Scott Young wrote a great tip on applying Pareto’s Principle to your Diet.
Food – Record your eating habits for a week. Calculate up the calories of the different items of food. I’ve done this before and I’ve found it surprising how some treats contribute a high percentage of your calorie pie for no nutritional value, when other vices consumed in smaller portions take up only a sliver but still offer a tasty treat.
Source: Scott Young, Twenty unique ways to use the 80/20 rule
Just in case, Here is Wikipedia’s definition of Pareto’s Principle:
80% of the effects comes from 20% of the causes