A few days ago, I had my first McGriddle. While I usually try to avoid McDonald’s meat products – that’s the benevolent influence of my wife, who rightly insists on eating humanely raised animal products – I was stuck in an airport and couldn’t bear the idea of another yogurt parfait. The “standard” McGriddle consists of bacon, a brick of bright yellow egg and neon orange American cheese served between two small pancakes that have been injected with maple syrup (or some sort of maple simulacrum) so that they taste extremely sweet and yet aren’t sticky to hold. The top of the griddle pancake is embossed with the McDonald’s logo.
Needless to say, the McGriddle is eerily delicious. If the human tongue has a secret password, then this sweet, salty and fatty breakfast sandwich is the code. The problem of course, is that all the deliciousness comes with a steep caloric cost: every McGriddle has 420 calories, 15 grams of sugar and 80 percent of the recommended daily allowance of cholesterol. As Elizabeth Kolbert recently described in the New Yorker, this preference for bacon wrapped in sweet pancakes is an unfortunate side-effect of human evolution:
Brains are calorically demanding organs. Our distant ancestors had small ones. Australopithecus afarensis, for example, who lived some three million years ago, had a cranial capacity of about four hundred cubic centimetres, which is roughly the same as a chimpanzee’s. Modern humans have a cranial capacity of about thirteen hundred cubic centimetres. How, as their brains got bigger, did our forebears keep them running? According to what’s known as the Expensive Tissue Hypothesis, early humans compensated for the energy used in their heads by cutting back on the energy used in their guts; as man’s cranium grew, his digestive tract shrank. This forced him to obtain more energy-dense foods than his fellow-primates were subsisting on, which put a premium on adding further brain power. The result of this self-reinforcing process was a strong taste for foods that are high in calories and easy to digest; just as it is natural for gorillas to love leaves, it is natural for people to love funnel cakes.
It’s now possible to observe this preference at work in the brain. While the McGriddle is a tasty creation, the most pleasurable thing about the sandwich isn’t the pancake or the bacon: it’s the calories. According to a recent paper in Neuron, the brain also receives rewarding input from metabolic processes that have nothing to do with the tongue. When you eat at McDonald’s, a big part of the pleasure comes from the fact that the food is sustenance, fuel, energy. Even mediocre food is a little rewarding.
The scientists at Duke came up with a clever paradigm for isolating this more indirect rewarding pathway: they studied mice without a functional TRPM5 channel, which is essential for detecting sweetness. As a result, these mutant mice showed no immediate preference for sugar water.
But here comes the cool part of the experiment. The scientists then allowed the mice to spend some time with the sugar water and normal water. After a few hours, it became clear that the mutant mice greatly preferred the sugar water, even though they couldn’t taste the sugar. (A control experiment with sucralose, an artificial sweetener, demonstrated that the rats were responding to the caloric intake, not the sweet taste.)
Finally, the scientists measured dopamine levels (via in vivo microdialysis) in the nucleus accumbens (a brain area that processes rewards) in the mutant mice and normal mice.* While normal mice exhibited an increase in dopamine in response to both fake sugar and real sugar – the reward was the sweet taste – the mutant mice only demonstrated a dopaminergic spike when consuming genuine sugar water. What they enjoyed were the calories. As the authors conclude:
We showed that dopamine-ventral striatum reward systems, previously associated with the detection and assignment of reward value to palatable compounds, respond to the caloric value of sucros in the absence of taste receptor signaling. Thus, these brain pathways…also perform previously unidentified functions that include the detection of gastro-intestinal and metabolic signals.
This is a troubling idea, since it reveals the very deep biological roots underlying the obesity epidemic. Let’s imagine, for instance, that some genius invented a reduced calorie bacon product that tasted exactly like bacon, except it had 50 percent fewer calories. It would obviously be a great day for civilization. But this research suggests that such a pseudo-bacon product, even though it tasted identical to real bacon, would actually give us much less pleasure. Why? Because it made us less fat. Because energy is inherently delicious. Because we are programmed to enjoy calories.