Many fitness and nutrition experts suggest using a journal to keep track of exercise and food intake. how can using a journal someone become more fit or change body composition?

It can stop you from eating to much, or to little, for example if you see you've eaten to many calories in a day it can stop you from eating more. It can also convince you too eat a little more if you're under the recommended calorie intake a day. If you're under working yourself it can convince you to work out a little more, but also tell you to take a break if you're overworking yourself.

hope this helps :)