You may have seen a YouTube video in which Dr. Robert Lustig of UCSF opines that calories are irrelevant and that sugar is, in essence, the root of all dietary evil. If you are among the few who have missed it, you have perhaps caught Gary Taubes' "what he said" article in the New York Times Magazine.
The notion that sugar is evil and the only dietary consideration that matters is, in a word, humbug.
Sugar, concentrated into the nectar of flowers, fuels the flight of hummingbirds. It is, in fact, the sole food source of these marvels of both aviation, and metabolic intensity. How evil can hummingbird fuel be?
Clearly not evil at all for hummingbirds! But that may not say much about us. So enough about hummingbirds; let's talk about you and me.
We were born with a preference for sweet because that has fostered the survival of not only Homo sapiens, but mammals in general, for perhaps as long as there have been mammals. Breast milk -- and I trust no one is foolhardy enough to suggest that breast milk is evil! -- is a sugar-sweetened beverage.
Human breast milk is, in fact, as compared to the milk of many other mammals, an especially concentrated source of lactose, or milk sugar. It is sweeter than cow's milk.
Breast milk contains sugar because a fairly concentrated dose of readily metabolizable fuel is apparently essential to getting us mammals off to a good start in life, growing body and brain. So suggesting that sugar is not just "a," but "the" premier dietary peril certainly risks tossing out the baby with the bathwater. Or, at least, the baby's food.
The fact that sugar is intrinsic to our first, best food tells us something about the sweet tooth with which we are all born, before ever we have teeth. Taste buds, you see, did not sprout all over our tongues to help us have a good time. In fact, beneath the cold analytical glare of natural selection, a good time is of no consequence. What matters is survival, or the alternative.
We like sweet because mammals who like sweet are more apt to survive than mammals who don't. Period.
Taste buds are sentinels. They stand at the threshold to our inner world, and distinguish friend from foe. A friend is something we can safely digest, supporting our various needs to grow, build cells, manufacture hormones, fight a rival, or flee from a tiger. A foe is food containing a toxin that just might kill us. Sweet tends to signify the former; bitter, an acquired taste children resist, portends the latter.
Clearly, members of the human clan -- or for that matter, any animal clan -- adept at distinguishing dietary friend from foe would be far more likely to fight, flee, enjoy a bit of anabolism, and live to see another day than a less adept counterpart. Those of us with taste buds best oriented toward survival would be most likely to survive, and pass on the genes to manufacture just those kinds of taste buds to our offspring.
The result, of course, is a prevailing palate pointing with the fidelity of a compass needle to true dietary north: eat what tastes good, and survive. (The compass only works in a world of natural foods, alas.)
But we do not speak "taste bud." The response of a taste bud to the chemicals in a food is itself a chemical reaction that must first be translated into a language we do speak before we can follow the potentially life-saving guidance of our tongues. The language into which chemical excitation of taste buds is translated is pleasure, or displeasure.
Recall that the eons, evolution, and natural selection are unconcerned with our pleasure. But pleasure proves to be a potent motivator of our behaviors, and thus figures most saliently in those behaviors most potently linked to our prospects for survival, and procreation. Not to put too fine a point on it, but it may be that eating and sex are on the very short list of activities virtually every human on the planet would agree are ... fun.
Fun is the means, survival is the ends. And the same is, of course, true of displeasure in reverse. By and large pain is a source of displeasure because pain signifies a threat to survival. A genetic abnormality that silences pain neurons means a life free of pain, but generally means a short life -- because no pain means no awareness of many imminent threats to life and limb.
Which leads us, albeit circuitously, to another bit of recent news about food having almost the impact of Dr. Lustig's anti-sugar diatribe. Namely, findings -- from researchers at my own university among others- suggesting that food can be addictive. Specifically, a paper by Gearhardt and colleagues in the Archives of General Psychiatry shows that those individuals scoring higher on a scale for "food addiction" have more intense responses in the reward circuitry of the brain, revealed using functional MRI scans, than counterparts scoring lower.
The media attention this study has generated would suggest that it delivers an epiphany. But I think the case is just as strong that it delivers a tautology: those who report receiving a stronger, more irresistible reward message from their brain in response to food have a stronger, more irresistible reward response in their brain in response to food.
With or without brain scans, we knew that food affects the brain. The brain is hard-wired to reward us for doing things that are good for our survival. What's good for our survival gets passed from generation to generation. The hard-wiring of the brain "most likely to succeed" determines the hard-wiring of the brain most likely to prevail. Success is survival, and vice versa.
If sugar fosters survival in a world where food energy is scarce, then those who get more of it are more likely to survive. Those who get more of it, all other things being equal, will be those who work harder to get more of it. And they, in turn, are apt to be those who want it most, and like it best.
So those most motivated to get the sugar they need wind up getting the most sugar. They, in turn, benefit from this by having more of the needed food energy -- and thus are more likely to survive. In particular, they are more likely to survive into adulthood, and to procreate. And thus they become our ancestors, who pass traits along to us. People who don't survive long enough to procreate make rather poor ancestors.
The reward mechanisms in the brain are set up to say 'good job!' when we do the things that increase our likelihood of surviving and passing our genes along.
