Intermittent fasting or IF is a practice involving alternating fasting time and/or calorie restriction with periods of feeding that has proven cellular benefits, metabolic gains and remission or reversal for a variety of symptoms and disease states. Time restricted eating is compressing an eating window to a specific number of hours each day. An example of this would be eating all the day’s food within a 6–8-hour window.
With the prevalence of obesity and chronic disease impacting our healthspan and quality of life, implementing the practices of intermittent fasting or time restricted eating may prove to be an important lifestyle tool for maintaining health and vitality as we age.
In Part One of this series, I went into detail about how intermittent fasting and time restricted eating works along with the long list of health benefits that have been linked to these lifestyle tools. In today’s article, Part Two takes a more practical view regarding the different ways to structure intermittent fasting and time restricted eating. We will also cover some of the most common questions about the safety and details of these two lifestyle practices. This will essentially be a guide to intermittent fasting and time restricted eating for beginners and experienced fasters alike.
This article covers the following topics:
- How to intermittent fast
- Does intermittent fasting work?
- Intermittent fasting times
- How to do time restricted eating
- Is intermittent fasting and time restricted eating safe?
- Are these two practices different for men and women?
- Can you drink coffee or tea?
- Does intermittent fasting and time restricted eating promote weight loss?
- Can a ketogenic diet be combined with intermittent fasting and time restricted eating?
By the end of this article, you’ll know if intermittent fasting and time restricted eating are for you and how to get started.
Time restricted eating meal plan hours – 16:8, 18:6, and 20:4
There are many ways to implement a time restricted eating and/or intermittent fasting plan. Let’s look at some of the most popular schedules for time restricted eating and intermittent fasting.
|Type of time restricted eating or intermittent fasting
|What to eat in your window
|Time restricted feeding (TRF)
|Fast for 16 hours overnight and condense meals into an 8-hour window
|Finish dinner by 8 pm then fast until 12 pm the next day
|May be practiced daily or a few times per week
|Time restricted feeding 18:6 (TRF)
|Fast for 18 hours overnight and condense meals into a 6-hour eating window
|Finish dinner by 6 pm and fast until 12 pm the next day
|May be practiced daily or a few times per week
|Time restricted feeding 20:4 (TRF)
|Fast for 20 hours overnight and condense meals into a 4-hour eating window
|Finish dinner by 6 pm and fast until 2 pm the next day
|May be challenging to meet nutrient needs if practiced daily
|One Meal A Day (OMAD)
|Eat only one meal per day and fast for 23 hours
|Eat between 12 pm and 1pm each day
|May be challenging to meet nutrient needs if practiced daily
|Alternate Day Fasting (ADF)
|24-hour fast every other day
|For example Monday – Fast Tuesday – Eat Wednesday – Fast Thursday – Eat
|Safe for several months, long-term challenges (1)
|5:2 fasting (periodic fasting)
|24-hour fast 2 days per week
|Monday, Tuesday – Eat Wednesday – Fast Thursday, Friday – Eat Saturday – Fast Sunday – Eat
|Fasting-Mimicking Diet (FMD)
|5 days of plant-based diet
|May be practiced monthly for between 3 and 6 months
|Plant-based diet of 800 to 1000 calories per day
|Food available through Prolon or Whole Food FMD program, available through the Hoffman Centre
With so many options, it may be challenging to determine how to start time restricted feeding or intermittent fasting. For example, do you just dive in or do you ease into it more slowly? I recommend starting with either time restricted feeding (TRF) or with the fasting-mimicking diet. (FMD). With that experience, you can then work with your provider or myself to determine if you’d benefit from other practices.
TRF may begin with a simple 12:12 schedule, meaning that you begin fasting overnight and then eat your regular diet within a twelve-hour eating window. For many people this isn’t that much different from their typical pattern, although they may have to be aware of any tendencies for late night snacking. A fast from 8 pm until breakfast at 8 am the following day is a good schedule to start with. Once you have this under your belt, you can expand your fasting window, in increments if needed, to a fourteen-hour fast with a ten-hour eating window. You can then potentially lengthen this to include a fast of sixteen hours or longer.
The fasting-mimicking diet (FMD) is a five-day program, typically practiced once per month for between three and six months, and then one time every 3-4 months as a maintenance program. During the five day fast, you follow a plant-based, calorie-restricted diet. The diet is derived from plant sources like vegetables, nuts, seeds, and fruit. The diet relies on plant foods for protein, olives, coconut and nuts and seeds for healthy fats. The diet constituents are carefully chosen by a nutritional expert. There is a commercially available program involving packaged constituents called ProLon.
