What is it?

Intermittent Fasting (IF) is a dietary approach that cycles between periods of fasting and eating, and doesn’t specify which foods to eat but rather when you should eat them. The most common methods include the 16/8 method, where you fast for 16 hours and eat during an 8-hour window, the 5:2 method, where you consume only 500–600 calories on two non-consecutive days of the week but eat normally the other 5 days, and Eat-Stop-Eat, which involves a 24-hour fast once or twice a week.

Who is it for?

IF is suitable for adults looking to improve their health and well-being, including those aiming for weight loss, improved metabolic health, and perhaps an increase in longevity. It’s particularly appealing to those who prefer a lifestyle approach over counting calories or specific diet restrictions. Additionally, some research suggests that IF may be more beneficial than other diets for reducing inflammation and improving conditions associated with inflammation such as:

• Alzheimer’s disease
• Arthritis
• Asthma
• Multiple sclerosis
• Stroke

However, IF is not for everyone; it’s generally not recommended for children and teens under the age of 18, pregnant or breastfeeding women, individuals with a history of eating disorders, or those with certain health conditions such as type 1 diabetes, without prior consultation with a healthcare provider.

Circadian Fasting

Similar to IF is Circadian Fasting (CF). In contrast to intermittent fasting, which allows for a flexible division of a 16-hour fasting period and an 8-hour eating phase, circadian fasting follows a schedule with an earlier and extended eating window. This approach typically advises concluding your fast by 7 p.m., dividing the day into two 12-hour cycles, culminating in a full 24 hours. A common circadian eating pattern might span from 7 a.m. to 7 p.m. However, it’s also possible to lengthen the fasting period to 14 hours, with both durations linked to positive health outcomes

Benefits of CF, in contrast to regular IF, arise from the fact that the body is more metabolically active earlier in the day and therefore primed to burn energy. Reasons for this include: improved movement of food through the digestive tract, enhanced absorption of nutrients in the intestines, improved ability to utilize glucose, and increased energy expenditure after eating. 

Fasting and Women

Recent research has revealed nuances to fasting when it comes to women. According to a recent article from the Cleveland Clinic ^[1], fasting is more effective and likely to cause less hormonal imbalance if it is timed up to your cycle. Ideal timing for fasting is one to two days after your period begins as well as a week after. It is best to exercise caution and limit fasting during the two weeks before your period is due (when you’re most likely to be ovulating), because this is when your hormones are most likely to be affected by fasting. Moreover, its best to avoid fasting the week before your period because this is when your body is the most vulnerable to stress; estrogen drops during your period, which leads to cortisol sensitivity. 

After menopause, although ovulation and menstruation will not be affected by fasting, you should exercise caution because of the symptoms associated with having low hormone levels.

What are the benefits?

• Thinking and Memory: Studies show that IF boosts working memory in animals and verbal memory in adult humans. ^[2]
• Heart Health: IF can improve blood pressure and resting heart rates as well as other heart-related measurements.^[3]
• Physical Performance: A study on young men who fasted for a 16 hour period showed that subjects were able to maintain muscle mass while losing weight. ^[4]
• Type 2 Diabetes and Obesity: In animal studies, IF has been shown to prevent obesity. In studies with obese adult humans, IF proves effective for weight loss. For those with type 2 diabetes, most available research shows that IF not only helps people lose body weight but also lower their levels of fasting glucose, fasting insulin and leptin while reducing insulin resistance, decreasing levels of leptin and increasing levels of adiponectin. Some studies even show that patients practicing IF, with the supervision of a healthcare profession, are able to reverse their need for insulin therapy.^[5]
• Tissue health: In animals, IF reduced tissue damage in surgery and improved post-surgery outcomes. ^[6]
• Improved Sleep: Fasting increases the production of the neurotransmitter orexin-A, which is tied to alertness, which has the effect of making one more alert during the day and increasing restful sleep. 
• Appetite and Weight Loss: Intermittent fasting can reduce appetite by lowering ghrelin levels (an appetite-controlling hormone), and therefore has the potential to help one lose weight. However, prolonged periods of not eating may also slow one’s metabolism and lead to overeating which can result in weight gain, increased inflammation and disrupted hunger cues. For this reason, it is important to choose the right fasting protocol, with some studies suggesting that the 12/12 plan (12 hours of fasting followed by a 12 hour eating window) is the safest and most sustainable.

*Note: due to the relatively recent interest in IF, many of these results come from animal studies. Continued research into the effects of IF on humans is needed. 

What are the costs?

One of the appeals of intermittent fasting is its cost-effectiveness. Since IF focuses on when to eat rather than what to eat, there are no specific food requirements or supplements necessary to start. Therefore, the costs associated with IF are minimal and primarily involve the potential need for nutritional guidance or consultation with a healthcare provider to ensure it’s a safe option for you. Some may also invest in apps or books for guidance and inspiration, which would be additional but relatively low costs.