What should we eat, and how much of it should we eat? The flux in official nutrition guidelines must be terribly frustrating to those who believe in philosophical realism. In other words, such a person would be disappointed that experts have not settled on the one and only one universally accepted truth with respect to what nutritional guidelines should really be.
But nutritional guidelines will never be etched in stone. Consider the interest groups putting pressure on governmental bodies to frame the guidelines in a manner friendly to their profits.
In other words, these guidelines are not the simple result of advances in knowledge. Rather they reflect the interplay between the work of scientists and the private interests who prefer particular portrayals of that work. In short, the guidelines are social constructions molded by the dominant voices in a culture honeycombed with inequality. The guidelines will repeatedly change because (1) the guidelines are not for the exclusive benefit of those who eat food, (2) food itself changes in quality, (3) politicians seeking reelection have a pronounced fear of industry trade associations, and (4) big pharmaceutical companies push drugs that allegedly reduce the health effects of eating certain foods. These factors wrestle for the right to tell us what we should eat for optimal health.
How naive we are when we think that production in a market system is for consumers and their well-being!