Even an excellent diet (according to the recommendations) may leave something to be desired. The United States Department of Agriculture developed the Healthy Eating Index (HEI), a single number that reflects how well one adheres to the food pyramid guidelines. A high value indicates adherence to the guidelines, and a low value indicates a poor diet according to the guidelines.
The Healthy Eating Index graphic is linked from http://www.nalusda.gov:80/fnic/HEI/results.html.
For some nutrients, adequacy of intake correlates fairly well with adherence to the guidelines (e.g., magnesium). Those who have a good diet as judged by the Healthy Eating Index (HEI) tend to get enough Mg, and those who have poor diets do not.
For other nutrients, correlation is low to very low.
It may seem strange that a good diet seemingly does not necessarily correlate with good nutrition. Here are some possible interpretations:
Explore: A 1995 report, The Healthy Eating Index , on 1989-90 data explains the methodology and the results in detail.