Anista,

I hope that it is ok for me to jump in here.
When one loses weight, one loses muscle and fat.

To reduce the loss of muscle one can eat eat more protein and do weight exercises--- progressive weight lifting, what YouTube calls "Body Weight"exercises, and likely some types of yoga.

If you eat eggs and animal milk, cheese, or yogurt, you will get enough B12.

For certain periods of time I have eaten a vegetarian diet and gotten enough protein by not eating any rice or bread.
I spent my calories on chic peas(garbanzo beans), lentils, eggs, dairy, lots of green leafy vegetables, and some other vegetables like summer squash and broccoli.

Green leafy veggetables have lots of calcium.

Mari