Is it bad to do kick-boxing when I weigh 42kg?

If you are determined to do kickboxing at this weight, I would severely control the time you spent on this particular activity.

You can put on weight though, while still kickboxing. Like all sports, you burn calories, so if you’re putting in more calories than you burn, you’ll start to gain weight. Essentially that’s the concept in bodybuilding, a strength-oriented and physique sport where the mass you put on is determined by your input of calories opposed to your output of calories.

So to begin putting on lean muscle mass at 42kgs, I would say you need to begin eating:

1,840 calories 184g of Protein 184g of Carbs 40g of Fat

This is a 40/40/20 ratio. Once you’ve put on a pound or two, adjust the calories higher by 250 to 500 and increase the macronutrients by 10% to help keep gaining more mass while kickboxing.

Although, that is if you can do that. If not, I would suggest taking my principles above and packing on some nice, clean, muscle mass first, then beginning kickboxing.

If you’re unsure, visit a physician and see what they recommend you to do since they are overall better than some kid’s advice on FA and a coach or two.