Suppose you are designing a race car; further suppose that you are very good at designing race cars, and so get everything right. You face a variety of tradeoffs. A larger engine will increase the car's power to accelerate, it will allow it to better overcome wind resistance—but it will also weigh more and require a larger gas tank, which will increase the car's mass, reducing the gain in acceleration and possibly making the car more likely to burst its tires or skid out on a turn. Similarly with the size and shape of tires, width of the wheel base, and a variety of other features.
Your car is designed, built, and it and its close imitators are winning races. A critic points out that you obviously have it wrong; the engine should have been bigger. To prove his point, he builds a car that is just like yours save that the engine is half again as large. Testing it on the straightaway, he demonstrates that it indeed has better acceleration than your car. He enters it in a race against your car—and loses.
He should not have been surprised; if the advantage of a larger engine had not been at least balanced by its disadvantages, your car would have had a larger engine. While there may be many ways of changing your design that produce a gain on one margin, each of them, if you have done your job right, will result in equal or greater losses on others.
Which argument helps to explain two recent news stories about human health.
There is a longstanding argument for reducing the amount of salt modern Americans consume, based on evidence that a high salt diet tends to produce high blood pressure. A recent European statistical study, however, reported just the opposite of what that argument suggests—evidence that lower salt intake was correlated with an increased risk of death from heart disease. Similarly, there is evidence that an increased consumption of omega 3 oils reduces the risk of heart attacks. But it has recently been reported that it also increases the risk of the more serious form of prostate cancer.
The logic of optimization provides an explanation for these results. The human body, like the race car, is a machine optimized for a purpose, although the optimization is by evolution rather than deliberate design. If it functioned better with less salt, the design would at some point of have been tweaked to consume less salt, excrete more salt more rapidly, or in some other way take advantage of that particular opportunity for improved design. If it functioned better with whatever metabolites fish oil produces, the very sophisticated chemical factory build into our metabolism would, presumably, have been modified over time to produce those metabolites without requiring that particular input. It is not surprising if changes produce improvements on some dimension of successful functioning for the human organism—but it is also not surprising if those changes, like changes in the design of a race car, produce at least equal worsening on other dimensions.
I do not want to overstate the point; there are at least two reasons why the design of my body might be suboptimal from my point of view, hence capable of improvement by, among other things, changes in my diet. To begin with, my objectives are not the same as the objectives of my genes. Evolution has optimized my design not for my longevity or happiness but for reproductive success; I am a machine for getting more copies of my genes into future generations. Living a long and healthy life is one means to that end, so optimizing for reproductive success will imply, among other things, changes that make my life longer and healthier. But where the two objectives are in conflict, evolution will unhesitatingly sacrifice welfare in favor of reproductive success. There might be ways in which my deliberate choices could reverse that preference.
Second, evolution is slow. We have had hundreds of thousands of years to optimize our bodies to function in the environment of hunter/gatherers, about ten thousand years to optimize against the environment of agriculturalists, and only a century or so to optimize against an environment where, in the developed world, most people have all the food they want and little need for physical labor. Hence it is not surprising if some of our behavior, and some of our biology, is poorly adapted to our present environment—making possible deliberate improvements in diet or life style.
But the implication of the argument I have offered is that we ought not to be surprised by results such as the two I just discussed. The fact that some change produces a gain in one measurable dimension that matters to us is very poor evidence that it produces an overall gain. Before altering behavior or diet, one ought to look for evidence of net effects on life expectancy or other reasonably final goals, not merely for desirable effects on one input thereto.