Saturday, May 21, 2011

Salt, Fish Oil, and the Implications of Optimization

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.

21 Comments:

At 9:30 AM, May 21, 2011, Anonymous Anonymous said...

I imagine high-salt diet is related to very recent invention of processed foods which have lots of salt and sodium even in unexpected dishes, to which we are not adapted.

Also evolution may find a local maximum but not necessary a global maximum. (consider race-car with a non-internal combustion engine from the future) Plus, some margins like appendix and toenails might suggest a saddle point rather than a local maximum.

 
At 1:49 PM, May 21, 2011, Anonymous Mat Stroller said...

Also evolution may find a local maximum but not necessary a global maximum.

Yes. But gradual changes are unlikely to be helpful reaching other global maxima and one should be fairly sure of something before doing a "great leap".

Plus, some margins like appendix and toenails might suggest a saddle point rather than a local maximum.

Vestigiality is one explanation. But there is some evidence that the appendix is "a solution to a forgotten problem". We need to be careful with removing things we consider "not useful".

 
At 6:54 PM, May 21, 2011, Blogger Neolibertarian said...

This post shows a clear ignorance of evolutionary science. As has been noted by other posters, evolution does not find global optima. It doesn't even find local optima. One might argue that natural selection is some sort of environmental "gradient descent", but even that seems a bit of a stretch.

This non-optimality is demonstrable in many different ways from the laryngeal nerve of the giraffe to different species filling the same niche in different geographic locations, to the threat of invasive species to native ones.

 
At 12:14 AM, May 22, 2011, Anonymous Anonymous said...

"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."

Not neccessarily, given the trade-offs you illustrate in the post. There would only be value in synthesizing fish oil metabolites if a) they were advantageous and b) there was a significant scarcity.

If they were advantageous but not scarce, we might expect problems if we reduce our intake of them, because the past abundance has meant that development of alternatives was wasteful, and therefore development in other directions was favoured.

 
At 2:50 AM, May 22, 2011, Blogger William B Swift said...

Evolution optimizes, to the extent that it does, only things directly related to differential reproduction; that is things that provide for more surviving offspring than your competition. Everything else it is satisficing, that is "just good enough". To the extent that evolution promotes long life, it is because it improves the survival rate of later generations.

Just because something is necessary for survival and health does not mean it will be provided internally. Vitamin C is a good example. Humans (and I think most primates) cannot produce their own vitamin C, though many other mammals do.

 
At 3:39 AM, May 22, 2011, Anonymous Ricardo Cruz said...

I don't understand what a discussion on food intakes has to do with local vs global optimum levels. What are you guys arguing about? That a little more salt is bad for you, but a lot more can turn out to be beneficial?

This post was not a fine discussion on evolutionary biology. I'd expect everybody here would understand basic points like evolutionary paths can and do get trapped into local maximums. If you want to make a on-topic point, explain what the implications of that are for our diet.

 
At 4:03 AM, May 22, 2011, Blogger William Barghest said...

Since both heart disease and prostate cancer kill you late in your reproductive life, evolution should not have effected a trade-off between the two.

 
At 5:48 AM, May 22, 2011, Anonymous Anonymous said...

Population-wide salt restriction is a triumph of public health policy over science. The best scientific estimates of the prevalence of salt-sensitive hypertension is about 10%, and it appears to be genetic in origin. Thus, if your parents had salt-sensitive hypertension you should be careful about salt intake.

On the other hand, at least 80-90% of the population are not salt sensitive (and some actually need more salt for optimal health). In the U.S., the highest incidence of salt sensitivity is in the African-American population. There was a strong genetic selection in the slave ships for salt retention, so if you are a descendent of African-American (or Caribbean) slaves, you have a higher chance of the 'wrong' genes. (Of course, in a sense, those genes were the 'right' genes because your ancestor(s) may not have lived long enough to produce your family without them.) Salt sensitivity does not seem to be higher (although it has not been studied extensively) in native African populations.

The public policy 'thumb on the scale' came because some people believed it would be too complicated (or racist, etc.) to target an underprivileged ethnic group, and they assumed there was no down side to salt restriction in the rest of the population.

Bottom line: If you don't have high blood pressure, and your parents did not (or had high blood pressure insensitive to salt, which is very common in the elderly) you can probably eat all you want--your kidneys will just excrete any excess (as long as they and your heart are healthy). If you have high blood pressure, you should try a period of salt restriction to see if it helps. And if you are a descendent of slaves from Africa, you should be careful with salt and check your blood pressure religiously.

 
At 9:58 AM, May 22, 2011, Anonymous Chris Hibbert said...

My take away from DDF's post is that we need to remain aware of what we're trying to optimize for, and so look for evidence of treatments that retain overall health and mental functioning while increasing the span of healthy life. So far, I think there is significant evidence for only two courses. One is Calorie Restriction, which has been shown in may species and conditions to increase healthy longevity. The evidence directly bearing on primates and humans is still sparse, but that's because the time scales are long to get results, not because there's been any evidence that it won't work as well for us.

The other promising results are from Michael Rose's fruit flies. These show longer-lived healthier flies from selection effects. Rose and others have been analyzing the results, looking for the differences that make a difference, and suggesting regimens that seem to produce analogous biological states. It's hard to evaluate the particular supplements they recommend, but it seems likely that we'll learn more over time about how to achieve the ends we're seeking. For now, Genescient proposes some specifics that may help. Others, like Kurzweil & Grossman have suggestions, but they're based more in the narrow evaluations that DDF warns against, so they're harder to take as strongly demonstrated.

 
At 10:08 AM, May 22, 2011, Blogger Neolibertarian said...

