Fasting for 24 Hours Doesn’t Do What You Think

Summary

Dr. Eric Berg argues that a 24-hour fast does not usually cause blood sugar to crash in healthy people. He says the body uses stored liver glycogen early in a fast, then increasingly relies on fat and ketones for energy. He also claims that the liver can make glucose during fasting and that muscle glycogen is reserved for the muscles.

Key points and strategies

• Fuel use during fasting: Berg describes fat as a major fuel source, rather than merely a backup. He says liver glycogen falls substantially over the first 24 hours, while the body can draw on much larger fat reserves.
• Blood sugar: He says blood glucose generally changes only slightly during a 24-hour fast in healthy people because the liver can produce glucose. He cites hormone responses—including glucagon, adrenaline, and cortisol—as helping maintain it.
• Fat burning and fasting: He says the shift toward fat burning and ketosis often begins around 12–24 hours after eating. He also associates fasting with autophagy, the recycling of damaged cellular material, and describes ketones as anti-inflammatory.
• Insulin resistance: Berg links frequent carbohydrate intake and insulin resistance with difficulty regulating glucose. For people with elevated morning glucose, he suggests a walk to use some of that glucose and recommends low-carb eating and longer-term intermittent fasting.
• Shakiness or irritability between meals: He argues these symptoms are not necessarily caused by low blood glucose and criticizes routine snacking as a solution. He attributes meal-related symptoms in some people to a high-carbohydrate diet.
• Product promotion: The video includes an advertisement for a black elderberry supplement with zinc, vitamin D3, and camu camu. Berg recommends taking three per day with meals, if possible.

These are the presenter’s claims, not individualized medical guidance. Fasting may not be appropriate for everyone, particularly people with medical conditions or medications that affect blood sugar.

Presenters and sources

• Presenter: Dr. Eric Berg
• Research cited by the presenter: A 1973 study of 19 healthy volunteers, a 2001 study in which 33 healthy people fasted for 72 hours, and the Whitehall study. The subtitles do not provide full citations.