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The Pros and Cons of Intermittent Fasting

Intermittent fasting has recently grown in popularity due to a fad diet called the 5:2 diet. It does not dictate certain foods to eat or avoid, but instead prescribes intermittent fasting, which is an eating pattern that alternates between periods of eating and fasting. On two non-consecutive days of the week, the participant drastically reduces intake to 500-600 calories. For the other five days of the week, there are no restrictions. There are other diets that include complete fasts, where no food is consumed for a length of time, commonly 16-24 hours, twice per week.

Fasting is not a new concept; humans have been required to fast due to the unreliability of access to food throughout history. Sometimes fasting occurred because food was simply not available. It has also been a part of religious holidays across different faiths. But does it make sense to purposefully deprive ourselves of food when it is readily available in modern society? Let’s examine the pros and cons of intermittent fasting.

The major appeal of intermittent fasting to those seeking to lose weight is that no foods are restricted. The eating plan does not discuss which foods to eat, but rather when you should eat them. By simply eating fewer meals over time, intermittent fasting can lead to an automatic reduction in calorie intake.

While eating healthy is simple in theory, it can be hard to stick to, due to the time required to plan and prepare meals. Fasting can help reduce the prep work, cooking, and cleaning required. As a result, fasting can help simplify your life. It is well known that compliance to calorie-restricted diets is low. For some people, fasting may be easier than cutting calories at every meal. By setting a schedule where you only have to worry about what you’re eating 2 days per week, adherence to this type of diet may be more successful.

Interestingly, fasting as little as 16 hours seems to cause changes in the body on a cellular and molecular level. For example, hormone levels change to make stored body fat more accessible. As your body recognizes that food is scarce, energy must be used from somewhere, so it readies itself to use stored energy (fat) instead of food.

Studies on fasting, though mostly in animals, have also shown improvements in inflammation, oxidative stress and insulin resistance. Unfortunately, many of the studies currently published were small, short in duration, or conducted in animals. Further research is required in humans to see if these benefits are directly caused by fasting.

While the benefits are still being studied, there are known negatives of intermittent fasting.

Hunger is the primary side effect of intermittent fasting, which may also lead to feeling weak and tired. While this may be temporary as the body adapts, some decide that these side effects are too disruptive to energy levels and even work performance. Increasing hunger may also lead participants to binge on non-fasting days, which could lead to weight gain as well as blood sugar control problems. This pattern may exacerbate binge-eating tendencies for those struggling with disordered eating.

Intermittent fasting has the potential to lower your metabolism by propelling your body into starvation mode, which can cause muscle to be broken down for energy instead of fat. Some studies have shown that this can occur in as little as 24 hours of fasting. One solution is to include resistance training in your routine in order to prevent muscle loss.

If you have a medical condition, consult with your doctor before trying intermittent fasting. Pregnant or lactating women, growing adolescents and the elderly have increased nutrition demands that may not be met through intermittent fasting.

You may consider trying intermittent fasting for weight loss, convenience, or metabolic effects. Make sure to monitor your symptoms and adjust the diet if you find it is causing undesirable side effects.

Carolyn McAnlis, RDN, is a Registered Dietitian Nutritionist who has a special interest in preventing chronic disease through nutrition. She graduated from Syracuse University with a Bachelor of Science in Nutrition Science & Dietetics and a minor in Psychology. After completing a full-time dietetic internship at the University of Virginia Health System, she has developed a passion for convincing others that healthy food can be delicious through her blog A Dietitian in the Kitchen.

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