Does a long 20 hours flight effect muscle mass and cause weight gain?

No and depends: A 20 hour flight should have no effect on muscle mass. It will only cause weight gain if you eat a large amount of fattening food (like the soft drinks they serve). The greatest hazard to health on a long flight is getting blood clots in your leg; this is best avoided by getting up and walking every couple of hours.Blood thinners like low-dose aspirin (81mg), ginger & fish oil can help prevent.