# Lose Fat, Gain Muscle – Anatomy Guide

By Yotam

Topic: Human Body & Medicine

[Open the interactive Jig](https://ask.jig.com/j/KJF5oj2VZEG9)

## Guide

### 1. One body, two tissues

This model is a slice through an upper arm\. The red bundles are muscle, which moves you\. The pale yellow balls are fat cells, which store energy\. Losing fat while gaining muscle means changing both tissues in different ways at the same time\.

Drag anywhere to turn the model\.

### 2. Fat cells store fuel

Each fat cell holds one big droplet of oil\. When the body needs more energy than food gives it, the cells let some oil out as fuel and get smaller\. Their number hardly changes\.

One kilogram of body fat holds roughly 7,700 kilocalories of energy\.

### 3. Muscle is made of bundles

A muscle is built from bundles called fascicles\. Each bundle holds long, thin cells called muscle fibres\. A fibre is thinner than a hair, yet it can be many centimetres long\.

A fibre is 10 to 100 micrometres wide\. A micrometre is a thousandth of a millimetre\.

### 4. Lifting sends a signal

Lifting something heavy makes the fibres pull hard\. That tells the body to repair them and add more of the tiny threads inside, called myofibrils\. Lifting a little more as you get stronger, called progressive overload, keeps the signal coming\.

Health guidelines suggest muscle\-strengthening activity on at least two days a week\.

### 5. Protein brings the bricks

Protein in food is broken down into amino acids, the building blocks of muscle\. The blood carries them to the fibres, which use them to repair and grow\. Without enough protein, the training signal has little to build with\.

Studies with adults often use at least 1\.6 grams of protein per kilogram of body weight each day\.

### 6. A small energy gap

Eat slightly less energy than you use, and the body takes the rest from the fat cells\. The gauge shows a gentle gap, not starving\. A big gap can cost muscle too, so keep it small\.

A common adult target is losing about half to one percent of body weight a week\.

### 7. Rest, repeat, wait

Muscles rebuild while you rest and sleep, not while you lift\. Repeat the cycle for weeks and both tissues change: fat cells shrink while fibres thicken\. Slide through the weeks to watch it happen\.

The scale may barely move, because the muscle gained can balance the fat lost\.

### 8. Who it works for

Beginners, people coming back to training and people with more body fat usually change fastest\. Fat never turns into muscle; the two tissues change side by side\. Children need energy to grow, so they should play and eat well, not diet\.

Talk to a doctor before big changes to food or exercise\.