Most of our clients want to lose fat around their belly and many of them do crunches in anticipation of reducing fat from that area. This is a mistake!!
Firstly, it is not possible to reduce fat from one region alone (spot reduction), by doing exercise for that area/muscles.
As you do crunches, the muscles of the abdomen (Rectus Abdominis) becomes stronger. However, the fat deposit over that muscles will go away only by planning your nutrition with the right calorie deficit
Secondly, our spine is not a straight bone; it has curvature and when you lie down and perform crunches, you are pushing your curved spine against the floor and exerting the hip flexor muscles, which run from the thighs to the lower back (Lumbar region). If your hip flexors are weak, the entire load goes to the spine itself. This action of crunching the abdomen can cause pressure on the spine as you sit all the way up.
Further, the core muscles is a group of muscles that includes gluteus maximum (hip muscles), obliques (lower side abdomen), Rectus Abdominis (Abs), Lattissiumus Dorsi (Back-sides) and more (Read ) By performing only crunches and thereby overworking only one part of the core is a recipe for asymmetry, overcompensation, and muscle strains & pains.
While doing crunches, it is important to tuck the tailbone (Sacral region of the spine) and also draw the naval inside and closer to the spine. However, many of those at beginner level cannot maintain this proper form, causing the lower spine to hyperextend which may cause the discs in your back to bulge, pressing on nerves causing lower back pain and sciatica
In short, do not do crunches, especially if you are a beginner. You are not going to lose belly fat by doing crunches, and there is a high chance of hurting your lower spine