To strengthen muscles, you need to nourish the organisms with those elements that your muscles need. How do you know this? Just make a blood test - general, biochemical and special (depending on what worries you). With the test results, go to the endocrinologist. He will advise you on what to take and how to eat. As the correct metabolism is restored, the body itself will tell you what to eat. If the tests are in perfect order, then eat varied and exercise.
Proper nutrition in the morning to eat porridge - healthy buckwheat wheat porridge. Drink water a day you need to drink 1.5 liters or 2 liters not immediately but during the day. Eat more vegetables, especially fresh cucumbers, make salads from fresh cabbage, carrots, vinaigrette. also get carried away with fried and salty food less. It is better to bake fish meat in the oven. If you can eat less bread, it is better to refuse it. Replace sugar and sweets with honey and dried fruits or nuts, they are healthier.
In order to strengthen the muscles, first of all, it is necessary that the body understands that we want to strengthen them, that is, we need to create an incentive. Muscles are stimulated during exercise. It doesn't matter what kind of training with free weights, or with your own weight, or running, or martial arts.
As soon as the muscles get the right stimulus. The body will start looking for "nourishment" for them, be it growth or recovery. You correctly asked the question "what to eat", because the body will take the main building materials from food. Vegetables and fruits provide vitamins and minerals necessary for muscle growth and strengthening. And the right protein food (dairy products: milk, cottage cheese, cheeses; poultry: veal; beef; fish; eggs; specialized sports nutrition products with a large amount of protein), will give the necessary amino acids to build new muscle structures and "seal" obtained during damage training!