MELROSE PARK, Ill. (UPI) -- Two-thirds of Americans say they are on a diet to improve their health but relatively few are actually decreasing in size, a U.S. expert says.

Dr. Jessica Bartfield, who specializes in nutrition and weight management at Loyola University Health System's Gottlieb Memorial Hospital, said dieting is a skill, much like playing a musical  instrument, and requires practice and good instruction.

"You're going to hit some wrong notes and feel frustrated, but eventually you will succeed and it will get easier," Bartfield said in a statement.

Bartfield said the top four reasons many dieters failed to lose weight are:

-- Underestimating the number of calories consumed. Write down everything you eat -- including drinks and "bites" or "tastes" of food -- to help increase self-awareness. Food eaten outside of the home tends to be much larger portion sizes and much higher in calories.

-- Overestimating activity and calories burned. Typically you need to cut 500 calories per day to lose 1 pound per week, which would require 60 minutes or more of vigorous activity every day. A more attainable goal would be to try to increase activity and get a total of 30 minutes of moderate to vigorous exercise most days of the week.

-- Poor timing of meals. You need a steady stream of glucose throughout the day to maintain optimal energy and to  prevent metabolism from slowing down. Try not to go longer than 5 hours without eating a healthy snack or meal to keep your metabolism steady.

-- Inadequate sleep. Studies show people who get fewer than 6 hours of sleep have higher levels of ghrelin, a hormone that stimulates appetite, particularly for high-carbohydrate/high-calorie foods.