Humans have fasted throughout history for many reasons. Lately, the practice of intermittent fasting* as a potential weight-loss tool and health benefit has gotten a lot of attention. But how does it work and is it truly healthy for us?
What’s intermittent fasting?
Intermittent fasting (IF) involves eating your meals during a specified window of time and avoiding food outside of that window. Examples of IF include:
- The 16/8 Method: Skip breakfast and restrict your daily eating period to 8 hours, for example, noon–8 p.m., then fast for the remaining 16 hours. This method is popular because it’s relatively easy to follow (and because many people skip meals anyway).
- Eat-Stop-Eat: Fast for 24 hours, once or twice a week, for example by not eating from dinner one day until dinner the next day. You can also fast from breakfast to breakfast, or lunch to lunch. This approach is more challenging since it can be difficult (both physically and mentally) to go a full 24 hours without eating.
- The 5:2 Diet: Eat only 500–600 calories on two non-consecutive days of the week, and eat normally the other five days. For example, you might eat normally on all days except Mondays and Thursdays, on which you eat only two small meals.
- Warrior: Fast for 20 hours a day and eat one large meal at night (or a similar variation). This means you need to try and fit all your nutritional needs (both in terms of macronutrients and vitamins, minerals and fiber) into a single meal, which can be a challenge.
What are some benefits of intermittent fasting?
A lot of the research is based on relatively short time periods or animal studies. More research is needed to assess its longer-term effects and if and how the animal research findings apply to humans. However, IF has still been linked to a variety of health benefits including:
- Weight loss – Generally speaking, restricting your “eating window” means you’re getting fewer calories. IF has also been shown to reduce insulin levels and increase levels of human growth hormone, which can both boost your body’s ability to burn fat. However, as with any dietary approach, it’s important to choose healthy, nutrient-dense foods. (The idea, of course, is NOT to eat fast food and candy during your eating window.)
- Reduced risk of diabetes – IF’s ability to reduce insulin levels and boost insulin sensitivity may help reduce the risk of diabetes.
- Reduced inflammation – Some research suggests that IF boosts production of anti-inflammatory compounds in the body.
- Cardiovascular health – IF may help reduce LDL (“bad”) cholesterol and triglycerides.
- Longevity – Animal research suggests that IF may play a role in healthy aging and an extended life span.
Are there downsides to intermittent fasting?
The short answer is yes. Here are several things to consider before trying any kind of fast.
- It’s not easy. You’ll likely be hungry at times and some people may find it difficult to avoid food during the fasting period. You may also feel tired (including not having enough energy to exercise), irritable and have headaches. (If you experience any negative effects, stop immediately.)
- There’s a risk of overeating. While most people find that they eat fewer calories overall when practicing IF, there’s always the possibility that you’ll end up indulging in unhealthy foods or eating more than you would on your regular schedule.
- It could interfere with the social aspect of eating. If you’re invited to dinner or brunch with friends at a time that’s outside your eating window it may mean missing out.
- It may reduce muscle mass. While IF may be beneficial for shedding unwanted pounds, some of that weight loss may include muscle, which can lower your metabolism. This could make it more difficult to maintain your weight loss long-term.
There’s limited scientific evidence. The idea of intermittent fasting in humans is relatively new and there’s no scientific evidence to tell us the impact (or overall safety) of intermittent fasting over longer periods of time.
Who should avoid intermittent fasting?
IF isn’t for everyone. If you want to give it a try, always discuss it with your health care provider first before starting. IF may not be right for you if you have:
- Any medical conditions. IF can lead to low blood-sugar levels during the fasting periods, so people with hypoglycemia or other blood sugar-related conditions should steer clear. People with digestive disorders may also want to avoid IF. It may prove more beneficial to eat at regular intervals rather than skipping meals to encourage regularity and make sure the body has enough time to digest between meals.
- Prescription medications. Many medications (as well as over-the-counter supplements) affect blood-sugar levels.
- High levels of anxiety or sleep issues. Fasting leads to low blood sugar levels. In response, the body may release stress hormones (i.e. cortisol), which can increase feelings of anxiety. Similarly, if your blood sugar levels drop low during the night, your body may release cortisol to bring it back up, which can disrupt sleep.
- History of eating disorders or chronic dieting. IF has the potential to lead to extreme calorie restriction, binge eating and ruminating and obsessing about food.
- History of amenorrhea (absence of menstruation) or irregular menstrual cycles or are pregnant, breastfeeding or trying to conceive. Calorie restriction isn’t recommended during pregnancy and breastfeeding. Some research suggests that women in general may not benefit as much from IF as men. For example, one study found that while insulin sensitivity was increased in men, blood sugar control was worse for the female participants. Because of the impact fasting can have on women’s hormone levels, it’s often recommended that if women fast, they do so for 14 rather than 16 hours to help maintain hormonal balance.
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