This article is an excerpt from the Shortform book guide to "Fast. Feast. Repeat." by Gin Stephens. Shortform has the world's best summaries and analyses of books you should be reading.
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What triggers insulin release? How does insulin cause weight gain?
Insulin is released every time you eat. Most people eat every few hours, which leads to chronically high insulin levels. Constant feeding means a constant influx of glucose, which is the body’s preferred fuel—there’s no need to tap into fat stores for energy.
Keep reading to learn about the role of insulin in weight gain.
Insulin and Weight Gain
When we eat, our blood glucose (blood sugar) levels increase. In response, our bodies release insulin, a hormone that stores excess glucose in the liver and muscles as glycogen. If any blood glucose remains after this step, insulin converts it into fat and stores it away. By storing this glucose-based energy, insulin lowers your blood glucose. When your blood sugar runs out and you need energy, the pancreas signals for your liver to release some of its stored glycogen to use as energy.
| How Does Insulin Cause Weight Gain? In The Obesity Code, Jason Fung explains this process in more detail. When you eat, the body breaks down each macronutrient—proteins, fats, and carbohydrates—into materials for the body to use. Carbohydrates are sugar molecules, so they become glucose, a simple sugar that every cell in the body can use for energy. This rise in blood sugar stimulates the release of insulin, a key hormone (molecules that deliver messages or resources to cells). Insulin acts like a key, fitting into the insulin receptor on a given cell and “opening the door” for glucose to enter the cell. If there’s more glucose than needed, insulin chains together glucose molecules into glycogen and stores them in the liver. Later, they’ll break back down into glucose when the body needs more energy. Fung compares glycogen stores and fat stores to your wallet and bank account, respectively. Glycogen is easier to access, but your liver stores a limited amount. Conversely, your fat stores are difficult to access but hold far more potential energy. Fung also explains that ketosis doesn’t activate as quickly as Stephens suggests. Your body won’t start breaking fat down into ketones until one to three days into a fast, he says. In the first 24 hours, your body instead breaks fat into glycerol, from which it produces new glucose. |
Modern Eating Habits Cause Weight Problems
According to Stephens, the modern habit of eating continually causes high blood glucose levels—that is, the body never gets a break from converting food into glucose. This leads to chronically high insulin levels, since insulin releases to store all that glucose. Since we’re never out of glucose or glycogen, the body can’t access its fat stores, burn them, and lose weight.
(Shortform note: In The Obesity Code, Jason Fung argues along these lines in more detail. Continuing the “wallet and bank” metaphor, he argues that, as with money, we prefer to use our wallets (glycogen)—and when we deplete them, we’d rather fill them back up than get money from the bank (fat stores). By eating without long breaks, you prevent your body from accessing fat stores, and at the same time you keep your insulin levels high, which promotes increased storage of glucose as fat. In Fung’s view, this leads to obesity.)
| Fasting May Heal Insulin Resistance In The Obesity Code, Jason Fung argues that periods of extended fasting (24 to 36 hours) are the key to healing insulin resistance and beating obesity. As he explains, our modern eating habits of three meals a day, plus regular snacking, never allow insulin to drop much. These chronically high insulin levels lead to insulin resistance. Fasting heals insulin resistance because it’s the most reliable way to lower insulin levels. The longer you fast, the more insulin levels decrease. Insulin levels must be both high and persistent to produce insulin resistance, so if you regularly fast, you prevent persistently high insulin. Over time, you’ll increase your insulin sensitivity, and that enables your body to make do with less insulin—which slows down fat storage and helps with fat loss. |
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- How intermittent fasting can help you lose weight, feel better, fight disease, and live longer
- An explanation of the cutting-edge science that supports fasting
- How to follow a four-week quickstart program to adapt to this new lifestyle
