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What is Intermittent Fasting?

There are many types of fasts. Previously I discussed a fasting-mimicking diet which is a short term fast that can be done periodically. Today we discuss a specific type of fast that can be done daily, and long term. Intermittent fasting or time restricted feeding is defined as restricting eating for at least 12 hours but less than 24 hours (1). One consumes all their daily calories in a 3-4 hour, 7-9 hour, or 10-12 hour window per day. This type of fast in studies has lower dropout rates (10%) so it is more tolerable than other fasts and is an easy intervention for most people (2).

Can I lose weight with Intermittent Fasting?

Yes! One study showed a pound per week weight loss by restricting eating to a 6 am to 7 pm window. Participants lost a pound each week over 2 weeks compared to a control. No other interventions or changes to food selection was required (3). Another study had participants eat in an 8 hour window from 10 am to 6 pm for 12 weeks and they lost an average of 7 pounds compared to controls. The time-restricted feeding group also decreased their blood pressure (4). This could be partly because with restricting feeding time, one inadvertently decreases caloric intake.

Does the time of day you eat during Intermittent Fasting matter?

Most all life forms have a circadian rhythm. This is an internally generated 24-hour cycle of biological processes that can be modulated by the environment. These circadian rhythms are important for sleep, eating patterns, brain wave activity, hormone production, cell regeneration, and other biological processes. One study showed that there are many circadian rhythm genes important in the body’s response to the hours one is fasting (5).

So what time of day is best for fasting?

One study showed that if nighttime feeding is restricted, usually less healthy food is reduced as people tend to eat more sweets and fatty foods later at night (6). Another study showed that when eating the same amount of calories at dinner versus breakfast, dinner causes more fat gain (7). Also, night time snacks are more fattening than snacks in the day (8). One study compared eating late in 1 meal during a late (4 hr) window versus eating the same amount in 3 meals. The 20 hr fasting group lost fat mass but increased blood pressure and cholesterol because they were eating at night (9). So it appears that if you are going to do Intermittent Fasting, having the feeding period during the day is better than feeding at night.

What are some other benefits of Intermittent Fasting?

When early time-restricted feeding was tested in pre-diabetics, it showed improvements in insulin sensitivity, immune function, blood pressure, and oxidative stress (10). Another small pilot study took overweight individuals and had them them reduce their calorie intake to any 10 hour window (but during a consistent time) over 16 weeks. They showed weight loss, increased energy, improved sleep, and the participants wanted to stay on the program after the study ended. They maintained the program and the benefits for at least a year (11). Avoiding late night eating can even reduce the risk of breast cancer and breast cancer recurrence(12).

Intermittent Fasting may also be one of the reasons that the longest living people in America, the Loma Linda, CA Seventh-day Adventist Church, have such longevity. In addition to being vegetarian, thin, active, nonsmokers, it is also tradition that they only eat 2 meals a day — breakfast and dinner. More recently this is not as well-practiced but most of this group still have their largest meals being breakfast and lunch and typically don’t snack much (13). Intermittent Fasting has been shown to decrease insulin, cause more fat reserves to be used for energy, reduce visceral (unhealthy) fat, lower blood pressure, reduce cholesterol, decrease the risk for Alzheimer’s, and reduce cancer risks (14,15). Intermittent Fasting can even be good for stem cells!

Who should and should not consider Intermittent Fasting?

Intermittent Fasting is good for most normal or overweight adults that want to optimize their weight and health. This may not be good for people with eating disorders, those that are frail or low body weight, pregnant or breastfeeding women, or children. Those with medical problems especially diabetics on medications, those with kidney problems, heart problems, or older than 70 years old should consult with their doctor first.

The Bottom Line

Intermittent Fasting or time-restricted feeding is an easy to follow tool that can help you lose weight and has many health benefits. Even independently from changing food choices, it has good benefits. It is best to do a feeding window earlier in the day and avoid eating at night. There is also benefit to being consistent with the feeding window and sleep patterns every day to coincide with our natural circadian rhythm. I recommend everyone starting Intermittent Fasting to try to go at least 12 hours without eating. Try to work your way to an 8-10 hour feeding window and a 14-16 hour fasting period. Avoid having the feeding window within 3-4 hours of going to sleep. You can, and should, drink water during the fasting period and can also have unsweetened tea and coffee as long as there are no calories, and no artificial sweeteners. Keep this program consistent every day including the weekends as best as possible. For extra credit eat healthy foods during the feeding period! See Nutrition 2.0 for more tips.

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2. Rothschild J, Hoddy KK, Jambazian P, Varady KA. Time-restricted feeding and risk of metabolic disease: a review of human and animal studies. Nutr Rev. 2014;72(5):308–318. doi:10.1111/nure.12104
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8. Nighttime snacking reduces whole body fat oxidation and increases LDL cholesterol in healthy young women. Hibi M, Masumoto A, Naito Y, Kiuchi K, Yoshimoto Y, Matsumoto M, Katashima M, Oka J, Ikemoto S.Am J Physiol Regul Integr Comp Physiol. 2013 Jan 15;304(2):R94-R101. doi: 10.1152/ajpregu.00115.2012. Epub 2012 Nov 21.
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12. Marinac CR, Natarajan L, Sears DD, et al. Prolonged Nightly Fasting and Breast Cancer Risk: Findings from NHANES (2009-2010). Cancer Epidemiol Biomarkers Prev. 2015;24(5):783–789. doi:10.1158/1055-9965.EPI-14-1292Circadian rhythm.
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15. Mattson MP, Longo VD, Harvie M. Impact of intermittent fasting on health and disease processes. Ageing Res Rev. 2017;39:46–58. doi:10.1016/j.arr.2016.10.005