces its metabolic rate to about 1,500 rather than using fat stores. Cutting to 1,500 while fasting periodically lets insulin fall, so he says the body can release about 500 calories from fat and keep burning roughly 2,000, lowering body fat.
• The Ancel Keys study (1944) gave volunteers 1,500–1,700 calories (about a 40% cut) and found metabolic rate fell about 40%, heart volume shrank about 20%, and heart rate and body temperature dropped; the speaker notes this resembles typical "1,500 calorie, low-fat, low-protein" weight loss advice.
• Dr. Fung cites the Leibel 1995 study (New England Journal of Medicine) as finding metabolic rate dropped by roughly 300–400 calories per day during weight loss.
• The speaker cites the 2021 American Heart Association scientific statement recommending adjusting energy intake and expenditure to maintain a healthy weight, presenting that as the calorie-model position.
• The speaker names Harvard scientists Dr. Walter Willett and Dr. David Ludwig as promoters of the carbohydrate-insulin model, and mentions cortisol as a smaller contributing hormone.
• Caveat: this is the speaker's interpretation of research, not professional medical advice.

Bottom Line

Weight loss depends on hormones as well as calories: cutting calories alone often triggers a drop in metabolic rate, while lowering insulin through fasting or lower-carb eating lets the body use stored fat.