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Human nutrition is complex.
There’s just no getting around the fact that human nutrition is a breathtakingly complex subject. There are tens of thousands of biologically active chemicals at work in the human body, and almost all of them are derived from food in one way or another. Scientists are only able to study one or two small pieces of the intricate puzzle of human metabolism at a time. All too often, they are unable to observe how these pieces are affected by other, unseen pieces, and as a result they draw conclusions that will have to be retracted or revised when these other pieces come into view. Good scientists understand that all of their conclusions are tentative and subject to later revision, but we often have little choice but to base our dietary decisions on the tentative conclusions of nutrition scientists, and it can be frustrating when they are in fact changed.
One example of this dynamic is the story of margarine. Scientists and doctors advocated margarine as a healthier alternative to butter in the 1960s because margarine is lower in saturated fats than butter, and a link between saturated fat and heart disease had been recently discovered. What these doctors and scientists did not know at the time is that trans fats, of which margarine is full, are far worse. The advice to replace butter with margarine has since been retracted.