How do we talk about food? In episode 2 of Frasier, Frasier Crane discusses his preferred breakfast with his father. A "low fat, high fiber" breakfast with (terribly expensive) plain black coffee. There's a lot to be said about the implicit social views of eating in this scene. But instead imagine this: out of the woodwork I wander onto the show and say "Actually Dr Crane, you would be better off substituting that bran muffin for bacon.".
What does that mean? A calorie neutral substitution? A mass neutral substitution? A subjective substitution? The second is not a joke - it's clear (is it?) that a person who is fed via IV would "feel hungry" and attempt to eat whether the IV was sugar or oil. In the language of economics such a person would hit their first order conditions for diet optimality (I.e. calories would be right) but not their second order conditions. They're at a minimum utility. This cannot last. Believing in diet advice unstable to perturbations is unscientific - completely methodologically unsound. The third option is not necessarily unscientific - subjective feelings of fullness are related to the health relavent properties of food. Attempting to lose weight via a simplistic, objective calorie/mass accounting system may again put you at an unstable equilbrium. These kind of yo-yo unstable diets aren't obviously healthy.
The market for food is broken, possibly irrevocably broken. There has never been, in all the history of mankind, a society where farm labor is economically valued. As a result, all industrial societies prop up production - often in highly distortionary ways. It is obvious that, for instance, the US overproduces corn carbohydrates. This is a Bad Thing.
On the consumer side, it must be admitted by any person who desires to be taken seriously that branding and monopolistic competition more generally are real. Government intervention has been ineffective at policing this, even when it has been pointed in the right direction. This is unsurprising - Coase on the right and Stiglitz on the left are always fond of pointing out that the conditions in which governments/markets can fail are the exact conditions markets/governments can fail.
This brings us back to the first point. How do we talk about food? There is an enormous signaling problem here. Just as every prospective worker has an incentive to appear to be valuable to a prospective employer, so every prospective meal has an incentive to appear to be healthy to a prospective eater. (Healthiness & productivity of course defined variationally) Eaters therefore statistically discriminate, choosing foods with a few easily observed outward signs of "healthiness". Sugary cereals put photos of nutritious meals on the box. Having blueberries and a green container turns a malted into a diet food. More informative statistics - carbohydrate and protein and fat measures, total calories, ingredients, etc. - are buried in confusing, neutrally colored, small type statistical abstracts.
Adding exercise to a given diet is generally good, since exercise to a large extent determines the distribution of variable masses for a person of a given mass. We may not be indifferent between being Akebono and Bob Sapp. As Kimball notes above & every bodybuilder knows - total weight balances the mass of food that comes in and goes out. (Also, cardio health is good, even though the mass of the cardio system isn't particularly variable in mass) But there are huge difficulties here. First, it isn't methodologically sound to assume a person can vary exercise and not vary their diet (it assumes that their old equilibrium was unstable, which is exactly what it is not for an obese person who can't lose weight). The market for exercises is not obviously healthy. Like with food, every prospective exercise plan has an incentive to seem healthy & sustainable even if it is not. Survivor bias is endemic here - everyone who keeps up My Super Special Program long enough achieves their weight & weight distribution targets and everyone who doesn't leaves.
You can't sit around thinking about your diet all day. Simplistic accounting techniques can beat advanced techniques just because they're easier to understand. It may not be easy to account for carbohydrate, protein or fat intake simply because their are so many kinds of carbohydrates, proteins and fats that eating the "wrong" kind may not give the eater noticeable feedback. Fasting can then empirically outperform theoretically superior modes because it's easy to notice when you cheat.
These are only the simplest economic metaphors. Beyond this there are cultural and even political factors. But that'll have to wait for another post.
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