With calories restricted to approximately between 750 and 1100 per day, with day one containing the most calories. This represents a réduction in calories of around 50 to 60 percent, this diet is designed to mimic molecular and cellular fasting while increasing patient compliance. The stomach sees food, while the cells see fasting. (2, 3)
The fasting mimicking diet has been clinically studied as a therapy for a variety of conditions including autoimmunity, breast cancer, and metabolic disease such as heart disease and diabetes. Extensive studies in mice have been completed, along with a few human clinical trials.
In the most recent randomized controlled trial from 2021, obese women received either a five-day fasting mimicking diet or their typical diet with a calorie deficit of 500 calories each day. This particular study didn’t indicate a difference in weight between the two groups, but the women following the fasting-mimicking plan showed reduced insulin resistance and improved appetite regulating hormones, along with preserved muscle mass and metabolic rate. (4)
At the Hoffman Centre, Justine leads a whole food fasting-mimicking program which I’ve personally undertaken three times and seen the dramatic results. An additional benefit to this structure is the group dialogue component and support provided throughout the process.
It’s important to note that many fasting trends such as juice fasting don’t have the same benefits and may even have risks. Prolonged fasting of more than two days without food may contribute to electrolyte imbalances, dizziness, exhaustion, and other symptoms, making compliance quite challenging. Both time restricted feeding and the fasting-mimicking diet offer the benefits of fasting with intermittent fasting rules that are easy to follow.
Frequently asked questions
Let’s dive into some of the most common questions that I’m asked about intermittent fasting and time restricted eating, who it’s recommended for and who it’s not recommended for, along with some details to help you feel more confident moving forward.
Are intermittent fasting and time restricted eating safe?
Intermittent fasting and time restricted eating are safe and effective practices for many people. However, it’s important to work with your doctor, especially if you have a medical condition or take any medications. A doctor should look at your medical history, complete a physical exam, and review any laboratory testing. Please however note that your doctor may not be that familiar with these approaches to nutrition nor know the science behind it. Be sure that you are practicing the most well informed kind of patient advocacy and be prepared to educate you doctor on the subject .
While intermittent fasting and time restricted eating might be beneficial in a variety of medical cases, as explained in Part One, there are many cases in which intermittent fasting and time restricted eating are not indicated including:
- Pregnancy and lactation
- Anorexia, underweight, or chronic malnutrition
- Type 1 diabetes or insulin-dependent Type 2 diabetes (as insulin requirements may plummet dramatically requiring a lowering of insulin dosing)
- Recent stroke or heart attack
- Pulmonary embolism or deep vein thrombosis
- Cardiac instability or atrial fibrillation
- Advanced kidney disease
- Advanced liver disease
- Advanced heart disease
- Porphyria, MCAD
- Inability to discontinue medications
- Inability to obtain adequate rest while fasting
- Active growth, such as with children or adolescents
- Current fever, cough, or signs of an active infection (5)
Alternatively, if you’re working on any of the following imbalances or disease states, it may be worth discussing intermittent fasting and time restricted eating with your personal doctor or with myself.
- Excess weight or obesity
- Elevated cholesterol
- Elevated blood pressure
- Cardiovascular disease
- Metabolic syndrome or type 2 diabetes
- Lymphoma and other cancers
- Digestive imbalance, including SIBO
- Autoimmune disease
- Dependency or toxicity
Fasting side effects may include fatigue, weakness, headache, dry mouth, menstrual irregularity, memory impairment, muscle pain, constipation, sugar cravings, and brain fog. Be sure to stay well hydrated and avoid strenuous exercise or extreme environments while fasting. Fasting is the ideal time for rest.
Is intermittent fasting and time restricted eating different for men and women?
While much of the initial intermittent fasting research has been conducted on animals and human men, we’re starting to learn more about the unique needs of women when it comes to fasting. Whereas men have similar hormonal patterns from day to day, women’s hormones fluctuate on a monthly cycle and then decline through perimenopause and menopause. You can learn more in my article on hormone replacement therapy.
Women seem to be more sensitive to over-fasting and restricting their food intake too much, too often. They might see imbalances in stress hormones, thyroid hormones, and sex hormones. In extreme cases, too much fasting may lead to amenorrhea or the loss of a woman’s period, especially when percentage body fat drops below a certain percentage. When it comes to intermittent fasting for women, it’s important to note that more fasting isn’t always better. A less-is-more-approach often applies.
