Intermittent Fasting and Time Restricted Eating — duration and timing

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Introduction

People have heard about the benefits of intermittent fasting and time-restricted eating; however, the duration and timing of fasting are important considerations. Intermittent fasting refers to specific times for ‘eating’ and ‘not eating’ (fasting) on a set schedule. There are two main types, which are (1) partial fasting or alternate-day fasting, and (2) time-restricted feeding (TRF).

Two Main Types of Intermittent Fasting

Partial Fasting or Alternate Day Fasting is a type of intermittent fasting where fasting days are set to a specific number of days out of the week and may include one day out of seven, or ‘alternate day fasting’ — where the usual amount of food is consumed every other day.

Time-Restricted Eating (TRE), also called time-restricted feeding (TRF), is a type of intermittent fasting where eating occurs during a specific period each day, and ‘not eating’ (fasting) occurs the remainder of the time.

The “5:2 diet” is a type of time-restricted eating that has people eating 25% of their normal caloric intake on two non-consecutive days per week (i.e., ~400-500 calories per day for women and ~500-600 calories for men), and consuming normal intake on the other five days. Since the focus is on restricted caloric intake and not intermittent eating, it has not been elaborated on in this article.

A common TRE schedule is 16:8, and is where all eating occurs during 8 hours, and the remaining 16 hours per day is a period of fasting. This is often done by people skipping breakfast and eating from midday until ~8 PM.

Periods of Fasting of 24 hours or more

Except as required for specific clinical or religious reasons, I do not recommend that people engage in periods of fasting of 24 hours or more based on the effect of fasting on lean body mass (muscle). For this, I refer to the work of medical doctor Dr. Stephen Phinney, MD, PhD, and Registered Dietitian Dr. Jeff Volek, RD, PhD.

Phinney and Volek have documented that, in periods of prolonged fasting (1-42 days), nitrogen loss, which is a marker of protein loss, begins on day 1 and reaches a maximum on day 3, then gradually declines[1].

“Net protein breakdown begins within the first day of fasting, reaches its maximum rate within 2-3 days – typically a pound of lean tissue lost per day.[1]”

While the human body has fat stores that can easily sustain us during extended periods where we don’t eat, the body does not have reserve protein stores to sustain us over long periods of fasting. All the protein in the body — whether as muscle, red blood cells, antibodies, or neurotransmitters is functional, so “whenever the body loses protein, it loses some of its functional reserve[1].“

Time Restricted Eating

Time-restricted eating (TRE), also called time-restricted feeding (TRF), is, for most people, the easiest form of intermittent fasting to do because the period of not eating only occurs for part of the day.

An early pilot study from 2019 in adults with metabolic syndrome which includes high blood pressure, elevated blood sugar, excess fat around the abdomen, and abnormal cholesterol levels found that limiting eating to only 10 hours per day (i.e. a 10 hour “eating window”) promoted weight loss, reduced abdominal fat, and led to more stable blood sugar and insulin levels in those taking standard medication to lower cholesterol and blood pressure [2].

The researchers from the Salk Institute, including circadian biologist Dr. Satchidananda Panda, concluded that the 10-hour time-restricted eating schedule supported an individual’s circadian rhythms, which results in health benefits evidenced by previous and subsequent mouse studies published by the Salk team[3].

Circadian rhythms are the regular 24-hour cycles of biological processes that affect nearly every cell in the body.

“Eating and drinking everything (except water) within a consistent 10-hour window allows your body to rest and restore for 14 hours at night. Your body can also anticipate when you will eat, so it can prepare to optimize metabolism.[3]”

A follow-up study from the researchers at Salk Institute that was published in 2024, found that people who ate within a consistent eight-to-ten-hour window each day for three months saw improvements in several markers of blood sugar and metabolic function, compared to those who received standard treatment [4].

Given that more than 1/3 of adults in the US [4], and more than 19% — or 1 in 5 adults in Canada have metabolic syndrome[5] which rises to ~40% in adults over the age of 65, the implication that simply changing the amount of time each day in which eating occurs can significantly improve markers or metabolic syndrome, is significant.

Erratic Eating Patterns in Adults

A landmark study from 2015 by circadian biologist Dr. Satchidananda Panda of Salk Institute had healthy, normal-weight adults who did not perform shift work track everything they ate each day for 21 days (3 weeks) by taking pictures of it using a smartphone app. The time-stamp on each photo enabled analyses of the time at which eating occurred.

In contrast to the popular belief that most people eat three meals per day within a 12-hour interval, this study found that eating patterns are much more erratic and differ between weekdays and weekends. The amount of time spent eating each day approached 15 hours per day for half the people in the study, and the only time they were not eating was when they were sleeping. In addition, less than 25% of calories were eaten before noon, and more than 35% of calories were eaten after 6 PM.

Supporting Time-Restricted Eating

It’s important to keep in mind that for some people, the idea of eating actual meals over a 10-hour period without eating snacks IS intermittent fasting when compared to the way that they usually eat.

For those accustomed to eating meals and snacks or grazing over 15 hours, the idea of eating meals with nothing between over 10 hours can seem daunting. Many are concerned they will be hungry, and others that their blood sugar will drop, making them feel lightheaded or dizzy. By ensuring that meals are made up of sufficient amounts of highly bioavailable protein and healthy fats, both of these concerns are easily addressed.

Implementing a 16:8 Time Restricted Eating Schedule

As mentioned above, a common time-restricted eating schedule is 16:8, where eating occurs during an 8-hour eating window, and the remaining 16 hours per day are fasting. Most people skip breakfast and begin eating from 11 AM or noon until ~7 or 8 PM. This is called a late time-restricted eating window (TREL). That, however, is not the only way to eat within a 16:8 window.

