Obesity is not a choice. Those with overweight or obesity face both stigma and increased risks for type 2 diabetes, cardiovascular disease, sleep apnea, osteoarthritis, and some cancers. In 2017–2018, 74% of U.S. adults over 20 had either overweight (31.1%) or obesity (42.5%) (CDC, 2021).

Losing excess fat and particularly keeping it off are notoriously difficult. Up to 80% of people who lose significant weight regain it within 2–5 years. This has led some to suggest dieting “destroys” metabolism. In reality, metabolism falls modestly with dieting and can persist after weight loss, but hormonal changes and an obesogenic food environment are greater barriers. Fortunately, scientific approaches to keeping weight off offer hope.

The Metabolism Drop – Adaptive Thermogenesis (AT)

Metabolism here refers to total daily energy expenditure (TEE), total Calories burned daily. This includes 1) resting energy expenditure (REE), 2) the thermic effect of food (TEF), energy for digestion, and 3) activity energy expenditure (AEE), which covers both exercise and non-exercise activity thermogenesis (NEAT), the calories burned in everyday movement.

When weight is lost, TEE falls for two main reasons. First, a smaller body simply requires less energy. For example, the Mifflin St. Jeor equation predicts a 40-year-old male at 200 lbs and 5’9″ burns about 1800 Cal/day at rest, but only 1700 Calories at 180 lbs. Less energy is also expended moving a lighter body.

Second, NEAT declines beyond this size effect. On very low-calorie diets, people often move less without realizing it, reducing expenditure by about 150 Cal/day.

The additional drop is adaptive thermogenesis (AT), which can exceed 150–200 Cal/day during restriction (Redman et al., 2018). But after weight stabilization, AT typically decreases to only about a 50–100 Cal/day reduced expenditure.

AT reflects entry into “starvation mode,” marked by reduced sympathetic tone. Sympathetic tone is what raises heart rate and energy expenditure, largely by activating brown adipose tissue (BAT). Unlike white adipose tissue (WAT), which stores energy and secretes leptin, BAT is packed with mitochondria, rich in uncoupling protein-1 (UCP1). In cold conditions, sympathetic tone drives UCP1 to burn BAT for heat.

Thus, weight loss lowers metabolism through body size, NEAT, and AT, but does not “destroy” it.

Hormonal Pushback: Leptin, Ghrelin, PYY, and Fat Tissue

Hormonal changes adjust appetite and hunger accounting for even more of the challenge in keeping weight off.

Leptin, secreted by fat tissue, signals satiety in proportion to fat mass. However, after weight loss, leptin falls more than expected, triggering the “starvation mode” mentioned above. This drop blunts satiety, reduces sympathetic tone, and suppresses BAT activity, conserving energy consistent with AT. The central role of leptin is shown by studies where leptin replacement restores satiety signaling, lowers food-cue reactivity, and reverses metabolic changes that promote regain.

Peptide YY (PYY), secreted from the gut in response to meals, also falls, further weakening fullness cues.

The hunger hormone ghrelin, released from the stomach when empty, rises after weight loss and can remain elevated for over a year, amplifying hunger.

Over time, leptin suppression and ghrelin elevation attenuate but often remain abnormal compared to never-obese, weight-matched individuals further explaining why maintaining weight loss is harder after obesity.

The Food Environment with Addictive Triggers

Without today’s addictive food supply, metabolic and hormonal changes alone might not explain the weight-regain observed today. Ultra-processed foods high in sugar, refined carbohydrates, and saturated fat activate the brain’s mesolimbic dopamine reward pathway, the same system involved in addiction. Those prone to addiction-like eating are especially vulnerable. Neuroimaging shows these food cues strongly stimulate craving circuits. Combined with higher ghrelin and lower leptin, resisting cravings is harder.

The Mitigating Role of Gut Microbes

Emerging evidence suggests the gut microbiota may help offset defenses against weight-regain. Fermentation of ingested dietary fibers produces short-chain fatty acids (SCFAs) that stimulate secretion of satiety hormones such as PYY and GLP-1, partly reducing post-diet cravings. Certain microbial profiles may also improve leptin sensitivity, support brown adipose tissue (BAT) thermogenesis, and influence the brain’s dopamine reward pathway, potentially reducing responsiveness to hyperpalatable foods. Dietary induced microbe changes may also mitigate adaptive thermogenesis (AT) and non-exercise activity thermogenesis (NEAT). While promising, all these effects require stronger confirmation in human trials.

Medical Intervention

However, with the remaining challenges, the rise in weight-loss medications and surgeries is unsurprising. Yet nutrition counseling from a registered dietitian (RD), especially one certified as a Certified Specialist in Obesity and Weight Management (CSOWM), through the Commission on Dietetic Registration remains an underused but powerful option.

Such an RD is specifically trained to work with clients to: 1) mitigate hunger and satiety hormone changes through individualized strategies, 2) rewire brain reward pathways by fostering preferences for whole foods while weakening ties to ultra-processed options, often with cognitive-behavioral methods, 3) preserve lean mass and resting expenditure through quality protein intake and resistance training, 4) navigate today’s food environment with structured, sustainable plans, and 5) incorporate intake strategies to support a gut microbiota profile favorable to satiety, energy balance, and long-term weight control.

Conclusion

Weight loss through dieting only modestly lowers metabolism, but more critically triggers hormonal and neuronal defenses against fat loss increasing susceptibility to addictive foods. These changes make maintaining weight after loss harder for those who have experienced obesity than for those who have never gained excess weight. While medications and surgery may be appropriate for some, the value of professional nutrition counseling is clear.

An RD offers an evidence-based, and often insurance-covered way to navigate these challenges. RD-CSOWMs can help clients counter hormonal adaptations, leverage microbiome-supportive nutrition, and build sustainable strategies for lasting health.

About the Author

Patrick Traynor, PhD, MPH, RD, CSOWM, CPT, is a registered dietitian and founder of MNT Scientific, LLC (MNTScientific.com), an insurance-based nutrition practice serving South Lake Tahoe, CA; Minden, NV; and Ashland, OR. He holds the Interdisciplinary Specialist Certification in Obesity and Weight Management (CSOWM) from the Commission on Dietetic Registration. Virtual appointments are available via telehealth. For inquiries or appointments, visit MNTScientific.com, dial (530)429-7363, or email securecommunications@mntscientific.hush.com.