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Intermittent Fasting: Does It Matter When I Eat?

The Shaw Cancer Center Dietitian Team
Intermittent fasting, or going without eating for at least 12 hours but less than 24 hours, has been a proven method for managing weight and reducing overall body inflammation. When we eat even healthy foods, there is an inflammatory response in the body that increases insulin, triglycerides, cholesterol and blood sugar levels. Additionally, eating late at night tends to lead to poorer food choices such as salty or sweet snacks that are more processed like chips and ice cream.

Intermittent Fasting Data

Studies show benefits of prolonged overnight fasting. With no other dietary changes, not eating between 7:00 PM and 6:00 AM led to a 1 pound per week weight loss compared to no time restrictions on eating. Another study showed that subjects eating from 10:00 AM to 6:00 PM lost 7 pounds over 12 weeks and reduced their calorie intake by 250 calories per day. 

Some of this weight loss can be attributed to less time in eating and avoiding eating when our food choices are typically poor, but it also can be due to a “metabolic” circadian rhythm. When consumed at dinner instead of at breakfast, the same foods and calories are more fattening and raise insulin, triglycerides and blood sugars more. The subjects in the study who ate the same calories at dinner instead of at breakfast lost less weight. 

Additionally, in weight loss studies that kept eating only in the evening hours, the subjects lost more weight than those eating the same number of calories over longer period, but those only eating in the evening had an increase in blood pressure and LDL (“bad”) cholesterol, which is negating the benefits of the weight loss.  

A 2018 study showed that when time restricting their feeding between 8:00 AM and 3:00 PM over 16 weeks, subjects lost 7 pounds and reportedly were sleeping better, had increased energy, and expressed desire to continue with this eating schedule even after the study was completed. Those who had a 12- to 14-hour overnight fast reduced their C-reactive protein levels by 8% (an inflammatory marker). Additionally, for every 3 hours increase in overnight fasting, hemoglobin A1C is reduced by 20% (a marker for blood sugar).

Intermittent Fasting and Cancer

For breast cancer survivors, fasting less than 13 hours per night had a 36% higher risk for recurrence than those who were fasting for 13 hours or more.  For cancer prevention and recurrence, there is some promising data on the benefits of a prolonged overnight fast. However, intermittent fasting during cancer treatment is not for everyone and should be discussed with your provider and dietitian to see if it is appropriate for you.

In conclusion, there seems to be a happy place of fasting between 13 and 14 hours overnight and still being able to get adequate fruits, vegetables, whole grains and lean proteins during the hours you are eating for weight management, reducing overall body inflammation, avoiding late night eating pitfalls and even reducing cancer recurrence.  

Information in this article was adapted from the following sources:
Marinac CR, Nelson SH, Breen CI, Hartman SJ, Natarajan L, Pierce JP, Flatt SW, Sears DD, Patterson RE. Prolonged Nightly Fasting and Breast Cancer Prognosis. JAMA Oncol. 2016 Aug 1;2(8):1049-55. doi: 10.1001/jamaoncol.2016.0164. PMID: 27032109; PMCID: PMC4982776.