The truth is that fat loss comes from a whole body workout and a healthy diet, and sit ups are one of the least effective core exercises to choose from.
Sit-ups target only a few muscles at once, imagine how long and inefficient a workout would be if you worked every muscle individually.
This is why we prefer the Plank; while both exercises improve core stability and strengthen abdominals, the Plank recruits a better balance of muscles by targeting the arms, upper back, obliques, mid and lower abdominals and lower legs.
The Plank is a more ‘functional exercise’, it requires the activation of several muscle groups at once just like a daily task or activity would. The basic sit up only activates some abdominal muscles and the obliques (side abs).
Lastly, crunches and sit-ups involve spinal flexion and too much flexion of the spine can lead to disc problems over time. Many people have jobs that already demand too much from their backs due to long periods of sitting, we certainly don’t need to add more stress to the back.
So if you love Sit-ups, throw in a few here and there if you wish, but for a really effective core workout there are much better exercises to choose from such as the Plank. The fact you don’t get to lie down while you do it is a tell-tale sign that you are going to get a better workout.