Erin Peisach, RDN, a registered dietitian nutritionist and wellness education specialist with Sharp Rees-Stealy, answers some common questions about intermittent fasting.
What is intermittent fasting?
Intermittent fasting is an eating strategy that fluctuates between restricted calorie (food) intake and normal calorie intake over designated time spans.
- Alternate day fasting — fasting for a full day followed by eating normally the next day
- Modified fasting (5:2) — eating 20% of energy needs on 2 fast days per week, followed by typical eating patterns for the other 5 days of the week
- Time-restricted feeding — eating normal amounts but during a restricted time window each day
- Extended fasting — fasting for 1 day or longer
- Religious fasting — fasting for religious or spiritual purposes
What are the benefits of intermittent fasting?
Findings appear promising for intermittent fasting to help with metabolic health. This may be because fasting allows the body to repair, replenish and recycle cells involved in metabolism. Think of it as a “metabolic tune-up.”
One predominant theory is that the body uses ketone bodies — chemicals made by the liver — as an alternative fuel source to glucose during fasting. Ketones are created from the breakdown of fats when glucose is in short supply, and act as a potent energy source, providing fuel to high-demand areas such as brain neurons.
Additional benefits of fasting come from its ability to reduce total calorie intake. It is well-established that eating less helps to support longevity and lower the risk of many chronic diseases. The question remains whether these benefits are experienced from the fasting, or if continuous calorie restriction alone (dieting) produces the similar effects.
Is intermittent fasting safe?
Intermittent fasting is appropriate and safe for most adults without preexisting medical conditions who may be looking to experiment with their diet in a safe and reasonable manner.
However, intermittent fasting is not appropriate for everyone, especially people who:
- Have chronic health conditions such as diabetes, hypothyroidism or electrolyte imbalances
- Are critically ill with conditions such as advanced kidney or liver disease, or an unstable heart or lungs
- Live with mental health conditions including eating disorders
- Are in developmental life stages such as pregnancy and early childhood
Can you exercise while intermittent fasting?
It is OK to engage in physical activity while fasting. However, bodies need to adjust to the new eating pattern. It’s recommended to start with lower-intensity workouts during a fast and eventually build up the intensity over time. Eating windows should begin shortly after a morning workout to help support workout recovery.
“Intermittent fasting is a flexible eating approach that can fit your needs and lifestyle,” says Peisach. “Keep in mind that you don’t need to stick to your intermittent fasting every day,” she says. “Plan your social calendar ahead to account for your fasting days and times each week.”