The 'Food Pyramid' was a well-intentioned attempt by the US Department of Agriculture (USDA) to provide simple guidelines for 'healthy eating' in a format that would be easy to popularise. Unfortunately, research evidence now strongly indicates that the guidelines embodied in the Food Pyramid are in urgent need of revision.
The reasons for this are explained in this excellent article by Walter C. Willett and Meir J. Stampfer, professors of nutrition and epidemiology respectively at the Harvard School of Public Health. Both are also professors of medicine at Harvard Medical School.
[Update 2004: See the Harvard School of Public Health website for further information on how and why the official US government guidelines for healthy eating are still unsatisfactory. Here, the Harvard researchers provide their own guidelines, Food Pyramids - What should you really eat? , which do a much better job of reflecting the latest scientific evidence.
The dietary guide introduced a decade ago has led people astray. Some fats are healthy for the heart, and many carbohydrates clearly are not