Intermittent Fasting and Time Restricted Eating — duration and timing

 

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. Participants were recruited through a newspaper advertisement, paper flyers, and online advertisements, and inclusion and exclusion criteria were determined by an online questionnaire and in-person interview.

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 (95% interval) 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.

While the sample set in this study was small, my clinical experience has found that at least half of adults eat this way. 

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. 

Studies support that eating and sleeping according to one’s natural circadian rhythms will, in and of itself, improve many health markers, and when one’s Meal Plan is designed to factor in desired fat loss and/or muscle gain, the benefits are additive.

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 (TRE_L). 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 (TRE_E).

Does it matter whether someone does an early or a late 16:8 time-restricted eating window? Chrononutrition studies seem to indicate that it can.

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. 

Circadian rhythms are regular fluctuations of physiological processes over 24 hours and include the production of hormones such as cortisol, insulin, and melatonin, as well as certain enzymes used in digestion.

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 other studies, breakfast skipping has been linked to greater type 2 diabetes risk, higher total and LDL cholesterol levels, body weight, fat mass, and abdominal adiposity, and lower HDL cholesterol [11,12]. In the same study, there was a 55% higher coronary heart disease risk in men who ate late at night [11]. 

In a 2014 study from Japan, breakfast skipping was associated with 28% and 57% higher odds of developing metabolic syndrome and obesity, but only when it was in combination with eating late-night dinners within 2 hours of bedtime [13]. While it is hard to tease out whether the issue is skipping breakfast or eating late into the evening, an earlier fasting window TRE_E solves both issues.

In the 2019 national Korean dataset, night eating, which was defined as eating after 9 PM, was associated with 48% higher odds of metabolic syndrome in men but not women, suggesting that there may be sex differences in these associations [14]. In ~900 middle-aged to older adults in the same Korean data set, a higher percentage of daily energy eaten within 2 hours of bedtime was associated with 82% higher odds of being overweight and obese, whereas a higher percent of of daily energy intake consumed during the morning window, within 2 hours of waking up was associated with 47% lower odds of being overweight and obese [15].

Early or Late Time-Restricted Eating – some considerations

To a large extent, for most adults deciding to adopt either an early or late time-restricted eating schedule comes down to a matter of personal preference and convenience.

For older adults however, the need to prioritize protein and the amino acid leucine first thing in the morning favours an early time-restricted eating window, but that will be a topic of an upcoming post. 

Adults who need to eat their dinner with other family members will often adopt a late time-restricted eating window (TRE_L). They will feed their family breakfast, but not begin to eat themselves until 11 AM, or noon. They will eat their second meal with their family around 5-6 PM, then eat their final meal of the day at ¬8 or 9 PM.  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. As well, eating within 2 – 3 hours of bedtime may be associated with some of the metabolic drawbacks observed in studies — especially for those who already have abnormal blood sugar, blood pressure, cholesterol, or weight.

Those who adopt an early time-restricted eating window (TRE_E), will usually have their breakfast at ¬7 AM, lunch at around 11:00 AM, and their last meal of the day around 4 PM. By having sufficient highly bioavailable protein at each of the three meals along with sufficient amounts of healthy fats, hunger is a non-issue, and blood glucose is well controlled because eating occurs when the body is the most insulin sensitive.

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.

Those who need to eat dinner with other family members may find adopting a late time-restricted eating window (TRE_L) the most convenient because it lets them feed their family breakfast while beginning to eat themselves closer to 11 AM, eat their second meal with their family around 5-6 PM, and eat their final meal of the day at ¬8 PM. While it can have an impact on sleep onset and be less than optimal in terms of metabolic improvements, it will likely be a significant improvement to how they were eating before.

Finally, those who can adopt an early time-restricted eating window (TRE_E), often enjoy having breakfast at ¬7 or 8 AM, lunch, if they eat it at around 11:00 AM, and eat their last meal of the day around 4 PM. Those who are responsible for making meals for their families do very well following this eating window by eating their dinner early, then sitting with their family while they eat dinner.  This way, they don’t miss out on this important time of connecting with the families.

As in other areas, there is no one-size-fits-all approach.

More Info?

If you would like to learn about how I can help support you with a style of time-restricted eating that best suits your lifestyle, please visit the landing page to learn about the services that I provide.

To your good health!

Joy

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References

 

1. 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, doi: 10.1016/j.cmet.2019.11.004
   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. doi:10.7326/M24-0859
   5. Rao DP, Dai S, et al, (2014) Metabolic syndrome and chronic disease, Chronic Diseases and Injuries in Canada (CDIC), Vol. 34, No. 1, https://doi.org/10.24095/hpcdp.34.1.06
   6. Gill, S., and Panda, S. (2015). A Smartphone App Reveals Erratic Diurnal Eating Patterns in Humans that Can Be Modulated for Health Benefits. Cell Metabolism. 22. 10.1016/j.cmet.2015.09.005.
   7. Raji OE, Kyeremah EB, Sears DD, St-Onge MP, Makarem N. Chrononutrition and Cardiometabolic Health: An Overview of Epidemiological Evidence and Key Future Research Directions. Nutrients. 2024 Jul 19;16(14):2332. doi: 10.3390/nu16142332. PMID: 39064774; PMCID: PMC11280377.
   8. Poggiogalle E, Jamshed H, Peterson CM. Circadian regulation of glucose, lipid, and energy metabolism in humans. Metabolism. 2018 Jul;84:11-27. doi: 10.1016/j.metabol.2017.11.017. Epub 2018 Jan 9. PMID: 29195759; PMCID: PMC5995632.
   9. Saad A, Man, CD et al, Diurnal Pattern to Insulin Secretion and Insulin Action in Healthy Individuals. Diabetes 1 November 2012; 61 (11): 2691–2700. https://doi.org/10.2337/db11-1478
   10. Cahill L.E., Chiuve S.E., Mekary R.A., Jensen M.K., Flint A.J., Hu F.B., Rimm E.B. Prospective Study of Breakfast Eating and Incident Coronary Heart Disease in a Cohort of Male US Health Professionals. Circulation. 2013;128:337–343. doi: 10.1161/CIRCULATIONAHA.113.001474.
   11. Paoli A., Tinsley G., Bianco A., Moro T. The Influence of Meal Frequency and Timing on Health in Humans: The Role of Fasting. Nutrients. 2019;11:719. doi: 10.3390/nu11040719.
   12. Witbracht M., Keim N.L., Forester S., Widaman A., Laugero K. Female Breakfast Skippers Display a Disrupted Cortisol Rhythm and Elevated Blood Pressure. Physiol. Behav. 2015;140:215–221. doi: 10.1016/j.physbeh.2014.12.044.
   13. Kutsuma A., Nakajima K., Suwa K. Potential Association between Breakfast Skipping and Concomitant Late-Night-Dinner Eating with Metabolic Syndrome and Proteinuria in the Japanese Population. Scientifica. 2014;2014:253581. doi: 10.1155/2014/253581
   14. Ha K., Song Y. Associations of Meal Timing and Frequency with Obesity and Metabolic Syndrome among Korean Adults. Nutrients. 2019;11:2437. doi: 10.3390/nu11102437.
   15. Xiao Q., Garaulet M., Scheer F.A.J.L. Meal Timing and Obesity: Interactions with Macronutrient Intake and Chronotype. Int. J. Obes. 2019;43:1701–1711. doi: 10.1038/s41366-018-0284-x.

 

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