We all strive to do things that not only help us live longer, but to live with a better quality of life. This is the difference in life-span vs. health-span, or the number of years we live vs. the number of years we live as healthy and disease free as possible. Essentially, the goal is to live long, better.

Research shows that energy restriction, better known as “fasting” is one way to improve your health-span.

Empty Plate

Fasting for health is not a new phenomenon. Hippocrates, the father of modern medicine, is said to have prescribed fasting to aid in healing. Benjamin Franklin stated, “The best of all medicines are resting and fasting.”

Current research points to the fact that fasting not only aids in weight loss, but also impacts diabetes and insulin sensitivity, improves cardiac risk factors, decreases inflammation, improves cognition, increases muscle mass, and can extend longevity. In addition, studies are finding that fasting slows or reverses aging at a cellular level and can extend longevity.

The Physiology of Fasting

The average American diet consists of 3 meals a day, plus snacks. Combined with little exercise, this means people are running solely on those calories rather than burning any excess calories stored throughout the body as fat. Therefore, fasting or intermittent fasting can provide a big bang for the buck in terms of health for many people.

To better understand the benefits of fasting it helps to have a general understanding of what happens when we eat.

  • After eating a meal, calories are broken down to glucose (sugar) to provide energy for the body’s cells. What is not immediately used is stored as glycogen (long chains of glucose) in the liver. The problem is that the liver has a limited storage capacity. So when the liver reaches that limit, it turns the excess glycogen into fat – which is stored Liverboth in the liver and in other fat deposits throughout the body.
  • When the body needs energy it first utilizes the glycogen stored in the liver. Once these stores are used up it then relies on stored fat elsewhere in the body. We store body fat in order to use as energy while we are in the non-eating/fasting state. This fat is ultimately converted to “ketone bodies”, and when food is not being consumed ketones provide a major source of energy for the body (including your brain).

Therefore it is essential to enable our bodies to be placed in a state where these fats/ketones are the major source of energy. This can best be accomplished by fasting. Fasting allows you to use your body fat.

If you think about the typical day for most Americans it is easy to see that burning the fat stores can be a challenge. If we start eating the minute we roll out of bed, and don’t stop until going back to bed at night, it is difficult to burn fat. If you are eating throughout the day, your body will just use the incoming food as energy. You are chronically in the “fed” state so have more than enough glucose stored in the liver to provide energy to the body. Whereas, if you spend more time in the non-fed or fasting state, your body will start to use the fat stores for energy.

This can be accomplished by most people though intermittent fasting. There are different ways to do this, but the general idea is that you eat normally some days of the week and drastically reduce your calories on other days.

What are the types of Intermittent Fasting?

The most common forms of intermittent fasting are:

  • Prolonged Fasting:
    Complete avoidance of food for two or more days. Though safe for most people, this is not a fasting plan most of us have the desire or discipline to follow. Luckily there are other options!
  • Alternate Day Fasting:  
    As the name implies you fast every other day.  Again not easy to maintain long term.
  • The 5:2 Plan:  
    Fast on two non-consecutive days of the week, and eat healthy during the other 5 days. The easiest way to accomplish this for most people is to eat supper, and not eat again until supper the next day. This plan has been shown to be equally effective as a modified fast. During the fasting days women can take in 500 calories a day and men, 600 calories a day.
  • Time Restrictive Feeding:  
    With this type of intermittent fasting, one completely avoids eating for 14-20 hours a day. This can be accomplished by skipping any meal, but most seem to find it easier to eat an early dinner and not eat again until the next day with an early lunch or late breakfast. To be effective, this needs to be done 4-6 days of the week.
  • Fasting Mimicking Diet:  
    This is a plant-based diet of 800-1000 calories a day for 5 consecutive days each month for up to 6 months. This diet plan is designed to mimic molecular and cellular changes seen in the body during a true fast. It provides the physiological benefits of fasting while still allowing small amounts of food intake. Typical foods eaten in this diet are low glycemic foods like vegetables, beans, lentils, and almonds. For those wanting to take the prep work out of this diet, there is a plan developed by Valter Longo, PhD at the USC Longevity Institute, called Prolon. The foods you need for the 5 diet days are pre-selected and available for purchase as a kit.

Any of the above plans can take 2-4 weeks for the body to become accustomed to fasting. Feeling hungry, fatigue and mild headache are common early on. Drinking green tea or black coffee during fasting periods can help with hunger sensations. As time passes, initial side effects abate and hunger cravings decrease. Most people actually feel better and have more energy compared to how they felt prior to starting the diet.

Advantages of Fasting Diets

These diets for the most part are attractive because of their simplicity, low cost, convenience, flexibility, and safety in combination with most any other diet plan.

In addition to the many positive effects listed earlier, there is evidence that fasting reduces inflammation, improves the sleep wake cycle, and strengthens the immune system in fighting disease and cancers. Clinical studies are currently underway to better understand the potential metabolic and neurologic benefits in Inflammatory Bowel Disease, Alzheimer’s, Rheumatoid Arthritis, Thyroid disease, Fatty Liver disease and pre-diabetes/metabolic disease.

Though most everyone can safely fast at some level, children, women who are pregnant or breast feeding, underweight individuals, and those with a history of eating disorders should refrain from fasting. Due to risk of hypoglycemia, those taking diabetic meds should talk with me before starting this type of diet.

For those of you wanting more information, research, and articles on fasting diets, the following are good resources.

Looking forward to a happy and healthy new year!

Kerry Rhodes MD



  • Cabo R, Mattson M. Effects of Intermittent Fasting on Health, Aging, and Disease. N Engl J Med 2019;381:2541-2551
  • Wei M, et al. Fasting Mimicking Diet and Markers/Risk Factors for Aging, Diabetes, Cancer, and Cardiovascular Disease. Sci. Trans. Med. 2017;9:1-12