Ask the expert: How does your body actually lose weight? 

Weight loss has always been described as standard and mathematical, suggesting that counting calories is the best way to shed weight. In reality, the human body is more complicated than that, and everyone will experience weight loss differently.

There are many factors that go into weight loss, such as genetics, metabolism and your environment. Many people believe that one factor determines your weight loss, but there’s more to the story.

Kim Pfotenhauer is an associate professor for the College of Osteopathic Medicine at Michigan State University. She is a diabetologist, or a physician who specializes in the diagnosis, treatment and management of diabetes and specializes in obesity medicine, including understanding how you can overcome your biology to change your weight.

Here, Pfotenhauer discusses the different models of weight loss and explains how factors like genetics, hormones, metabolism, environment and your body’s natural defense mechanisms all affect how much you weigh.

Answers are excerpts from an article originally published in The Conversation in English and Portuguese.

What is set point weight and how is it determined?

Set point weight is the idea that the body tries to maintain a certain amount of fat, or adipose tissue, by changing hunger levels and the number of calories it burns. The level of fat is determined by genetics, physiology and environmental factors. Set point weight is determined by observations that after weight loss, appetite increases and the number of calories burned decreases until the weight is restored. In theory, this process prevents the body from starving, even with significant weight loss.

How does metabolism change as weight changes? Can the change affect a person’s weight loss journey?

As you lose weight, you burn fewer calories than expected for someone at the same weight who has not undergone recent weight loss. This is called metabolic adaptation. Metabolic adaptation manifests as an increase in appetite and a decrease in resting metabolic rate, which is the energy a person burns to sustain background processes such as heartbeat, temperature regulation, respiration and digestion, even if you lie in bed all day.

In metabolic adaptation, resting metabolic rate decreases after approximately 5% weight loss. The energy burned from exercise decreases after around 10% weight loss. This means that as a person loses weight, the amount of energy used for background processes to maintain life decreases. Furthermore, a person must increase exercise as they lose weight to see continued weight loss. So, the more weight a person loses, the harder it is to lose more. This decrease in energy expenditure may persist for years after weight loss. However, some studies have found that metabolic adaptation is less significant than it was once thought.

Are there strategies to lose weight without metabolic adaptation?

There are several strategies to overcome set point weight and the metabolic adaptations expected with weight loss. Bariatric surgery — a surgical procedure for weight loss — appears to change set point weight, reducing hunger without decreasing calories burned, and patients rarely become underweight. GLP-1s and similar medications may not affect metabolic adaptation while reducing weight. Nutritional strategies include increased protein intake, reduced glycemic load and increased intake of high-fiber foods, although evidence for their effectiveness varies.

What is the settling point model? How does it differ from other theories?

The settling point is an alternative theory to set point weight. This model suggests that weight changes naturally based on a person’s behaviors and surroundings, rather than being actively controlled by biological processes. The settling point is the weight a person reaches when the calories they eat match the calories they burn. This is influenced by the energy needed to maintain a person’s body weight. People with greater body weight expend more energy because they need more energy to move and maintain a larger body. Therefore, people living in larger bodies would have greater food intake needs.

Settling point may sound like the old ‘calories in, calories out’ model, but it also considers environmental and societal influences. For example, if you have a job where you are on your feet all day and eat home-cooked foods most of the time, your weight might be stable. If you switch to a desk job and start eating more calorie-dense foods and larger portions, your weight may increase until it becomes stable again. In both scenarios, your weight eventually stabilizes at different settling points depending on your current circumstances.

However, the settling point theory fails to explain biological and genetic aspects of weight.

What is the dual intervention point model? How does this explain why some people gain and lose weight more easily than others?

The dual intervention point model integrates both the set point weight and the settling point. This theory proposes an upper and lower threshold that defines the boundaries of each person’s ‘acceptable’ body weight, called the zone of indifference. The lower threshold is the point where starvation is prevented while maintaining all biological and metabolic needs.

Within the zone of indifference, settling point concepts prevail: The body will adapt to its energy and environment. If a person’s weight drops below that limit, the body responds by increasing hunger and slowing calorie burning to prevent starvation. The body’s hormonal systems increase appetite and reduce the calories burned. When body weight exceeds the upper threshold, biological mechanisms should engage to prevent further weight gain. However, this process is more evident in animals than in humans, as it is more closely related to predators and survival.

The dual intervention point model also suggests that the zone of indifference varies widely between individuals. Some may recognize this as the old struggle of ‘losing the same 10 pounds over and over again.’

Which body weight regulation theory is shown to be the most accurate?

The answer is none of them fit real-world experiences exactly. But there seems to be a difference in how your metabolism responds to active weight loss versus weight maintenance, so the approach to each goal may be different.

Decreasing food intake seems to be the most beneficial for attaining weight loss. Conversely, exercise seems to be key to weight maintenance.

Overall, the big takeaway is that weight balance is complex. It is not a simple math problem. Effective obesity care includes nutrition, exercise, sleep, stress and other factors that can affect body weight. Changes in these factors can be combined with medication or surgery to achieve sustained weight loss.

Weight loss is often not linear, and plateaus are expected. Each case is individual, and one size — or theory — does not fit at all.