Why fasting can help, or hurt, your chances of longevity and weight loss

Aug 19, 2017

Not eating, deliberately, has been a way to make a political statement for centuries. And for even longer, it’s been a normal part of some religious practice. But far more recent uses of fasting are for weight loss and other health benefits -- scientifically proven benefits like lowered cholesterol and reducing systemic inflammation.

Dr. Valter Longo joins us on “Take Care” this week to discuss the benefits, challenges and problems associated with fasting. Longo is a professor of gerontology and biological science at the University of Southern California Leonard Davis School of Gerontology. He’s also director of the USC Longevity Institute.

There are three major fasting practices, according to Longo.

  1. Time restrictive feeding: Eating for a certain number of hours a day and reserving the other hours for fasting. Longo says restricting yourself to less than 11 or 12 hours of eating a day (six, for example) creates more problems than benefits and isn’t the best option.
  2. Intermittent fasting: A method of alternate day fasting, where you eat one day and then don’t eat or eat very little the next day; or eat normally for two days and then either don’t eat or eat very little the third day.
  3. Periodic fasting: Not eating for 2 or more days, periodically.

Short term effects of fasting

When you begin to fast, your body makes a metabolic switch.

“There is no more sugar basically coming into the body,” Longo says. "There are no more proteins coming in, so the body switches from a mode in which carbohydrates are the main source of fuel to fats. So the body starts breaking down the fats.”

Autophagy can also occur, where cells start to destroy themselves, only to replace them later.

“There’s also breaking down of entire cells, so killing of entire cells, and then eventually the regeneration of the cells that have been killed during fasting when we return to the normal food,” Longo says.

Long term effects of fasting

Longo says the idea of fasting should be respected, because it has large consequences. He says that within just five days of fasting the brain switches from using just sugar to make fuel to using both sugar and ketone bodies at a 50:50 ratio.

Just like you don’t take drugs because they worked for someone else, you shouldn’t try fasting without the guidance of a medical practitioner, Longo says. Water only fasting, for example, should only be attempted at a clinic with nurses and doctors on staff.

Longo does cite fasting-mimicking diets as a safe way to achieve positive, long-term effects (diets that mimic fasting usually allow you to eat between 100 and 750 calories per fasting day). These effects include:

  • lower cholesterol
  • lower fasting glucose levels
  • relief from systemic inflammation
  • lower triglyceride levels
  • fewer biomarkers for cancer

What you can do

“Keeping in mind compliance, I would say there are three major things that can be done,” Longo says.

He says for those who have problems with weight, the first two options are best. Time restrictive eating is a good option for many people, allowing them to eat for a certain period of time, say 12 hours and then restricting eating for the following 12 hours.

“Most people can do it. Most people around the world who get to 100 years of age -- that was normal [for] them,” Longo says.

Another option is to just cut back on the number of meals you eat per day, he says.

“You can go from let’s say three, four, five, six meals a day, reduce it to two major meals per day and one snack,” Longo says “If you look around the world, it’s very common for people to have two meals a day.”

A periodic fasting mimicking diet also can reap most of the benefits associated with the practice. A normal, healthy person, who exercises regularly can eat a restricted calorie diet of 750 to 1,100 calories a day for five days in a row, according to Long0, and can do this once or twice a year.