Will I still have to count calories to lose weight?

One of the benefits of time restricted feeding (based on a 16:8 fasting schedule, for example) is that many people experience natural caloric restriction and weight loss without counting calories. In a study of time-restricted feeding (fasting for 16 hours per day), Dr. Krista Varady found that most research participants naturally reduced their daily caloric intake by 300 calories on average and lost 3% of their body weight after a three-month period, even while being instructed to eat normally.

Can I have a cheat day?

It’s important to be flexible. Intermittent fasting is ideally a lifestyle, not a fad diet. In Dr. Krista Varady’s studies of alternate day fasting, she tells people not to worry if they miss two or three fast days in a month. Sometimes your fast days might land on a holiday or during another family event, and it may be more stressful to stringently stick to your fast than to enjoy a piece of cake.

However, some people find that “cheating” derails their whole fasting schedule. If you are the type of person who needs a lot of structure to succeed at a healthy behavior, Varady suggests not skipping your fast days for family events or other occasions. However, for most people the occasional cheat day won’t significantly impede weight loss progress or metabolic health.

Should I be worried about potential negative impacts of meal skipping on my blood sugar levels?

A small-scale study published in 2017 raised concerns about breakfast skipping (front-end fasting) creating an acute state of muscle glucose intolerance upon refeeding. There is some evidence, based on natural daily cycles of insulin sensitivity, that it is better to skip dinner or start a prolonged overnight fast early in the evening as opposed to skipping breakfast or fasting until late afternoon. However, acute fasting studies that look at the impacts of 24 to 48-hour fasts on individuals who may not be accustomed to fasting periods of this duration are not necessarily indicative of what would happen for individuals who practice fasting on a regular basis, Dr. Krista Varady says. After a month of alternate day fasting, research participants in intermittent fasting studies overwhelmingly experience reductions in glucose levels and insulin resistance, based on cellular adaptations to the stress of fasting.

Are there any supplements I can take to improve the efficacy of my fasts?

There are ketone ester supplements available for mass consumption today that can raise your blood ketone levels and help you achieve ketosis. For athletes, these supplements may help the body produce ketones that can be advantageous as an energy source during strenuous workouts. However, unless you are already on a ketogenic diet these supplements give your body mixed signals in terms of whether it should be burning primarily fats or sugars for fuel. These supplements may not be effective or necessarily safe for metabolic health long term, says Dr. Krista Varady.

How long is it safe to fast for?

Water fasts up to 24 to 36 hours in duration are generally safe and well tolerated based on clinical studies, Dr. Krista Varady says. However, from a weight loss and maintenance perspective, 24-hour water fasting on a regular basis can be difficult to stick to and adopt as a long-term health practice. People with eating disorders should not to practice IF without oversight from a physician. However, alternate day fasting has been found to decrease feelings of depression and binge eating behavior in obese subjects, while improving body image perceptions.

It’s best for your metabolic health to stick to a fasting regimen you can easily maintain over time, with the caveat that you should stop fasting and see a physician if you experience light-headedness or significant discomfort, or if you are at risk of becoming underweight. Intermittent fasting is not the only way to improve your metabolic health; it’s important to find the meal composition and timing plan that works best for you long term.

There is limited research on the impacts or safety of long term periodic fasting for 3–5 days at a time.

Is intermittent fasting safe for me if I am undergoing cancer treatment?

You should consult with your physician or oncologist before practicing intermittent fasting during any type of cancer treatment. Intermittent fasting can be problematic during treatment especially if you are experiencing muscle wasting, poor nutrition or other side effects from treatment. However, there is evidence from in vitro studies, animal studies and early clinical studies in humans conducted by Dr. Valter Longo of the USC Longevity Institute and colleagues that periodic fasting or low-protein fasting-mimicking diets may sensitize cancer cells to the impacts of chemotherapy while protecting healthy immune cells and other cells against side effects. The IGF-1R (Insulin-like growth factor 1 receptor) pathway appears to play a key role in the impact of dietary restriction on tumor progression and treatment sensitivity.