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Many health trends come and go, but one dietary trend that has seemed to endure the craze is intermittent fasting. The primary reason intermittent fasting (IF) has remained so popular is that it offers flexibility to fit a busy schedule, and (while I don't typically advise mindless eating as long as you're fasting) it doesn't ultimately demand restricted eating.

What is intermittent fasting, and how is it done? Intermittent fasting is defined as a diet regimen that cycles through a period of time in which the body is driven into a catabolic state through fasting, where no significant calorie intake occurs, met with periods of fed states where no calorie restriction occurs. The windows for fasting are typically around 16 hours, met with an 8-hour fed state.

Intermittent fasting can be used in a few different ways:

  • Time-restricted feeding: You restrict the time you are allowed to eat to set hours of the day. For example, you eat all of your meals in an 8-hour window, from 9-5 pm. During the window, no calories are restricted, and outside of this window, no calories are consumed.

  • Alternative day fasting: You alternate the days on which you restrict your caloric intake with ones where there are no restrictions. For example, Mon-Wed-Fri you fast for 24 hours/limit calories to 25% of your daily intake, while alternative days have no restrictions

  • Full Day Fast: You only consume water and other liquids for 24 hours1-2 days per week.

The most common approach is time-restricted feeding. Now, most guidelines state that the timing of these fed/fasted windows won’t matter – ie a fed state from 9 am – 5 pm will produce the same results as a fed window from 1 pm – 9 pm. We’ll talk about why that’s not always true in a moment. But first, let's look at the research that demonstrates the health benefits of intermittent fasting. Are There Benefits to Intermittent Fasting?

  • Weight Loss: A systematic review of over 40 studies on intermittent fasting has shown it to have benefits on weight loss in the short term, with average weight loss around 9-11 lbs at 10 wks (1). This occurs for two reasons. #1 – during fasting, the body goes into a catabolic state to continually feed the brain. This means that when we run out of bioavailable sugar and glycogen stores, the body will start breaking down our fat as fuel, hence, fat loss. This mechanism typically peaks at about 12-14 hours, with some evidence suggesting the benefit is lost after the 16-hour mark (hence the common 16 hours fasted/8 hours fed approach). #2 - during fasted states, the body increases the production of human growth hormone (HGH), which helps maintain lean muscle mass. Therefore, in catabolic states in the presence of HGH, the body prefers fat breakdown instead of lean muscle mass (again, favouring fat loss). To further avoid muscle loss and promote fat mass loss, I typically recommend incorporating some form of daily exercise and maintaining a high fat/high lean protein diet while using intermittent fasting.

  • Reduced Cancer Risks: The risks for certain types of cancer development and reoccurrence can be lowered through the use of intermittent fasting. Studies have shown higher leptin levels in the body do contribute to the development of certain cancers. Through intermittent fasting, leptin levels can not only be lowered but adiponectin levels can also be increased, which is a hormone that has been shown to be protective of metaplastic cell development. A trial published in JAMA Oncology in 2016 demonstrated that fasting 13 hours or more per night resulted in a statistically significant improvement in glucoregulation and a reduction in the reoccurrence rates of breast cancer (2), making it a simple non-pharmaceutical approach to minimizing breast cancer reoccurrence.

  • Improved Cardiovascular Health: Intermittent fasting helps reduce low-density lipoprotein (LDL) or “bad” cholesterol and triglyceride levels. Having high levels of both LDL cholesterol and triglycerides increase the risks of heart disease, heart attacks, and strokes. Using a combination of intermittent fasting methods and eating a healthy diet can naturally reduce these risks.

  • Better Regulation of Insulin Levels: The amount of sugar people consume in their diets has become a major cause of insulin resistance, which is one of the contributing factors of diabetes. By incorporating fasting methods just a few days each week, Intermittent fasting helps us become more sensitive to insulin, the fat-storage hormone, and leptin, our satiety hormone, ultimately improving sugar metabolism, while reducing insulin resistance.

  • Improved Mental Stability: People who practice intermittent fasting report energy, as well as lower levels of anger, confusion, tension, and stress. Additionally, they found they were less prone to mood swings with longer periods of mood stability throughout their days. This likely has a correlation to better blood sugar stability, as well as improved mitochondrial function. Fasting has been shown to boost the quality of mitochondria (the energy producers of every cell) by speeding the rate at which old and damaged mitochondria are cleared.