@Ricardo, when someone spends the first 30% of your essay butchering evolutionary theory with a false analogy, it's hard to take the rest of the rest of what they write very seriously.

As anyone who understand basic logic knows, you can prove anything with a false premise.

 
At 3:21 PM, May 22, 2011, Blogger Brandon Berg said...

Fish oil supplementation isn't really about trying to improve on nature. Omega-3 fat is an essential nutrient, and the modern diet is somewhat deficient in it due to grain-feeding of livestock, which lowers the omega-3 content of foods derived from them. This is exacerbated by high intake of omega-6 fats from grain and seed oils. The two are antagonistic, and high intake of one raises requirements for the other.

Most people have a genuine deficiency and need supplementation to get up to the range in which the human body evolved to function.

The prostate cancer link is interesting, because omega-3 supplementation has generally been shown to be protective against cancer. The most plausible explanation I've seen for this is that metabolic syndrome lowers testosterone levels. Normally this is a bad thing, but it does suppress the growth of prostate tumors.

Omega-3 fat protects against metabolic syndrome, so it may paradoxically enable aggressive growth of prostate tumors. This is also consistent with the finding that trans fat (a factor in metabolic syndrome) had a protective effect.

Here's another example of how a normally beneficial effect of omega-3 supplementation can be harmful under certain conditions.

Perhaps a more straightforward example of the principle you're trying to illustrate would be the blood-thinning effects of omega-3 supplementation. As the blood becomes thinner, the risk of myocardial infarction and ischemic stroke, both of which are usually caused by blood clots, decreases. But the risk of hemmorraghic stroke, caused by failure to clot, increases.

 
At 3:28 PM, May 22, 2011, Blogger Brandon Berg said...

By the way, it is quite common for medical studies to look at all-cause mortality instead of or in addition to mortality from specific causes.

 
At 2:15 PM, May 23, 2011, Anonymous RKN said...

To begin with, my objectives are not the same as the objectives of my genes.

How can your objectives be different than the objectives of your genes? Your brain - where your objectives obtain - was built entirely by gene products. Genes that build brains that in turn produce objectives contrary to the gene's objectives should have been deselected for a long time ago, no?

 
At 3:13 PM, May 23, 2011, Blogger Brandon Berg said...

Genes that build brains that in turn produce objectives contrary to the gene's objectives should have been deselected for a long time ago, no?

No, because of the rapid changes in the environment. For example, advances in contraception have weakened sex drive as a predictor of reproductive success as it once was. In the future, evolution will favor those who actively choose to many children, but not so much those who just really like to have sex.

 
At 10:39 PM, May 23, 2011, Anonymous RKN said...

@Brandon:

A rapid change in the environment does not explain why the so-called "objectives" of the genes would be opposed to the objectives of the human organ they build.

It's also worth pointing out that contraception for males has been around for about five thousand years, possibly longer in females.

 
At 3:36 PM, May 24, 2011, Blogger Milhouse said...

Suppose your race car is optimized for those European countries where gasoline costs US$6/gallon, and you then take it to one of the Middle Eastern countries where government-subsidized gasoline costs 20c/gallon. Suddenly it's not so optimized, is it? And if a local tries to feed it too much gasoline the consequences might not be good.

 
At 4:28 PM, May 24, 2011, Anonymous Andrew said...

Humans are evolutionary shit. If humans were evolutionary cars, they'd be Yugos.

Humans have only been around for about 100k years while most other species have survived for over a million years.

A small handful of humans have the capability to extinguish human life on earth with the press of a couple buttons.

We are not a evolutionary success.

 
At 9:40 AM, May 25, 2011, Blogger neil craig said...

I don't actually believe the salt scare is anything but another of those scares the government and media exploit us with, but:

Precisely because the body evolved to suit cavenab condition in many cases it is not optimised for the life we live now. For example fat, salt and sugar are all useful in limited quantities but not widely available on the plaines of Africa. Therefore there was evolutionary pressure to make us seek it out - namely to make it taste good to us. That means that in a society where they are widely available in the shopping malls of America we are programmed to eat more than is good for us.

An intyeresting variant on that is the evidence of evolutionary pressure for increased intelligence in the urbanised and inbred Jewish comminity (& thus probably the same to a lesser extent among larger communities that have been less urbanised. This suggests that high intelligence, at least philosophical intelligence rather than the ability to spot lions, may have countervailing disadvantages in the veldt (easy boredom, argumentativeness, energy used in thinking), but that in urbanised societies the advantage/disadvantafge balance is seriously different.

 
At 2:00 PM, May 25, 2011, Blogger $9,000,000,000 Write Off said...

"The human body is a machine optimized for a purpose, although the optimization is by evolution rather than deliberate design"

By your reckoning, evolution is designed to optimize? That sounds deliberate, either by the designer or more likely by the theorist.

 
At 10:24 AM, May 27, 2011, Blogger Berna said...

@RKN
My genes have gone and tricked my brain
By making fucking feel so great
That's how the little creeps attain
Their goal to fucking procreate.
But brain has tricks itself, you see
To get the bang without the bite
I got me a vasectomy
My genes can fuck themselves tonight!

Oops, looks like I didn't remember it 100% correctly, but anyway, that's one obvious way one's goals can be different from one's genes'. ;-)

 
At 1:48 PM, May 27, 2011, Anonymous RKN said...

@Berna

I see. The "objective" of some brain genes is to build a portion of a brain whose objective will be reproductive success. The "objective" of other brain genes is to build a portion of a brain whose objective will be to subvert reproductive success (vasectomy). Got it.

 

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