And while each woman is different, it’s challenging to provide advice for fasting in women on a worldwide basis. For example, some women with autoimmune disease do very well with implementing intermittent fasting practices, while others might do more poorly. Remember that fasting is a stressor on the body and this can be a good stressor that leads to autophagy, detoxification, and cellular rejuvenation. Yet if the system is already stressed, fasting can sometimes be the straw that breaks the camel’s back. Often, if a woman is exhausted, overwhelmed, and feeling burnt out this isn’t the time to add even more stress.
In a study of obese women, intermittent fasting combined with calorie restriction was shown to reduce weight over a ten-week period. (6) However, many restrictive methods work in the short-term and we may need to learn more about the long-term results of fasting for women.
In another study comparing men and women in a forty-eight-hour fast, it was noted that women tend to accumulate triglycerides in their muscles, while men accumulate these in their livers, although other physiological aspects during the fast were similar. (7) We certainly need more research to further establish the differences related to long-term fasting practices and the different types of intermittent fasting between men and women regarding the potential benefits fasting.
As always with functional medicine, a personalized approach is best. As discussed above, I recommend starting with gentle time restricted feeding or with the fasting-mimicking diet.
Can I drink coffee or tea during fasting hours?
This question about hot drinks usually leads to hot debate! Whether you can drink coffee while intermittent fasting may depend on what works best for you as an individual.
Experts in the fasting field recommend “complete abstinence from all substances except pure water.” (5) Biological fasting is the absence of anything that triggers nutrient-sensing pathways. (3) This certainly means no protein, carbohydrates, or fats, but most likely no vitamins, minerals, or plant compounds either.
While black coffee or tea, doesn’t contain any calories, it does contain caffeine, which can influence the hormones cortisol and insulin. It also contains phytonutrients, the antioxidant compounds that are absorbed and which rely on digestion and metabolism.
So, what can you drink during intermittent fasting? If you want to be a purist, stick to only water during your fasting window then enjoy coffee or tea with your first meal of the day or at any time within your eating window.
After that, you can experiment with plain coffee or tea within your fasting window and see whether it improves, or deters from, your results. Coffee or tea with added fat, such as bulletproof coffee, should be enjoyed during the eating window.
Does intermittent fasting and time restricted eating help with weight loss?
Weight loss is difficult and traditional strategies are largely based on reducing calories and increasing exercise. However, these strategies, especially extreme versions, typically only produce short-term results. Many factors contribute to weight, including hormones, sleep, stress, nutrient levels, toxin exposure, mindset, and so much more. Simply looking at calories doesn’t always address the situation and a short-lived fast may only result in a Band-Aid effect. Yet for some, even a quick boost in hope and confidence that the body can lose stubborn weight can be a catalyst for deeper change. That’s why discussing how to use fasting with a trained professional is key.
Using intermittent fasting and time restricted eating for weight loss might be a solution, or just part of the weight solution, especially for someone who spends the majority of their time in the fed state. Fasting might provide the metabolic balance that will address some of the underlying physiology contributing to weight gain, such as inflammation, elevated insulin, and oxidative stress.
In a review of different types of intermittent fasting, IF produced similar weight loss results to those derived from caloric restriction. 5:2 fasting was similar to restricting daily calories in nine out of eleven studies. In addition, the majority of the weight loss occurred in the first three months before weight hit a plateau and results were similar with different distributions of macronutrients. Time restricted feeding and caloric restriction also seemed similar as far as weight was concerned. (8)
In a long-term study that compared alternate day fasting or ADF with daily calorie restriction in obese adults, weight loss after one year was 6 percent in the ADF group compared to 5.3 percent in the calorie restriction group, so there wasn’t a huge difference. (9)
When examining human studies involving individuals with diabetes, those practicing time restricted feeding as opposed to consuming six small meals per day lost more weight. The studies also showed more results with intermittent fasting in terms of decreasing A1C and blood glucose, which are markers of diabetes, compared to a common recommendation of eating frequent small meals. (10)
The definitive answer to this question regarding the intermittent fasting weight loss diet may not be clear in the science. However, I’ve seen it used successfully in my practice for patients who are good candidates, along with other functional medicine interventions.
Does intermittent fasting and time restricted eating work while following a ketogenic diet?
Ketogenic diets, time restricted eating, and intermittent fasting are often discussed as going hand in hand. Keto, which is an abbreviation for the ketogenic diet, is a high fat, low carbohydrate eating pattern that in its own way mimics the fasting state through the restriction of dietary glucose. The ketogenic diet, time restricted eating, and intermittent fasting all have the potential to increase ketones in the blood that can be used as fuel by the cells instead of them employing glucose. The ketogenic diet combined with time restricted eating and intermittent fasting may also have similar benefits related to a treatment approach to chronic and metabolic diseases.