Where the research falls short: The first shortcoming to intermittent fasting is that when you compare apples to apples, research to date shows that intermittent fasting provides no further long-term benefit over calorie restriction, and calorie restriction provides no long-term benefits in weight loss. In fact, most long-term evidence shows calorie restriction has negative impacts on long-term weight loss. Now let me be clear here – daily calorie deficits are a good thing. Eating the same/slightly less and exercising more could never be a bad thing. However, when we examine some of the major clinical studies conducted on weight loss (the TODAY study, the Women’s Health Initiative study, and the Diabetes Prevention Program), data shows that despite initial weight loss compared to control groups, continuous calorie restriction did not amount to long term changes in weight or body composition. In fact, the members of these trials not only gained back all their weight but also now have to follow a calorie-restricted diet to maintain it. (4,5,6) With this being said, the long-term data for intermittent fasting benefits just simply doesn’t exist yet. Understanding that calorie restriction and intermittent fasting both have underlying catabolic actions call for the need for more long-term studies with follow-ups greater than 1 year would be needed before drawing conclusions about the long-term safety of intermittent fasting on metabolism. The second biggest pitfall to intermittent fasting research is that the majority of it is done in men. No large-scale, long-term follow-up trials have yet to be conducted on female subjects, which is a problem. Numerous trials to date show higher negative sequelae to dietary restrictions in women, such as carbohydrate deprivation inducing under-functioning thyroid in as little as 6 wks, and ketosis caused menstrual dysfunction in 45% of women after 6 months (7). And while we don't have great human trials for intermittent fasting for women, rat trials show significant impacts on reproductive hormones in as little as two weeks following IF (8). This information should be extrapolated with caution when deciding if intermittent fasting is right for you. When should intermittent fasting be avoided? While health benefits from IF exist for many people, the following people should not engage in restrictive dieting without the counsel of a medical professional:

  • Type I Diabetes – while I do use IF in some of my insulin-dependent patients, it is recommended on a case-by-case basis, and with close supervision. Uneducated users can risk hypoglycemic events and severe health risks.

  • Eating disorders that involve unhealthy self-restriction (anorexia, bulimia nervosa, orthorexia)

  • Use of medications that require food intake or result in hypoglycemia

  • Active growth stages, such as infants or adolescents

  • Pregnancy, breastfeeding – there’s just not enough research to confirm its safety in pregnancy, and restrictive dieting during breastfeeding usually leads to decreased milk supply.

My general advice remains to assess things on a case-by-case basis to determine if intermittent fasting is right for you, and for how long. Until more research confirms more long-term benefits and conclusive benefits in women, I usually advise sticking to IF 2-3 days a week and maintaining a whole foods, plant-rich diet the rest of the time. To find out if intermittent fasting and which methods are right for you, please feel free to contact Dr. Courtney Holmberg, ND at 647-351-7282 to schedule an appointment today! References:

  1. Seimon RV, Roekenes JA, Zibellini J, Zhu B, Gibson AA, Hills AP, Wood RE, King NA, Byrne NM, Sainsbury A. Do intermittent diets provide physiological benefits over continuous diets for weight loss? A systematic review of clinical trials. Mol Cell Endocrinol. 2015 Dec 15;418:153-72

  2. Catherine R. Marinac, BA; Sandahl H. Nelson, MS; Caitlin I. Breen, BS, BA; Sheri J. Hartman, PhD; Loki Natarajan, PhD; John P. Pierce, PhD; Shirley W. Flatt, MS; Dorothy D. Sears, PhD; Ruth E. Patterson, PhD. Prolonged Nightly Fasting and Breast Cancer Prognosis. JAMA Oncol. 2016;2(8):1049-1055. doi:10.1001/jamaoncol.2016.0164

  3. Seimon RV, Roekenes JA, Zibellini J, Zhu B, Gibson AA, Hills AP, Wood RE, King NA, Byrne NM, Sainsbury A. Do intermittent diets provide physiological benefits over continuous diets for weight loss? A systematic review of clinical trials. Mol Cell Endocrinol. 2015 Dec 15;418:153-72

  4. Trepanowski JF, Kroeger CM, Barnosky A, Klempel MC, Bhutani S, Hoddy KK, Gabel K, Freels S, Rigdon J, Rood J, Ravussin E. Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection Among Metabolically Healthy Obese Adults: A Randomized Clinical Trial. JAMA Internal Medicine. 2017 May 1.

  5. Johnstone AM. Fasting–the ultimate diet?. Obesity Reviews. 2007 May 1;8(3):211-22.

  6. Harvie M, Howell A. Potential Benefits and Harms of Intermittent Energy Restriction and Intermittent Fasting Amongst Obese, Overweight and Normal Weight Subjects—A Narrative Review of Human and Animal Evidence. Behavioral Sciences. 2017 Jan 19;7(1):4.

  7. Mady MA1, Kossoff EH, McGregor AL, Wheless JW, Pyzik PL, Freeman JM. The ketogenic diet: adolescents can do it, too. Epilepsia. 2003 Jun;44(6):847-51.

  8. Kumar S, Kaur G (2013) Intermittent Fasting Dietary Restriction Regimen Negatively Influences Reproduction in Young Rats: A Study of Hypothalamo-Hypophysial-Gonadal Axis. PLOS ONE 8(1): e52416.

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