Time-Restricted Feeding: Fitter Through Fasting

By Ashley Reaver, MS, RD, CSSD, October 26, 2021

time restricted feedingWhen you think of holidays, you probably don't think of food deprivation. But, plenty of cultures have integrated fasting into their traditional observances, for millenia. This month, Muslims around the world are celebrating Ramadan, a holiday which emphasizes fasting during daylight hours. Since the holiday essentially mandates time-restricted feeding, with food only being consumed before sunrise and after sunset, Ramadan has piqued the curiosity of the research world. What's more, we also recommend time-restricted feeding. Therefore, it only seemed appropriate to dive into the topic of fasting this month and discuss the potential benefits that time-restricted feeding can have on our health and how it can help us reach our goals.


5 blood biomarkers all athletes should knowThe old way: alternate day fasting

In recent years, Intermittent fasting has gotten a fair amount of attention. A simple Google search brings up nearly 1 million results, and here at InsideTracker we even devoted a four-part blog series to it last year. While intermittent fasting has a rather broad definition, it essentially refers to an eating pattern that cycles between periods of fasting and non-fasting. Typically, intermittent fasting is conducted through alternate day fasting, which entails regular eating for most days of the week with 2 to 3 days of severely restricted caloric intake on non-consecutive days. The major downside of this method is the simple fact that those fasting days are hard. Dropping down to just 500 calories a day usually causes intense feelings of hunger and subsequent grumpiness — ultimately causing individuals to revert back to regular eating.


A new solution: time-restricted feeding

This method, time-restricted feeding, encourages eating only within a defined time interval, usually an 8-hour window each day, and can be thought of as extending the normal overnight fast that occurs naturally with sleep. For example, with an 8-hour feeding window, food is only consumed between 10AM and 6PM. Since the required fasting time is much more manageable than an entire day, people are able to stick with it. In fact, in research studies conducted on time-restricted feeding (TRF) and alternate day fasting (ADF), the participant dropout rate for TRF is half that of the ADF studies!1

The concept of cutting off calories after a certain time is not a new one, however, the scientific support for it is — and for more reasons than just caloric restriction. Both feeding/fasting methods impact circadian rhythm, our body’s internal clock. Our circadian rhythm is mainly controlled by light and dark stimuli in an area of the brain’s hypothalamus called the SCN, suprachiasmatic nucleus. On the most basic level, humans have a diurnal rhythm, or two general periods: wake during the day and sleep during night.2 For all of human history, eating has primarily occurred in the wake period (daylight) and fasting in the sleep period (nighttime)…until recently. Hello, electricity!

intermittent fasting

With extended illumination, our eating patterns shifted to later in the day even though wake times remained relatively the same. This has resulted not only in an increase in feeding frequency, but has also caused shifts in our circadian rhythm, since eating late in the day or at night can disrupt our natural internal clock.2,3,4 Circadian rhythm, however, is responsible for a lot more than just falling asleep. It also dictates the release of hormones (such as cortisol and testosterone), our metabolic response to food (like insulin sensitivity), and influences body temperature and heart rate.

Time-restricted feeding can help to restore our natural diurnal rhythm, and therefore, the associated metabolic, hormonal, and physiological processes it controls. For this reason, TRF has a practical use for much more than weight loss and calorie restriction.


How time-restricted feeding can help you reach your health and fitness goals

Lose fat and optimize metabolism

Consume the majority of your calories earlier in the day.

  • It is associated with greater weight loss and a lower body weight.5
  • Insulin sensitivity decreases throughout the day.5 Our bodies aren’t able to respond to glucose as efficiently later in the day, especially right before bed. Eating earlier in the day, and therefore, restricting food intake later in the day can help to improve fasting blood glucose. TRF trials have reduced fasting blood glucose by 10-27% when conducted for as little as 4 weeks.1
  • LDL cholesterol and triglycerides respond favorably to TRF with reductions ranging from 9-37% and 4-37%, respectively.1

Fasting 2 - Square – Education image-minBuild power & strength, build endurance, gain muscle

Work out in the early evening and have your final meal after.
  • Humans experience the greatest cardiovascular and muscle strength near the end of their wake/feeding period.4
  • Hormones associated with muscle building and recovery such as testosterone, cortisol, and growth hormone follow a circadian rhythm. Having an aligned circadian rhythm may help to regulate those hormones to ensure they function properly.4
  • Unfortunately, there are no TRF experiments conducted on exercising subjects to determine the best timing of exercise; however, in animal models, mice showed improved muscle coordination, increased activity, and increased energy expenditure at the end of their feeding period.4

Sleep better

Stop eating a few hours before you go to bed.

  • Nighttime eating is associated with reduced sleep duration and poor sleep quality.4
Better digestion

Follow a consistent eating pattern by sticking with your TRF interval each day.
  • TRF can help to restore and maintain the alignment of circadian rhythm with the gut clock. Consistent, rhythmic feeding helps to maintain the symbiotic environment of the microbiome. 20% of all bacterial species in the microbiome undergo diurnal fluctuations.6
  • The microbiome helps to manage an individuals’ response to particular substances, including food. A disrupted microbiome can worsen digestive problems and inflammation in the GI tract.6

Fight aging

Consume less than 30% of your daily calories in the evening.

  • In females, reduced energy intake in the evening, starting the nighttime fasting interval earlier in the evening, and eating more frequently is associated with reduced CRP. Each additional hour of fasting was associated with an 8% decrease in CRP.8 Another fasting trial in men showed that CRP was reduced by 13% after 12 weeks.9
  • No large-scale studies have been conducted in humans. All published research specific to longevity was conducted using animals. However, many factors that attribute to aging, and are included in InsideTracker’s InnerAge, can be positively affected by TRF such as blood glucose and hsCRP.

Time-restricted feeding is a relatively easy way to improve your health and reach your goals. We recommend an 8-10 hour feeding window. For most people, it is a combination of small changes that lead to successfully improving health. Learn what other tips we have for you at InsideTracker.



[1] https://pubmed.ncbi.nlm.nih.gov/24739093

[2] https://pubmed.ncbi.nlm.nih.gov/26706567 

[3] https://pubmed.ncbi.nlm.nih.gov/26226640 

[4] https://pubmed.ncbi.nlm.nih.gov/25857868 

[5] https://pubmed.ncbi.nlm.nih.gov/25404320 

[6] https://pubmed.ncbi.nlm.nih.gov/25901892 

[7] https://pubmed.ncbi.nlm.nih.gov/23000010 

[8] https://pubmed.ncbi.nlm.nih.gov/26305095 

[9] https://pubmed.ncbi.nlm.nih.gov/24215592 


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