A health expert has outlined a 20-hour daily fasting method combined with fasted high-intensity interval training (HIIT) that he claims can help shift "stubborn" belly fat "if all else fails". Dr Eric Berg DC, who specialises in healthy ketosis and intermittent fasting, shared the approach in a YouTube video titled "How to actually lose belly fat".
Why belly fat can be hard to lose
Dr Berg explained that the body stores fat around the midsection as a survival mechanism, with nine times more receptors for fat storage than fat burning in that area. Adrenaline helps activate fat burning, while high insulin levels can prevent belly fat loss.
He said: "Our bodies have evolved to store body fat around the midsection as a survival mechanism. There are nine times more receptors for fat storage than fat burning in this area. Adrenaline helps activate the fat-burning process, which can help shift the body from storing belly fat to burning it. High insulin levels can prevent you from losing belly fat."
The 20-hour method and HIIT
The method involves spending 20 hours each day without food, or at least 18 hours, to lower insulin levels and unlock fat cells around the midsection. During the remaining four-hour eating window, Dr Berg recommends eating just two meals without snacks, such as a late lunch at 2pm and dinner around 6pm.
He said: "Fasting for 20 hours a day can help lower insulin to your baseline. A HIIT workout during your fast can significantly increase adrenaline to boost belly fat loss." Dr Berg stresses trying this combined method "when all else fails" because standard weight-loss approaches do not trigger the specific hormonal shifts required.
To maximise fat burning, he advises short, intense HIIT workouts while still fasting, noting this triggers a massive spike in adrenaline and requires only 8 to 15 minutes of intense exercise per week. Weight training two or three times a week can boost adrenaline further, he said.
Food choices, sleep, and who should avoid it
The plan also relies on strict food choices and daily habits. Dr Berg says to avoid sugars, starches, industrial palm oil, and seed oils, focusing instead on healthier fats like olive oil. He also emphasises managing stress and cortisol by getting eight hours of quality sleep, spending time in sunlight, and stopping food intake at least three hours before bed.
He said: "There are also more cortisol receptors around the midsection than in other areas of the body, so chronic stress could be another reason you're having trouble losing belly fat. Adequate sleep, sunlight exposure, and avoiding food for three hours before bed are also important for losing belly fat. If you want to lose belly fat, it's vital to avoid industrial palm oil, seed oils, sugar, and starches."
Experts generally suggest beginning with shorter fasting periods and speaking with a doctor if you have a health condition or take medication that affects blood sugar. Extended fasting and fasted high-intensity exercise are generally not recommended for people with diabetes or blood sugar disorders, pregnant or breastfeeding women, people with a history of eating disorders, people with cardiovascular issues or unmanaged high blood pressure, and underweight people or those with high daily energy demands.
For those who have first spoken to a doctor, experts recommend easing in gradually rather than jumping straight into a 20:4 schedule, starting with a gentler 12:12 or 14:10 schedule to allow the body to adapt safely. They also urge people to prioritise hydration and electrolytes, as fasting flushes water and sodium from the body, and to stop exercising immediately if they experience chest pain, severe dizziness, confusion, or heart palpitations.
