I always suggest keeping a food journal to my clients. Those looking for weight management are directed to journal as much as possible and the journal is recorded in their file. Athletes are also set up with a food journal. The results are influenced by the fuel. And diet is becoming recognized as one of the variables that can be manipulated very easily.
All of the answers have merit. To me the answer to the question depends on several variables: your client’s goals, current dietary habits and your client’s education in nutritional science. It is an effective tool for a client to learn to assess eating habits–types of foods and fluids they generally consume, as well as quantity. When I was doing clinical research, especially with regard to weight loss and weight management, all subjects kept detailed dietary records because the data was essential in the statistical analysis of the physiological responses with regard to the study design’s hypothesis.
I don’t and I only ask those clients of mine who have trouble being accountable and lack self-discipline. Most of them get in the habit of counting their calories and paying better attention to what they are eating once they start keeping a food journal, because now they have a better idea of what they really put in their mouth. They become more self-conscious and they are grateful for that.