Guide · Long term · 3 min read

Protecting your muscle

By the CalJab team, with sources · Last reviewed October 21, 2026

Fast weight loss, with or without a GLP-1 medicine, often means losing both fat and muscle. ¹ In the STEP 1 trial, 38% of the weight lost was lean mass, which includes muscle; since muscle is about half of lean mass, that is roughly a fifth of the loss. ¹ Body scans in the same trial showed that more fat than lean mass was lost, so most of the weight that went was fat. ³

Who is at higher risk

• Eating little protein because of a small appetite can add to muscle loss. ¹
• Risk is higher with older age, menopause, lower testosterone, sitting a lot or no strength training. ¹
• Fast, large weight loss can also affect bone: losing 14% or more of your weight over 3 to 4 months has been linked to bone loss. ¹

Strength training

• Protein alone is not enough: more protein without strength training is unlikely to keep muscle. ¹
• How much: the Advisory suggests strength training at least 3 times a week plus at least 150 minutes of moderate aerobic exercise a week, and the NHS suggests strengthening activities on at least 2 days a week. ^1·2
• Aerobic activity alone helps less with keeping muscle during fast weight loss. ¹
• What counts: lifting weights, resistance bands, exercises that use your own body weight such as push-ups and sit-ups, carrying heavy shopping and heavy gardening. ²
• Start at your level: a plan should match your fitness, and if you have not exercised for some time, speak to your GP first. ^1·2

Protein when appetite is low

Smaller protein foods such as fish, eggs, Greek yogurt, cottage cheese, and nuts or nut butters make it easier to reach your protein when you eat less. ¹ Some people use protein shakes or bars. ¹

In CalJab

The protecting muscle card in Body shows how many days you reached your protein target and, if you connect Apple Health, your strength workouts. Your protein target is set with keeping muscle in mind, and your plan is built protein first.

Ask your doctor or a physiotherapist which exercise suits you.

Written by the CalJab team, with sources. It does not replace your doctor's advice. Found a mistake? Write to support@caljab.app.