Hormones & obesity — explained plainly
Your fat cells keep a running note of how much fuel is stored, and they send it to your brain. In obesity, that message stops getting through. Almost everything else — the hunger, the regain, the slowed engine — follows from that one lost signal.
When it works
When it doesn't
01 — The basics
Your body has trillions of cells. They cannot shout across the room to each other. So they send messages instead.
A hormone is one of those messages. It is made in one part of the body, it rides through the blood like a letter in the post, and it is read somewhere else. That is the whole idea.
Four of these messengers matter a lot for body weight. Here they are, in the simplest words possible.
Messenger 01
The key. After you eat, sugar floats in your blood. Insulin unlocks your cells so the sugar can get inside and be used.
Messenger 02
The fuel gauge. Made by fat cells. It tells the brain how much energy is stored, so the brain knows when to stop being hungry.
Messenger 03
The thermostat. It sets how fast your body burns fuel while you are just sitting still doing nothing.
02 — The central problem
Fat cells make leptin. More fat cells means more leptin. So in a bigger body, there is a lot of leptin floating around.
You would expect that person to feel very full all the time. The opposite happens. The brain stops listening to leptin. Scientists call this leptin resistance.
Here is the part worth sitting with. It is not that the tank is empty. It is that the wire between the gauge and the dashboard is broken. The tank is overflowing, and the dashboard still reads empty.
So the brain does what any sensible brain would do when it believes fuel is running out. It switches hunger on and keeps it on.
03 — Why it repeats
One broken signal would be a small problem. The trouble is that it feeds itself. Each step makes the next step more likely, and after five steps you are back where you started — only heavier.
04 — The scoreboard
Here is what happens to each hormone in a larger body, and what happens when weight comes off. Read the last column carefully — it is where the surprise is hiding.
| Messenger | In obesity | In plain words | After weight loss |
|---|---|---|---|
| Insulin Too high | About 8 in 10 people have too much insulin in the blood. | The locks on the cells are worn out, so the body keeps making more keys. | Recovers well The locks start working again. Sugar gets into cells properly. |
| Thyroid Usually normal | Hormone levels are normal. There may just be fewer "antennas" on cells to catch the signal. | The thermostat itself is fine. A slow thyroid does not explain most weight gain. | Turns down Active thyroid hormone (T3) drops. You burn fewer calories at rest. |
| Growth hormone Lower | Reduced — and this may be on purpose, not a fault. | One of its jobs is to pull fat out of storage. With plenty stored, the body sends less of it. | Back to normal Levels return once the situation changes. |
| Leptin High, ignored | Lots of it, but the brain has stopped responding. | The fuel gauge is shouting and nobody is listening. | Still off Falls, but stays higher than in someone who never had obesity — and the brain still under-reacts. |
Read the pattern
Insulin and growth hormone recover. Thyroid and leptin do not. Those two point in the same unhelpful direction: burning less while feeling hungrier.
05 — The twist nobody warns you about
Imagine two people. Both weigh 70 kg today. One has always weighed 70 kg. The other started at 90 kg and worked hard to get here.
They look the same on the scale. They are not the same underneath. The person who lost weight burns noticeably fewer calories doing nothing at all.
Meanwhile the fuel gauge has not been repaired. Leptin falls when fat falls — it has to, because fat cells make it — but it does not settle back to a normal reading, and the brain still does not fully trust it.
This is worth saying clearly, because a lot of people blame themselves for it. Regaining weight is not a collapse of willpower. It is a body running a very old survival program that treats fat loss as a famine and fights it.
06 — What to actually do
If the body's defences are hunger and a slower engine, then good food choices are the ones that answer those two problems directly. Keep the muscle. Keep full for longer. That is basically the whole strategy.
Food gives us three big nutrients, called macros. Here is roughly how a day is usually split.
Macro 01 — protein
When you eat less, your body burns some muscle along with fat. That is bad news, because muscle is what keeps your engine running fast.
Enough protein tells the body: burn the fat, leave the muscle alone. It also keeps you fullest of the three.
Macro 02 — fat
Fat carries more than double the energy of the other two per gram, so it is easy to overdo.
But you cannot cut it out. You need fat to absorb vitamins A, D, E and K. Choose liquid oils over ghee, butter and fried food.
Macro 03 — carbs
Your brain runs mostly on glucose. If carbs run out, the body makes glucose by breaking down muscle.
So enough carbohydrate helps spare your protein for repair instead of burning it as fuel.
Fibre is the part of plant food your body cannot digest. It sounds useless. It is one of the most useful things on the plate — about 20 to 35 grams a day — and it works in two ways.
It adds bulk without adding energy. Same calories, far more food. Your stomach stretches, and a stretched stomach sends its own fullness signal — one that still works fine even when leptin does not.
200 kcal of vegetables & dal
200 kcal of fried snack
It slows things down. Fibre makes food leave the stomach more slowly, so fullness lasts longer. It also flattens out blood sugar spikes — which means less insulin — which loops straight back to the very first messenger on our list.
07 — Corrections
Usually not.
Thyroid hormone levels are normally normal in people with obesity. A genuinely underactive thyroid can add a few kilos, mostly water, but it does not explain large weight gain. Worth testing, rarely the answer.
Almost certainly a typo — check any slide that says this.
Every real guideline says a percentage of total energy, not of fat energy. The current figure is under 10% of your total calories. Read literally, the version above would work out near 2%, which nobody could actually eat.
That is the number for people who are not dieting.
When you are eating less, the evidence points higher — roughly 1.2 to 1.6 g per kg — to hold on to muscle. Not every study agrees, but that is where the weight of it sits.
It is basically a tie.
A year-long trial with over 600 people found the two produced about the same weight loss. Bigger reviews give low-carb a small edge on weight and triglycerides, and low-fat a small edge on LDL cholesterol. The diet you can actually keep doing is the one that wins.
The one thing to remember
It is defending you, using rules written long before food was easy to find. Storage full, gauge broken, hunger on, engine slowed — every step is a survival instinct doing its job in a world it was not built for.
And it points at what actually helps: enough protein to keep your engine, enough fibre to feel full through a signal that still works, and food you can keep eating next month. Not a fight against your body. A workaround.