THE SCIENCE OF METABOLISM
How your body decides what to do with energy.
Appetite, insulin, fat tissue, liver and muscle form one connected system. Here is how it works, where it breaks down and how modern treatment targets each step.
REGULATION, NOT WILLPOWER
Your brain runs a weight thermostat.
The hypothalamus and brainstem integrate signals from fat tissue (leptin), the gut (GLP-1, PYY, ghrelin) and the pancreas (insulin, amylin) to set hunger and energy use. When weight falls, that system pushes back: hunger hormones rise and energy expenditure drops.
In a landmark study, people who lost weight on a diet still had higher ghrelin and lower satiety hormones a full year later (Sumithran et al., NEJM 2011). That biology, not a lack of discipline, explains why weight so often returns, and why treatments that act on these signals work.
SIGNAL
Leptin
Made by fat cells in proportion to fat mass; tells the brain how much energy is stored.
SIGNAL
Ghrelin
The main hunger hormone from the stomach; rises before meals and after weight loss.
SIGNAL
GLP-1 & PYY
Released by the gut after eating; slow the stomach and signal fullness.
SIGNAL
Insulin
Moves glucose into tissues; chronically high levels promote fat storage.
THE INCRETIN REVOLUTION
Medicines that speak the gut’s own language.
GLP-1 and related medicines amplify hormones your gut releases after a meal. Tap each signal to see where it acts.
Glucagon-like peptide-1
Semaglutide, liraglutide and orforglipron. Also one of the signals in tirzepatide and retatrutide.
Simplified schematic for education. “Proposed” marks effects supported mainly by animal or mechanistic studies.
WHERE FAT SITS MATTERS
The same weight can carry very different risk.
Fat stored in and around organs drives far more metabolic harm than fat under the skin. These are the tissues we watch.
VISCERAL FAT
Around the organs
Metabolically active fat inside the abdomen that releases inflammatory signals and free fatty acids straight to the liver. Waist size tracks it better than BMI.
LIVER
MASLD
Fat in the liver now underlies the most common chronic liver disease worldwide. It worsens insulin resistance and can progress to inflammation and scarring (MASH).
MUSCLE
The glucose sink
Skeletal muscle takes up the large majority of glucose after a meal. Less muscle, or fat stored inside it, means higher blood sugar.
PANCREAS
Beta-cell stress
Excess fat around the pancreas strains the cells that make insulin. In DiRECT, remission of type 2 diabetes came with falling liver and pancreatic fat.
WHAT WE MEASURE
The markers that tell the real story.
Weight alone misses most of the picture. These are measurements clinicians commonly use to judge metabolic health, with reference points from the guidelines that define them.
BODY SHAPE
Waist-to-height ratio
Keep waist under half your height. NICE (2022) uses 0.5 as the threshold for increased cardiometabolic risk.
GLUCOSE
Hemoglobin A1c
Three-month average glucose. ADA: 5.7–6.4% is prediabetes, 6.5% or higher is diabetes.
INSULIN
Fasting insulin & HOMA-IR
Early markers of insulin resistance. No universal cutoff exists, so trends over time matter most.
LIPIDS
ApoB
Counts the particles that cause atherosclerosis. Targets are individualized to overall cardiovascular risk.
LIPIDS
Triglycerides & HDL
Metabolic syndrome criteria: triglycerides 150 mg/dL or higher, HDL under 40 (men) or 50 (women) mg/dL.
LIVER
FIB-4 score
Built from routine labs. Under 1.3 suggests low risk of advanced liver scarring (AASLD).
COMPOSITION
DXA body composition
Separates fat, lean and visceral fat. Essential for tracking muscle during weight loss.
CIRCULATION
Blood pressure
ACC/AHA guidance targets under 130/80 mm Hg for most adults on treatment.
FITNESS
VO2 max & grip strength
Among the strongest predictors of healthy aging and survival. Both respond to training.
Reference points are from the named guidelines. Your clinician interprets every result in context; none of this replaces individual medical advice.
CHRONIC DISEASE, CHRONIC CARE
Stop the medicine, and the biology returns.
Obesity behaves like blood pressure or cholesterol: treatment works while you take it. Withdrawal trials show the body steadily defends its old weight once the medicine stops, which is why plans should cover maintenance from day one.
≈2/3
of lost weight was regained within a year of stopping semaglutide
STEP 1 extension · Wilding et al., Diabetes Obes Metab 2022
14%
weight regained over a year when tirzepatide was switched to placebo, while those who continued lost 5.5% more
SURMOUNT-4 · Aronne et al., JAMA 2024