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What Happens Inside Our Body and Brain When We Lose Weight and Weight Regain

August 9, 2021

90% of people who lose significant weight end up regaining some or all of their weight, and often end up at a higher weight than before their weight loss attempt.

We all know this as the all-too-common yoyo-dieting effect. Why is that? Why is it such a challenge to lose weight and keep it off? Have you been in this situation? Did you blame yourself for not trying hard enough or not having the willpower to see it through? Maybe it’s not you. Maybe it’s your biology.

The Biology of Our Body and Brain When We Lose Weight

The human body and brain developed over millions of years to protect us from weight loss since this was a common reason for our demise for most of our human history. Your brain does not know whether your weight is a “healthy” weight for you. All it knows is what your weight is. This becomes your “setpoint” weight and the weight that your brain will fiercely defend if your weight is reduced below your setpoint.

Your brain does have defenses against weight gain as well, but these defenses are not nearly as strong. We see the same thing with blood sugar: you can’t feel high blood sugar unless it becomes severe, but low blood sugar? You feel that right away! The same is true for weight.

When you lose weight, especially once you get to around 10% body weight loss, your brain starts to freak out. It can tell that you are losing weight because as your fat tissue declines, your fat cells produce less leptin. Leptin is the hormone that informs your brain of your fat stores, so when these levels decline, and they decline rapidly with weight loss, your brain begins its efforts to protect you from further weight loss and tries to encourage you to regain the weight you lost. 

As is commonly seen with weight loss, reductions in muscle mass also give your body the signal that it’s time to slam the brakes on this weight reduction and time to ramp up effort to regain weight.

Why Do You Regain Weight?

How does your brain try to make you regain weight? It increases the hormones that make you hungry and reduces the hormones that make you feel full. The result is an increase in appetite. But, no matter how much willpower you have, your biology usually wins out.

Like holding your breath: you can consciously hold your breath for so long, but eventually, the biological signals become too strong, and you take a breath. Your appetite signals are equally as strong but less immediate. And unlike breathing, we make over 200 decisions a day about what we eat, when we eat, how much we eat, and when to stop eating. We assume that these choices are made under our total, conscious control, but most of these decisions occur without our full awareness.

While we still have a personal responsibility to make the healthiest food choices for ourselves, we also must recognize that there are other, strong, factors at play. Besides our appetite hormones, food marketing and highly processed foods that are designed to be addictive can also hijack our brain, resulting in cravings and overeating.

So, how long do these changes in hormones that make us hungrier after weight loss last? A month? Six months? Until we regain the weight? We don’t exactly know yet, but from the studies we do have, it appears that these changes last for over a year, even when the weight is regained.

Changes in appetite hormones are just one way that the body tries to protect us from weight loss. The other mechanism with which the body fights weight loss and promotes weight regain is by affecting our metabolism, or in other words, how many calories our body expends.

Metabolism slows down with weight reduction, approximately 15% below what would be expected from just the weight reduction itself (in other words, the less you weigh, the less energy you need, but the reduction in metabolism is 15% below what would be expected for the person’s new weight). Again, this is because our muscles become more efficient, so we burn fewer calories doing the same activities.

"The Biggest Loser" and the "Metabolic Handicap"

The longest study on this phenomenon, often referred to as the “metabolic handicap,” lasted 6 years and was performed with contestants from the reality show “The Biggest Loser.”  They found that the metabolic handicap persisted, even 6 years after the initial weight loss, and even if the weight was regained. There was also a great deal of variability between the contestants regarding how much their metabolism slowed.

So it’s no surprise that those whose metabolism slowed the most regained the most weight, and those whose metabolism didn’t slow were able to maintain much of their weight loss, and some were even able to continue to lose more weight. So again, was it willpower that allowed some contestants to fair better long term? Or was it biology?

For some people, hearing this news gives them great relief. For years they just blamed themselves and beat themselves up for their inability to lose weight and keep it off, despite multiple serious efforts. For others, this information makes them feel hopeless.

Real Solutions to Lose Weight

We now have evidence-based solutions that can help us overcome this biology. These solutions come in the form of nutrition therapy, physical activity, medications, devices, and even surgery. There IS hope. There are REAL solutions to these problems. Not everyone’s body will respond equally as strong to weight reduction, but if you are one of the 90% of people who have tried to lose weight in the past only to regain it, know that there is hope and there are answers for you and that it’s not all your fault.

Find an obesity medicine specialist under the “Find a clinician” tab on the Obesity Medicine Association website and Find Obesity Treatment. You can search for providers on the Obesity Action Coalition website.


  1. Fothergill E, Guo J, Howard L, Kerns JC, Knuth ND, Brychta R, Chen KY, Skarulis MC, Walter M, Walter PJ, Hall KD. Persistent metabolic adaptation 6 years after "The Biggest Loser" competition. Obesity (Silver Spring). 2016 Aug;24(8):1612-9. doi: 10.1002/oby.21538. Epub 2016 May 2. PMID: 27136388; PMCID: PMC4989512.
  2. Van Baak MA, Mariman ECM. Mechanisms of weight regain after weight loss - the role of adipose tissue. Nat Rev Endocrinol. 2019 May;15(5):274-287. doi: 10.1038/s41574-018-0148-4. PMID: 30655624.
  3. Wansink, B. Mindless Eating. Why We Eat More Than We Think. © 2006 Brian Wassink
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Karli Burridge


Karli Burridge, PA-C, MMS, FOMA is a nationally recognized expert in obesity medicine and is founder of Gaining Health. She is passionate about educating other providers on how to incorporate obesity management into their every-day practices and helping those affected by obesity. She is comprehensively trained in medical obesity treatment, as well as in bariatric surgery, and has a background in psychology and exercise physiology.