Some will eat breakfast at ~7 or 8 AM and finish eating at 3 or 4 PM (with one or no meals in between), and this is called an early time-restricted eating window (TREE).

Chrononutrition

Chrononutrition is a field of study that examines the complex relationship between when eating occurs relative to normal human circadian rhythms and metabolic health, and studies indicate that the timing of eating matters — both the timing of the first eating occasion, and how late the last meal occurs.

[Image of circadian rhythm and meal timing]

Studies have shown that glucose tolerance as well as insulin sensitivity are at their peak in the morning and decrease significantly later in the day. As a result, when meals are eaten late in the evening, when insulin production is at its lowest, there is an increase in blood glucose, insulin release, appetite, and the risk of obesity [8,9].

When We Eat Matters

In the 16-year prospective Health Professionals Follow-up Study, which had almost 27,000 middle-aged male subjects, men who skipped breakfast had a 27% higher risk of coronary heart disease found to be mediated by higher BMI, high blood pressure, diabetes, and high cholesterol [10]. In the same study, there was a 55% higher coronary heart disease risk in men who ate late at night [11].

Early or Late Time-Restricted Eating – Some Considerations

For adults who need to eat their dinner with other family members, they will often adopt a late time-restricted eating window (TREL). A drawback to this is that eating this late can delay sleep onset because one of the signals required for the body to release melatonin from the brain is a drop in core body temperature. Since digestion of food produces a lot of heat, late consumption of food delays the release of melatonin and, subsequently, delays sleep.

Final Thoughts…

For those who are used to eating or grazing over 15 hours, having a Meal Plan that distributes meals over a 10-hour eating window may be the best place to start. This will allow for improvements in metabolic markers, fat loss, and muscle gain, while being able to eat their meals with family members. As in other areas, there is no one-size-fits-all approach.

More Info?

I discuss the effect of light exposure and sleep with all of my clients taking Routine Health services because achieving optimal metabolic health necessitates it. I am very comfortable building in periods of time-restricted eating into your lifestyle. Learn about me and the Comprehensive Dietary Package that I offer, which will enable me to support you with these dietary and lifestyle changes.

To your good health!

Joy

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References

  1. Virta Health, Phinney S., Volek J., To Fast or Not to Fast: What are the Risks of Fasting, December 5, 2017, https://www.virtahealth.com/blog/science-of-intermittent-fasting
  2. Wilkinson MJ, Manoogian EN, et al Ten-Hour Time-Restricted Eating Reduces Weight, Blood Pressure, and Atherogenic Lipids in Patients with Metabolic Syndrome, Cell Metabolism, Volume 31, Issue 1, 92 – 104.e5. [https://pubmed.ncbi.nlm.nih.gov/31813824/]
  3. Salk News, December 5, 2019, Clinical Study Finds Eating Within a 10-hour Window May Stave Off Diabetes, Heart Disease, https://www.salk.edu/news-release/clinical-study-finds-eating-within-10-hour-window-may-help-stave-off-diabetes-heart-disease/
  4. Manoogian ENC, Wilkinson MJ, et al. Time-Restricted Eating in Adults With Metabolic Syndrome: A Randomized Controlled Trial. Ann Intern Med.2024;177:1462-1470. [https://pubmed.ncbi.nlm.nih.gov/39348690/]
  5. Rao DP, Dai S, et al, (2014) Metabolic syndrome and chronic disease, CDIC, Vol. 34, No. 1.
  6. Gill, S., and Panda, S. (2015). A Smartphone App Reveals Erratic Diurnal Eating Patterns in Humans. [https://pmc.ncbi.nlm.nih.gov/articles/PMC4635036/]
  7. Raji OE, et al. Chrononutrition and Cardiometabolic Health: An Overview. [https://pubmed.ncbi.nlm.nih.gov/39064774/]
  8. Poggiogalle E, Jamshed H, Peterson CM. Circadian regulation of glucose, lipid, and energy metabolism in humans. [https://pubmed.ncbi.nlm.nih.gov/29195759/]
  9. Saad A, Man, CD et al, Diurnal Pattern to Insulin Secretion and Insulin Action.
  10. Cahill L.E., et al. Prospective Study of Breakfast Eating and Incident Coronary Heart Disease. [https://pubmed.ncbi.nlm.nih.gov/23877060/]
  11. Paoli A., et al. The Influence of Meal Frequency and Timing on Health in Humans. [https://pubmed.ncbi.nlm.nih.gov/30925707/]
  12. Witbracht M., et al. Female Breakfast Skippers Display a Disrupted Cortisol Rhythm. [https://pubmed.ncbi.nlm.nih.gov/25545767/]
  13. Kutsuma A., Nakajima K., Suwa K. Potential Association between Breakfast Skipping and Late-Night-Dinner Eating. [https://pubmed.ncbi.nlm.nih.gov/24982814/]
  14. Ha K., Song Y. Associations of Meal Timing and Frequency with Obesity and Metabolic Syndrome. [https://pubmed.ncbi.nlm.nih.gov/31614924/]
  15. Xiao Q., et al. Meal Timing and Obesity. interactions with macronutrient intake and chronotype. Int J Obes (Lond). 2019 Sep;43(9):1701-1711. doi: 10.1038/s41366-018-0284-x. Epub 2019 Jan 31. PMID: 30705391; PMCID: PMC6669101 [https://pubmed.ncbi.nlm.nih.gov/30705391/]
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