To answer the question, yes, intermittent fasting and time restricted eating can be combined with a ketogenic diet. Those following a ketogenic diet that are in a state of ketosis, where the body is efficient at turning fat into ketones and using them as fuel, may have a better experience with fasting and fewer negative side effects. Similarly, those with an existing fasting practice might have an easier time transitioning to a ketogenic diet because their metabolism is already primed to use ketones.
So, while intermittent fasting or time restricted eating combined with a keto diet may certainly be an important dietary approach for some people healing from chronic disease or working to promote longevity, it may be too restrictive for others. This is another reason why working with an experienced practitioner can be so helpful. You can dial in your nutrition plan and then have support adjusting, and even expanding, the diet over time.
We all want to remain healthy and high-functioning as we get older, but it’s about more than living a long time. It’s about improving our quality of life. Intermittent fasting is meant to mimic the balance between feast and famine that humans have always experienced throughout history. Regular feasting is a relatively recent development and this excess time in the fed state may deter us from experiencing all of the important health and longevity benefits that come from fasting. The best part about intermittent fasting is that it makes fasting simple, gentle, and fit into modern life.
To learn more about working with me individually or to join our next group fasting-mimicking diet, please contact my office.
- Stekovic S, Hofer SJ, Tripolt N, et al. Alternate Day Fasting Improves Physiological and Molecular Markers of Aging in Healthy, Non-obese Humans [published correction appears in Cell Metab. 2020 Apr 7;31(4):878-881]. Cell Metab. 2019;30(3):462-476.e6.
- Di Francesco, A., Di Germanio, C., Bernier, M., de Cabo, R. A time to fast. Science. 2018;362(6416),770-775.
- Hong, K. Intermittent Fasting and Fasting Mimicking: Clinical Applications. Presentation. University of Southern California.
- Sadeghian M, Hosseini SA, Zare Javid A, Ahmadi Angali K, Mashkournia A. Effect of Fasting-Mimicking Diet or Continuous Energy Restriction on Weight Loss, Body Composition, and Appetite-Regulating Hormones Among Metabolically Healthy Women with Obesity: a Randomized Controlled, Parallel Trial [published online ahead of print, 2021 Jan 9]. Obes Surg. 2021;10.1007/s11695-020-05202-y.
- Goldhamer, A. Can Fasting Save Your life. TrueNorth Health Center.
- Klempel MC, Kroeger CM, Bhutani S, Trepanowski JF, Varady KA. Intermittent fasting combined with calorie restriction is effective for weight loss and cardio-protection in obese women. Nutr J. 2012;11:98. Published 2012 Nov 21.
- Browning JD, Baxter J, Satapati S, Burgess SC. The effect of short-term fasting on liver and skeletal muscle lipid, glucose, and energy metabolism in healthy women and men. J Lipid Res. 2012;53(3):577-586.
- Rynders CA, Thomas EA, Zaman A, Pan Z, Catenacci VA, Melanson EL. Effectiveness of Intermittent Fasting and Time-Restricted Feeding Compared to Continuous Energy Restriction for Weight Loss. Nutrients. 2019;11(10):2442. Published 2019 Oct 14.
- Trepanowski JF, Kroeger CM, Barnosky A, et al. Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection Among Metabolically Healthy Obese Adults: A Randomized Clinical Trial. JAMA Intern Med. 2017;177(7):930-938. doi:10.1001/jamainternmed.2017.0936
- Muñoz-Hernández L, Márquez-López Z, Mehta R, Aguilar-Salinas CA. Intermittent Fasting as Part of the Management for T2DM: from Animal Models to Human Clinical Studies. Curr Diab Rep. 2020;20(4):13. Published 2020 Mar 12.
Dr. Bruce Hoffman, MSc, MBChB, FAARM, IFMCP is a Calgary-based Integrative and Functional medicine practitioner. He is the medical director at the Hoffman Centre for Integrative Medicine and The Brain Centre of Alberta specializing in complex medical conditions. He was born in South Africa and obtained his medical degree from the University of Cape Town. He is a certified Functional Medicine Practitioner (IFM), is board certified with a fellowship in anti-aging (hormones) and regenerative medicine (A4M), a certified Shoemaker Mold Treatment Protocol Practitioner (CIRS) and ILADS trained in the treatment of Lyme disease and co-infections. He is the co-author of a recent paper published by Dr. Afrin’s group: Diagnosis of mast cell activation syndrome: a global “consensus-2”. Read more about Dr. Bruce Hoffman.