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Why fasting often backfires, leading to longer-term weight gain

Some claim fasting is the pathway to sustainable weight loss, optimal health, and even longer lifespans, but is fasting really worth all the hype?

Throughout history, fasting has served as a means of sober reflection, healthy reform, protest, and prayerful contemplation. Technically, fasting is defined as “abstaining from all or some kinds of food or drink, especially as a religious observance” or to “be deprived of all or some kinds of food, especially for medical or experimental reasons”, with the reason, type, and duration of the fast left to the interpretation of the practitioner. Various religious groups regularly or periodically engage in individual or communal fasts – Mahatma Ghandi famously fasted for 21 full days in protest of the British rule of India.

More recently, intermittent fasting and “cleanses” are being increasingly used and marketed for management of weight, diabetes, or cardiovascular disease. A number of variations exist such as time-restricted (abstaining from food and calorie-containing beverages for 12-16 hours intervals); alternate-day (fasting every other day); and periodic fasting (eg. five normal eating days followed by two consecutive fasting days). On “fasting days”, very small amounts of calories – for example, 25-percent or less of one’s normal daily energy requirements – are consumed. Fasting limits short-term energy stores, thus, it is thought that during periods of fasting the body taps into fat reserves, thereby contributing to fat metabolism and weight loss, along with other health benefits.

The average Canadian consumes more calories than they need. Consequently, the rates of obesity, diabetes, and cardiovascular disease are rising at alarming levels. Since intermittent fasting ultimately translates to consuming fewer calories over a period of time, it would make sense that a person who regularly fasts would also consume less refined sugar, unhealthy fats, and processed foods over that same period of time, explaining the potential health benefits of fasting.




Most studies exploring the potential risks and benefits of fasting have only looked at animal models. In these studies, the data looks promising – albeit with mixed results – that intermittent fasting may offer benefits as it relates to the management or mitigation of diabetes, cardiovascular disease, cancer, and neurological disorders including Alzheimer’s disease. Studies on humans that have looked at the potential benefits of fasting have focused on weight loss, cholesterol reduction, and blood sugar management. So far, these studies have only looked at short term results, and have involved a relatively small number of participants. Recently, a year-long, randomized clinical trial1 – the largest and longest-duration study of its kind – concluded that alternate-day fasting did not produce better results in terms of weight loss, weight maintenance, blood sugar control, or cardiovascular benefits when compared to subjects who consumed a reduced amount of Calories overall, without fasting. Based on preliminary results of this study and other research, it’s probable that intermittent fasting approaches such as alternate day fasting are no more effective as simply reducing Calorie intake.

It is important to keep in mind that the participants of studies like the one referenced above are often cared for by a team of health professionals from whom they receive nutrition counseling even during the off-fast periods. Their eating patterns are therefore modified not only during fasting days, but every day. Researchers claim that the key to intermittent fasting is to avoid overeating on non-fasting days. In reality, however, it is more likely that individuals who practice intermittent fasting end up overeating and consuming unhealthy foods on non-fasting days.


All things considered, fasting may help some individuals break the pattern of eating out of habit instead of hunger; assist some individuals in learning how to feel fuller with smaller amounts of food; or help some individuals become more tolerant of feeling hungry. However, most health professionals agree that fasting is not a healthier way to lose weight and has not been shown to be more efficient than conventional approaches of maintaining a healthy balanced diet combined with regular physical activity.

It is important to consider the various repercussions of drastically reducing one’s intake for any duration, including the risk of nutrient deficiencies. Fasting, at least at first, can cause headaches, fatigue, dizziness, light-headedness, irritability, and nausea. The significant and/or abrupt reduction in certain foods can also exacerbate these symptoms.


Before considering a drastic "solution" such as fasting, try to change your daily habits! Join our free 30 day no refined sugar challenge.



“Although, we may not be clinically addicted to any one food, we are all hardwired to seek out food” explains Dr. Farrell Cahill, Research Lead for the Medisys Health Group. “When severely reducing our intake of any food(s) for a long or short time period, we shouldn’t underestimate the potential impact this can have on our psychological wellbeing” ads Cahill. Additionally, while fasting can induce fat loss, it can also induce the loss of muscle mass – particularly in long-term fasts if daily protein requirements are not met. Fasting also makes it difficult to maintain the energy levels required to exercise, resulting in decreased physical activity, which is also counterproductive.

"Moreover, fasting causes the body to enter into a “selfpreservation” mode, altering your metabolism to conserve energy when Calorie intake is severely restricted. Consequently, when you return to normal eating patterns the body begins stockpiling energy, resulting in rapid weight re-gain".

While intermittent fasting may provide a structured dietary pattern that keeps some individuals from overeating, for many, this approach backfires or is unsustainable. Fasting can be particularly risky and unadvisable for children; women who are pregnant, trying to conceive, or breastfeeding; individuals who are diabetic or hypoglycemic; the elderly; individuals who have had a history of disordered eating patterns; as well as those on certain medications.

In today’s society, eating is about much more than a means to satisfy our daily energy and nutrient requirements – it responds to a range of physical, emotional, and social needs. A restrictive eating pattern, like fasting, influences our perception of hunger and satiety signals which impedes our ability to respond to these signals appropriately. Fasting is likely to ultimately rob us of the enjoyment of the eating experience.


Consult with your local Medisys doctor or dietitian before considering any dietary approach, it’s important to ensure that any new diet approach is adapted to meet your unique needs and takes into account your current health status.





Instead of entering into a pattern of deprivation, seek to achieve healthy, sustainable eating habits. Listen to your body, and be mindful about what you eat and when you eat. The most successful paths to a healthy lifestyle have the following things in common: adequate Calorie intake (not too much, not too little); optimal nutritional quality; and regular exercise. Find something that works with your schedule and lifestyle, and most importantly, find something you can sustain. My advice is start listening to your body – it will tell you when you are hungry, it will tell you when you are full. Your body is wired to tell you what it needs; all you need to do is listen up!



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About the author: A member of the Order of Professional Dietitians of Quebec (OPDQ), Rachel Malcangi is a Registered Dietitian based out of the Medisys clinic in Montreal. Rachel graduated from McGill University with a Bachelors of Science in Dietetics and Human Nutrition following the completion of a degree in Anatomy and Cell Biology, and has since played an integral role in the patient care process, interning in such specialties as internal medicine, gastrointestinal surgery, and kidney failure, as well as consulting in areas including geriatric care, weight management, sports nutrition and preventative health and wellness. At Medisys, Rachel provides personalized nutrition counselling as well as corporate conferences to educate and empower clients of all ages and health conditions, helping them make informed decisions when it comes to their eating habits, and guiding them towards the best choices to help them achieve their goals. Rachel is a believer in the enjoyment of food, and leads her patients toward balanced nutrition for optimal health in a way that is simple, natural, and sustainable. To book an appointment with Rachel Malcangi at the Medisys clinic in Montreal, please email Rachel.Malcangi@medisys.ca.

1) John F. Trepanowski, Cynthia M. Kroeger, Adrienne Barnosky, Monica C. Klempel, Surabhi Bhutani, Kristin K. Hoddy, Kelsey Gabel, Sally Freels, Joseph Rigdon, Jennifer Rood, Eric Ravussin, Krista
A. Varady. 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