Is my body a trust from Allah?

Yes, scholars describe the body as an amanah, a trust from Allah. You didn't create it and you will be asked how you used it. That means caring for it with good food, movement and rest, and avoiding what seriously harms it, without becoming obsessed with it.

What "trust" means

An amanah is something entrusted to you that you must look after and return. Your body, health and senses fit that description. The Quran says that hearing, sight and the heart will all be asked about:

إِنَّ ٱلسَّمْعَ وَٱلْبَصَرَ وَٱلْفُؤَادَ كُلُّ أُو۟لَـٰٓئِكَ كَانَ عَنْهُ مَسْـُٔولًا "Indeed, all will be called to account for [their] hearing, sight, and intellect." Quran 17:36

Looking after the body is therefore an act of gratitude, not vanity. The Prophet ﷺ said, "There are two blessings that many people are deceived into losing: health and free time" (Sahih al-Bukhari 6412).

Your body has rights over you

Salman al-Farisi once visited his brother in faith, Abu ad-Darda, who was fasting that day and getting up to pray at night. Salman told him, "Your Lord has a right on you, your soul has a right on you, and your family has a right on you," and urged him to give each its due. When Abu ad-Darda told the Prophet ﷺ, he said, "Salman has spoken the truth" (Sahih al-Bukhari 1968). Worship is important, but not at the cost of your health and relationships. See Does Islam encourage exercise?

Don't harm it

The Quran says, "And do not kill yourselves" (Quran 4:29), and the Prophet ﷺ taught, "There should be neither harming nor reciprocating harm" (Sunan Ibn Majah 2341). Scholars use these texts as a basis for avoiding things that seriously damage your health.

What this means in practice

Food: eat well and in moderation, as the Quran says "eat and drink, but do not waste" (Quran 7:31). See What is the Prophet's one-third rule for eating?
Exercise: regular movement is part of looking after the trust, with prayer and modesty kept intact.
Sleep: Salman made Abu ad-Darda sleep before the night prayer, and the Prophet ﷺ told Abdullah ibn Amr to "offer prayers and also sleep at night" (Sahih al-Bukhari 1975). Skipping sleep for years is not a virtue.
Steroids: most contemporary scholars say using anabolic steroids to build muscle is not allowed because of the serious harm they can cause. A medicine prescribed by a doctor for a real condition is a different matter. See Is bodybuilding haram?
Smoking and drugs: many scholars treat them as harmful and therefore not allowed.

Not an idol either

Caring for the body doesn't mean worshipping it. Spending hours on your appearance or comparing your body with others can become a distraction. Illness and ageing are not failures either; they are part of life and can bring reward when borne with patience. Do what you reasonably can, ask your doctor, and leave the rest to Allah.

Key points

The body is an amanah, a trust you will be asked about.
Your body, soul and family all have rights over you.
Avoid what seriously harms you, including steroids for muscle building.
Look after yourself without obsession or neglect.

Tip: Pick one habit that protects the trust, such as a regular bedtime or a short walk after Isha. Make the intention "to keep my body fit for worship" and it becomes a good deed too.