In the ever-shifting lineup of dietary villains, sugar has taken a commanding lead. Even New York City's mayor, Michael Bloomberg, has fingered sugar as the major culprit in the obesity epidemic and proposed a ban on the sale of supersize sodas in city restaurants and fast food chains. It's as bad as exposure to asbestos or smoke, he insists, and it's creating outsize medical bills for the city. He doesn't know the half of it.
The brain runs on sugar, specifically the simple sugar glucose, which is what carbohydrates become in the course of digestion. But increasingly, the American diet supplies another variety of sugar: fructose. Until about 1970, fructose entered our bodies in relatively small amounts, primarily in fruits, and then in combination with fiber and antioxidants, which moderate its biological effects. Now, it is dumped wholesale into our food supply, thanks to its extraction in concentrated form from corn for use as a cheap sweetener in processed foods, from cereals to baked goods and especially soda. As scientists are discovering, the body handles fructose very differently from other sugars. And that, increasing evidence suggests, is particularly bad for the brain as well as for the rest of the body. It's best avoided in any portion size.
Glucose can enter the bloodstream directly from the gut, and its uptake by body cells is regulated by the hormone insulin. Fructose, however, is metabolized mainly by the liver, and the process significantly ups production of fatty triglycerides. It's as if the body is consuming a high-fat diet. The triglycerides create fatty liver disease. Pumped into the bloodstream, triglycerides create a risk for heart disease. Circulated to the brain, research shows, they contribute to an array of cognitive deficits.
Dumbed by Diet?
Consuming a high-fructose diet not only undermines body metabolism but compromises mental health, UCLA researchers report. Male animals drinking fructose-laced water can't remember landmarks placed to help them escape from a maze. Their brain cells have trouble transmitting signals from one neuron to the next. The explosive surge in metabolic syndrome, marked by insulin resistance and leading to an epidemic of diabetes, is likely taking a toll on the nation's cognitive and emotional capacity.
The memory-impairing effects of a high-fructose diet may primarily afflict males—and males are by far the biggest consumers of fructose, notably in soft drinks from fast-food chains. Call it the McDonald's effect. A 60 percent fructose diet enlarges the liver in female rats but does not consistently increase their triglyceride level. Nor is their memory impaired on maze tests, as it is in males, whose memory deficits are highly correlated with triglyceride level. Estrogen may blunt some of fructose's effects.
Brain cells develop insulin resistance, just as body cells do, say Michigan researchers. In fact, insulin resistance induced by triglycerides may cause the memory impairment of fructose-fed animals. It also may abet age-related cognitive decline. Insulin helps brain cells absorb and use their favorite fuel, glucose. But high levels of insulin, as occur with insulin resistance, also curtail the brain's ability to clear out beta-amyloid, greatly raising levels of the problem protein responsible for the brain plaques of Alzheimer's disease.
Expanding their view of the metabolic syndrome, scientists now invoke the term "metabolic-cognitive syndrome" to refer to a complex relationship between metabolic disorders and brain disturbances. Deficits in insulin signaling in the brain, caused by the metabolic syndrome, may foster the development of depression. By depriving brain cells of the ability to take up glucose, insulin resistance may also impair the functioning of neurons, diminishing energy levels and the ability to produce and respond to neurotransmitters.