Guest Post: Reasons For Weight Regain

Today’s Guest Post comes from my friend and colleague David Macklin, MD, Toronto

Not long ago I received a message from a colleague looking for help with a patient who was regaining weight. As I thought about my response, it occurred to me that there should be a comprehensive list of why this happens, yet I could not remember coming across one. The following is a more detailed reproduction of the list I sent back to my colleague that day.  I’d like to thank Arya for suggesting that I share this list with his readership.  

An important note regarding this list:  Reason number one is the most important and most common reason for weight regain. The other reasons can make the primary reason more complicated.


The primary reason for weight regain is biology. The brain defends against weight loss because of an old biological play book.  If our ancestors lost weight, it was not to look good for a wedding or because of bathing suit season. Back then, weight loss was either because of illness or an interrupted food supply. Simply put, defending against weight loss was defending against death.  

In the last 30 years we have learned how the brain does this. The brain is expert at 1) recognizing fat loss, 2) defending against fat loss, and 3) promoting weight regain. The brain does this by:  

a) increasing appetite – the motivation for calorie intake

b) decreasing metabolic rate

Increased appetite seems to be more complicit than slower metabolism in weight regain. Increased appetite, in the form of an increased motivation to eat, leads to increased overall calorie intake, which in turn leads to weight regain.

A reminder, the remaining reasons for weight regain operate through the main mechanism, biology. 


The next common reason for weight regain relates to dieting.  Note that dieting is not an effective method of preventing weight regain. Instead, the three pillars to preventing weight regain are behavioural therapy, medication, and surgery. Simply put, the risk of weight gain is greater the more “diet-like” the weight-loss method. Specifically:

a) if the weight-loss effort involved a commitment to a reduced calorie intake that was unsustainable.

b) if the weight-loss effort involved a commitment to a level and type of effort that was unsustainable.

c) if the weight-loss effort did not accept and involve a conscious commitment to the value of fun, food, drink, friends, socialization, and travel.

The greatest predictor of thwarting weight regain is sustained adherence to the method used to lose weight. The above methods predict poor adherence.  They also predict that for someone losing too much weight, going to a weight lower than what is sustainable, weight loss will be overwhelmed by biology.

Thus, we have the definition of best weight. One’s best weight is arrived at by committing to the opposite of dieting. Best weight is discovered by committing to 1) a sustainable calorie level, 2) at a sustainable effort, 3) while accepting a conscious commitment to the value of fun, food, drink, friends, socialization, and travel. Best weight is the weight you softly land at when committing to a lifestyle that is, by definition, sustainable.


The next and rather poorly understood reason for weight regain is a sustained change in one’s overall environment. A common current example of this would be someone who was working in an office and is now working in their home, or the opposite: someone who was working at home and now is working at an office.  Another example is someone who, during weight loss, avoided high-risk settings for the purpose of weight loss and now inevitably is re-engaging in these setting. For example someone who avoided social setting or restaurants temporarily just for the purpose of weight loss. The cues in our physical and social environments trigger the biological response of appetite – wanting – the subconscious motivation to eat.  If the cues experienced during weight loss change, weight regain can follow.


Anti-obesity medications (AOMs) work by defending against an individual’s biology. AOMs defend against increased appetite and dampen the subconscious motivation for calorie intake (wanting).  If AOMs are discontinued or poorly complied with, the defence against weight regain is lost and the biology does the rest.  


A highly underestimated reason for weight regain is in an individual’s response to “weight loss setbacks.” Common weight loss setbacks include 1) the aftermath of off-track eating and/or drinking episodes, and 2) the moments after seeing a number on a scale not in one’s favour. Unless effective behavioural therapy has been provided, these setbacks may often be followed by negative self-critical thoughts and negative emotions including demotivation.  Yet it is precisely when one is at a lower weight that they are most subject to their biological drivers of weight regain and most likely to experience over-consuming and unfavourable scale results. Screening for and treating these unhelpful cognitive responses is an important part of understanding and addressing weight regain. To reiterate, this is probably the most underestimated non-biological complication of weight loss.


Weight gain can be a possible serious side effect of many commonly used medications. Some medications more commonly cause weight gain than others, and significant inter-individual differences exist. Weight regain may be caused by the addition of a weight-promoting medication and if possible, a substitution can be made for an alternative medication that is more weight neutral.


Finally, there is a finite list of internal states that may promote weight regain.  It is intuitive to most that stress, fatigue, lack of sleep, depressed mood, and lower levels of activity may promote weight regain. For many individuals, especially in a state of reduced body weight, each of these factors can increase wanting, the motivation to eat, and decrease self-regulation skills (restraint). If someone is regaining weight, it is imperative to screen for each of these modulators and to recognize how a change in any of these may be leading to higher calorie intake and weight regain.

For clinicians, and for those who struggle with weight, I hope this list is helpful.

David Macklin, MD

Toronto, CND

About the author: David A. Macklin, MD, is a lecturer at the University of Toronto and a University of Toronto trained family physician. He has practiced obesity medicine since 2004. He is the Medical Director of the Weight Management Program at the Toronto Medcan Clinic and co-author of the Canadian ACTION Study and the Psychology and Behavioural Treatment Chapter of the 2020 Canadian Clinical Practice Guidelines for the treatment of Obesity in adults.