The Skinny on Intermittent Fasting

Time Out–Make it Safe

Aug 28, 2019

By Carla Yaldezian, MS, RD – CareerSmart® Learning Contributor

While obesity has long been attributed to excessive calorie intake and lack of physical activity, a growing body of research suggests that the timing of meals, especially the timing of the last meal of the day, may be more influential in weight management than calorie consumption alone. Additionally, meal timing has been found to affect insulin regulation and cell regeneration. Thus, the practice of fasting is notably becoming a sought-after method of weight loss while also being recognized for its benefits, such as its beneficial effects on diabetes, heart disease, cancer, and even cognitive and brain health. The way intermittent fasting works to aid in weight loss is by changing the physiological responses of the body by what is known as a “metabolic switch.”  This switch shifts the body’s preferred method of burning glucose to burning fat. This metabolic switch occurs when stored glucose in the liver is depleted, which generally occurs 12 hours after the last meal or food consumption. Eventually, the body becomes more efficient in utilizing the energy given at each meal and reduces storage of unused energy. Cell regeneration is also thought to occur more easily, as fasting is the body’s internal mechanism designed for detoxing and cleansing.

Fasting is often thought to be the act of going without food and, in some instances, without beverages. However, there are various types of fasting, and most studies have included water during periods of fasting. Popular forms of fasting include fasting for a certain timeframe during the day, on certain days during the week, or even per month; this is referred to as “intermittent fasting.” Combing through the research on this topic can be confusing as even literature varies in defining intermittent fasting, and findings can be inconsistent. Therefore, we will review popular methods of intermittent fasting.


Time-restricted fasting includes fasting for a period of 12, 14, or even 16 hours during a 24-hour period. Individuals have the freedom to set the time period, as long as the duration of the fast is 12,14, or 16 hours. For example, the first meal can be at 10 a.m. and the last meal at 6 p.m. Alternately, the first meal can be at 11 a.m. and the last meal at 7 p.m. Studies have found this method to be an effective tool in insulin management, especially in diabetic populations.(1) After a period of fasting, insulin sensitivity rises and insulin levels fall. This results in improved blood sugar control before and after meals as well as less adipose tissue growth due to lower levels of circulating insulin. Improved insulin sensitivity can also decrease inflammation in the body, LDL levels or “bad” cholesterol, and other metabolic factors that can contribute to the development of heart disease.(1)


Alternate-day fasting involves alternating fasting days and feeding days. During fasting days, no calories are consumed. During feeding days, calories are consumed ad libitum or as desired. Studies using human and rodent models found a 2.5% weight loss and reduction in total cholesterol and triglyceride concentrations, reduced inflammatory gene expression, and improved cell proliferation.(2) A caveat of this meal timing strategy is practicality, as subjects reported extreme hunger and lower mood during fasting. Thus, this may not be a realistic approach for most.(2)

5:2 DIET

The 5:2 diet involves restricting total energy needs to 20-25% on two non-consecutive days per week and unrestricted eating during the other five days of the week.(2) This is a modified version of alternate-day fasting. The primary difference is that this method does not involve days of zero calorie intake but rather significantly restricts calorie intake two days out of the week. Studies found similar results of weight loss and improved metabolic outcomes. One of the main differences reported was a smaller percentage of negative side effects, such as increased hunger, low energy, or low mood.(2). One study found that, compared to a typical caloric restricted diet (1,200 to 1,500 kcal restriction), weight loss was about 4% more effective with the 5:2 intermittent diet over an 8-weeks period, coupled with an exercise regimen. (2)

Intermittent fasting can be a useful short-term option for healthy weight loss, but the evidence is still not conclusive as to which method is superior and what health risks may arise, if any at all. Individuals with multiple comorbidities should seek advice from their health care provider before starting any method of intermittent fasting.

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(1) Grajower, M. M., & Horne, B. D. (2019). Clinical Management of Intermittent Fasting in Patients with Diabetes Mellitus. Nutrients11(4), 873. MDPI AG. Retrieved from

(2) Patterson, R., & Sears, D. (2017). Metabolic Effects of Intermittent Fasting. Annual Review Of Nutrition37(1), 371-393. doi: 10.1146/annurev-nutr-071816-064634. Retrieved from

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