Dr. Mitch Shulman: Why it’s so tough to lose weight and keep it off

You’re not alone if you’re struggling to lose weight and keep it from coming right back. You’re also fighting evolution, no wonder it’s so tough!

Weight loss is not simply a test of willpower. The human brain evolved to protect body fat during times of scarcity, and it can treat a recent higher weight as the new “normal”, triggering stronger hunger, cravings, and lower energy use when calories are reduced or weight has been lost. This helps to explain why many people regain weight after dieting and why medications such as semaglutide (trade names Ozempic and Wegovy, among others) and tirzepatide (Mounjaro and Zepbound, among others), help by reducing appetite signals, although their effects may fade when treatment ends.

It’s only recently in terms of evolutionary time that we have an abundance of easily obtainable food (and calories). To add to that, the food industry has done an excellent job (from their point of view) making food that our brain’s crave because they’re sweet and/or high in fat, targeting our evolutionary adaptations.

For most of human history calories were scare and periods of starvation were the norm. That’s why our brains evolved in this way to save us in that type of environment. Extra calories were good. Sweet foods were ripe and therefore more nutritious. Fatty foods were a quick source of calories. Every gram of fat packs nine kilocalories compared to four for starches and sugar. So, a desire for these types of foods became part of our brain. Based on what happened while the baby was in the womb and during its first seven years of life, the brain established how much fat there should be in the body.

How the brain continued to manage appetite, fat storage and established how much body fat it needs is then modified by the child’s diet, exercise habits and sleep patterns. As part of the adaptive response, if there isn’t enough food, hormones are released that create cravings and the impetus to search for the needed calories. At the same time the body’s metabolism shifts to reduce energy expenditure and conserve as many calories as possible.

This is part of the reason why losing weight is so tough. Your body is actively conspiring against you, and even if you do manage to lose the weight, your body will do everything in its power to get back to where it was.

This doesn’t mean that weight loss is impossible, but it does mean that you need to be aware of what you’re up against. When you use a GLP-1 receptor agonist medication, such as semaglutide or similar medications such as tirzepatide, they block some of the messages that would trigger cravings and the need to eat. While you’re on these medications you have a chance to reset the “fat thermostat” in your brain and also your daily habits that enabled you to maintain your heavier weight. It’s an opportunity to stock healthy, nutritious food in the house and to get moving, for example. Even something as simple as a walk during the day after breakfast, lunch or supper can help to establish a healthier body weight.

It may not be easy but if you understand what you’re up against you can set things up to give yourself the best chance to succeed.

Dr. Mitch Shulman is an Associate Professor in the Department of Emergency Medicine at McGill Medical School as well as an Attending Physician in the Emergency Department of the McGill University Health Centre. He’s also the CJAD AM 800 Medical Consultant.