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What is intermittent fasting?
At its simplest, intermittent fasting (IF) means cycling through periods of voluntary abstinence from food, interspersed with normal food intake.
The exact number of Calories consumed on a fasting day varies, but usually falls somewhere between 0 and 1000kcal/day, depending on the individual and the IF regime.
One of the most common intermittent fasting protocols is 16:8 which entails eating all of your daily Calories in 8 hours and fasting for the remaining 16 hours.
The 5:2 diet is another popular IF approach and an example of alternate-day fasting. On the 5:2 diet, people eat normally 5 days of the week and fast for the remaining 2 days.
The idea behind IF is that periods of restriction help reduce total Calorie intake, promote fat breakdown, and foster cellular regeneration. This is thought to lead to a variety of metabolic and neurological benefits.
What are the benefits of intermittent fasting?
Weight-loss and insulin resistance
Results vary by protocol, however, in general, IF appears to help with weight loss, fat loss, and insulin sensitivity — particularly in those who are overweight or obese. Being overweight or obese is a risk factor for several chronic diseases, including type II diabetes, heart disease, some forms of cancer, sleep apnea, and osteoarthritis.
Preliminary evidence suggests that fasting promotes autophagy, the cellular self-cleaning process that breaks down and recycles damaged molecules and cells. Autophagy is thought to help protect the neurological system and mediate inflammation — a factor associated with aging and many chronic diseases. However, larger human trials are still needed to confirm these results.
Brain function and cognition
Emerging human and animal studies indicate that IF may enhance cognitive capabilities such as memory and attention. These brain benefits are believed to be related to improvements in insulin sensitivity and inflammation brought about by IF.
Furthermore, animal trials suggest that IF may help spur neurogenesis — the production of new neurons — and help guard against neurodegeneration. Again, these results are only preliminary and still need to be confirmed by larger human studies.
Cholesterol and inflammation
IF appears to improve inflammatory markers and help fend off oxidative stress. In addition, some small human studies indicate that IF may be useful for reducing cholesterol and triglyceride levels, especially in people who are overweight or obese. Elevated cholesterol and triglycerides are both considered risk factors for heart disease.
What are the side effects of intermittent fasting?
Although IF is often well-tolerated, it may cause feelings of hunger, changes in mood, irritability, difficulty concentrating, fatigue, constipation, hypoglycemia, food cravings, and overeating during non-restricted days.
In addition, it may have a negative effect on exercise performance. Most studies to date have evaluated IF on a short-term basis and little is known about the long-term effects.
Are there groups that should avoid IF?
Intermittent fasting may be helpful for some, however, it can be counterproductive or even dangerous for others. IF is not recommended for people with type I or insulin-dependent diabetes without medical supervision. For this group, IF can lead to dangerously low blood sugar levels and potential loss of consciousness.
Fasting is also not recommended for professional and endurance athletes because of their increased Calorie and carbohydrate demands. IF may make it difficult for athletes to meet total Calorie and carbohydrate needs, resulting in a drop in performance and potentially a loss of muscle mass.
Those with a history of disordered eating should also consult their doctor before starting IF. IF is restrictive by nature and can increase preoccupation with food, and promote binge-purge cycles among vulnerable individuals.
Finally, IF is not recommended for pregnant women because of the metabolic demands of pregnancy and the nutrients required to support normal growth and development. If you’re pregnant and/or nursing, be sure to consult your doctor before starting IF.
The bottom line
As with many weight-loss approaches, intermittent fasting may benefit some and not others. Research on IF is still in its infancy and while it shows promise, we still have much to learn how it works, and who it’s right for.
At this point, IF is not recommended for pregnant women, athletes, and/or diabetics without medical supervision. For these groups, IF may be harmful rather than healthful. Furthermore, people with a history of disordered eating should approach IF with caution and seek advice from their medical provider before starting an IF protocol.
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