I read somewhere that nuts are not good source of protein because only 50% of their protein are usable by body, Is it true? And if it is, why?
Not true at all. The protein you get from nuts are great. The only drawback is that nuts are an incomplete protein meaning that they do not contain all the essential amino acids. All you need to do is combine nuts with a grain of some sort (whole grain bread, rice, etc) and you will get the missing amino acids you are looking for. The truth is, you probably get the missing amino acids from other foods that you have eaten throughout the day that are still floating around in your system.
Hope that helps,