The surprise, then, is not that foods are addictive (nor, I suppose that some refer to sex that way) but rather that anything else is! Why are drugs addictive, if they DON'T promote survival?
The answer is mistaken identity. The reward pathways in the central nervous system developed in a world where 'false positives' were not a big issue. It was unlikely that Stone Age people would fail to find food, but would manage to find tobacco, or cocaine. They certainly didn't find artificial sweeteners.
Natural selection is parsimonious -- it does not tend to embellish once it settles on a solution that effectively addresses the challenges of survival. So our various metabolic defenses are no more sophisticated than is required to get the job done.
And therefore, our fairly simple reward mechanisms may be misleading. They react to morphine the same way they react to our own native pain-fighting, pleasure-providing endorphins. There is evidence that tobacco, cannabis, and food may share some of the same neural networks of reward.
The prevailing definition of addiction has three salient criteria: yearning, tolerance, and withdrawal. We need to need, or at least want, something for it be addictive. Tolerance means the more you get, the more you want. Withdrawal, of course, is the tendency to feel really crummy when you can't get the needed 'fix.' Whether feeling crummy is unpleasant, or truly dangerous, varies with the substance.
With or without brain scans, we could say that various foods and food components -- sugar clearly among them -- readily satisfy the first two criteria. Food in the aggregate certainly satisfies the third -- there is an unpleasant withdrawal from it known as starvation. Whether there is a withdrawal syndrome for specific foods or food components is more debatable, and probably idiosyncratic.
But it doesn't truly matter. Of course food is addictive. Again, the surprise is that anything other than food and sex qualifies.
Demonstrating this on brain scans may have two important implications. First, the technological may better motivate public policy than the merely obvious. If cutting-edge brain imaging shows what foods can do to us, it may help justify regulation to make the food supply safer and more salutary. Second, it may provide comfort to those who subjectively contend that their reward mechanisms are getting the better of them that objective brain scans corroborate their position.
But there is, in my view, a very serious dark side to such brain imaging; it casts three long shadows. First, it suggests we place more emphasis on a picture of a brain than on what the person whose head it's in is telling us about how they feel. That is, at best, unsettling.
Second, the use of such technology implies the need for such technology before we take action. So despite the fact that it is perfectly obvious that food is among the reasons why the human body is even capable of addiction, we now await brain imaging studies to prove it, and facilitate responsive policies.
And finally, what's good for the goose is good for the gander. There is evidence that elements in the food industry have used just this sort of technology to determine how to make food excite the greatest possible reward in the human brain.
Which brings us back to sugar.
If anything in our food is potentially addictive, it is sugar- in its various forms, and under its diverse aliases. This is where Lustig, Taubes, and I converge. An excess of sugar- fructose or any other- is harmful. That is what "excess" means. The dose makes the poison.
Because sugar (or better still "sweet" which also encompasses artificial sweeteners) is addictive by any reasonable definition, getting to excess is easy. The more sugar there is in our food, the more we habituate to high levels, and the more we want. The more we want, the more manufacturers provide -- because keeping the customer satisfied, and outselling the competition, are the business of business. But the food industry can't just blame the excess of sugar in our food on our palates; they helped create that palate in the first place.
But this is where the common ground I share with Taubes and Lustig ends. Lustig takes particular aim at fructose, which may sound reassuring -- because fructose is not the sugar in breast milk. Lactose is broken apart into glucose, and galactose. But fructose is the predominant sugar in fruit. It may be that the reason our taste buds continue to reward us for eating sugar after we are weaned is all about fruit.
So while fructose as an ingredient excessively engineered into processed foods is, indeed, a problem- I find it far-fetched at best to suggest the native composition of, say, berries is "evil." Lustig seems to be tossing out the the strawberries with the soda. You find me the person who can blame obesity or diabetes on eating strawberries, and I will give up my day job and become a hula dancer.
A diet can contain sugar, and specifically fructose, and be optimal for health. A diet could be low in sugar, but high in sodium or trans fat, or deficient in fiber and omega-3 fat -- and be far from optimal.
Lustig and Taubes are propagating the ONAAT fallacy. Like Atkins and others who have come before them, they appear to be dualists who divide the spectrum and subtleties of food into good vs. evil; and iconoclasts who get attention by challenging conventional wisdom.
The redundant aspirations of dietary dualistic iconoclasts over a span of decades have done us no favors. This good vs. evil foodview invited us all to cut fat and eat Snackwell cookies; then to cut carbs while ignoring trans fat. We could waste a lot of time and squander a lot of health finding more, equally silly places to go.
Calories, of course, do count; they are a measure of energy, and their role is rooted in the laws of thermodynamics. It is the overall quality, and quantity, of our diet that matters to health- not just one villainous or virtuous nutrient du jour. We should, indeed, eat food, not too much, mostly plants. The work we need most urgently is about what it will take to get there from here.
As dietary guidance, the vilification of one nutrient at a time has proven as flighty as hummingbirds, propelling us from one version of humbug to another. My advice is to grasp firmly your common sense, and stay grounded.
Dr. David L. Katz; www.davidkatzmd.com
Follow David Katz, M.D. on Twitter: www.twitter.com/DrDavidKatz