The Science of Fasting

When you fast, your body stops using glucose for fuel and starts to metabolize fat. This helps drive blood sugar and insulin levels down, reducing your likelihood of gaining weight and developing diabetes or heart disease. Intermittent fasting also increases the release of human growth hormone, which regulates your metabolism and preserves muscle mass while burning fat.

Beneficial neurochemical changes in the brain can occur as well when you fast for short periods of time. Intermittent fasting can lead to improved concentration, better memory and even boosted energy levels.

Intermittent fasting increases the production of a protein in the brain called brain-derived neurotrophic factor, which stimulates the production of new neurons in the brain. Neurons transmit and receive information, so having more of them improves your memory and ability to learn.

People also report having more energy when they fast due to improved mitochondrial function. Studies show that intermittent fasting stimulates the production of ketones, which act as an energy source for neurons. Upon the release of these ketones our bodies increase the number of mitochondria in our neurons, which are responsible for taking in nutrients and creating energy. The result: a brain and body that’s more capable of producing energy and burning fat.

How to Fast for Health

Learning to fast intermittently can be less challenging if it’s already part of your culture, but for the uninitiated it can be difficult. Dr. Sinha says the key is to take it slowly and make sure you’re eating a balanced, healthy diet in the first place.

“If you’re used to eating sugary, carbohydrate-rich meals, fasting is going to make you feel very deprived and want to binge eat,” he says. “You need to first fix your metabolism by eating nutrient-dense meals that keep you satiated.”

Follow a diet that eliminates processed foods and focuses on high quality proteins, fruits and vegetables, healthy fats and small amounts of whole grains. Once you’ve corrected nutrient deficiencies and any hormonal or metabolic imbalances, then you can try incorporating intermittent fasting into your routine.

Try fasting for 12 hours overnight as a start, such as from 7:00 pm — 7:00 am. Once you’re comfortable, you can delay your morning meals and start intermittently fasting for 14 to 18 hours a day. Just make sure you don’t reduce the total number of calories you’re eating. The goal is to eat the same number of calories, just less frequently throughout the day. Fasting for three hours before bedtime and for at least three hours between lunch and dinner can also be helpful.

Eat the same number of calories as you would normally, but confine your eating to a smaller period of time in the day. Make sure you’re following a healthy diet and not eating things like hamburgers, pizza and soda during the hours you do eat.

For the experienced individual, an intermittent fast of 24 hours once a week may be beneficial, but anything more than that can be dangerous. Not eating enough food can lead to nutritional deficiencies that cause headaches, fatigue, anemia, dizziness and even mental confusion. In addition, fasting for too long can lead to intense food cravings, causing you to binge eat and undo all the work you’ve done to develop a new eating pattern.

Changing your eating habits can be difficult for the first few weeks, so limit intense exercising while your body adapts. Dr. Sinha recommends walking and light activities such as yoga and stretching during the adjustment period.

Cautions for Fasting

As with anything in life, what works for one person may not work for you. Make sure to talk with your doctor before making any drastic dietary changes.

Certain medications and chronic conditions may not be compatible with intermittent fasting. Women who are pregnant or breastfeeding should avoid fasting for long periods of time, as well as people with significant nutrient deficiencies who need to eat every few hours.

Low blood sugar can trigger a migraine, so fasting may not be a good idea for people with migraine headaches.

If you are taking insulin or diabetes medications, talk to your doctor before trying intermittent fasting or any form of restrictive caloric intake. While you still may be able to fast, your doctor needs to monitor your blood glucose level closely and adjust your medications appropriately to avoid life-threatening hypoglycemia. If you become hypoglycemic, you can feel dizzy, shaky, sweaty, confused and lightheaded.

Emerging medical research about intermittent fasting reveals that a one-size-fits-all nutritional plan is no longer realistic in modern society, Dr. Sinha says, so it’s important to personalize how you fast.

“If you’re under chronic stress or not sleeping well, both of which can increase hunger, intermittent fasting may not be the best option for you,” he says. “If I’ve had a sleepless night, I’ll go ahead and eat breakfast instead of pushing out my next meal time to later in the day.”

Intermittent fasting takes some getting used to, but it gets easier with practice, Dr. Sinha says.

“You have to build up your tolerance just as you would if you began a new exercise program. Make slow, gradual changes and soon you will start seeing results.”