杏吧原创

MIDDLE-AGED janitors rarely make their mark on science. But Caesar Barber looks like breaking the mould. Last July, Barber, a 56-year-old diabetic and double heart-attack victim from Brooklyn, sued McDonald鈥檚, Burger King, KFC and Wendy鈥檚, claiming that his illnesses were partly their fault. He had eaten in their restaurants for years, he said, without ever being told that the food was damaging his health.

Barber鈥檚 class-action lawsuit was the first volley in a long-awaited legal assault against the fast-food industry and its role in the obesity epidemic that is swamping the US health-care system (see 鈥淔at facts鈥�). Inspired by the success of Big Tobacco, the lawyers behind it believe they can force fast-food chains to meet their fair share of the enormous cost of caring for obesity. Pulling the strings is John Banzhaf, of George Washington University Law School in Washington DC, who masterminded the Big Tobacco crusade.

That campaign won him plaudits all over the world. But 鈥淏ig Fat鈥� is a different matter. To many 鈥� including a federal judge who last month dismissed a similar lawsuit against McDonald鈥檚 鈥� it seems blatantly absurd. Surely people who become fat and ill because they have eaten too much fast food only have themselves to blame?

Perhaps not. New and potentially explosive findings on the biological effects of fast food suggest that eating yourself into obesity isn鈥檛 simply down to a lack of self-control. Some scientists are starting to believe that bingeing on foods that are excessively high in fat and sugar can cause changes to your brain and body that make it hard to say no. A few even believe that the foods can trigger changes that are similar to full-blown addiction. The research is still at a very early stage, but thanks to Caesar Barber it is about to be thrust firmly into the limelight.

Taking on the fast-food industry was always going to be a much tougher assignment than beating the cigarette barons. Tobacco is obviously addictive. Nobody needs to smoke. And the tobacco companies knew their products were addictive yet covered it up. None of these accusations can be levelled at food.

Banzhaf maintains that he can win regardless. He points out that he doesn鈥檛 have to prove that the fast-food chains are entirely responsible for obesity. All he has to do is convince a jury that his clients鈥� health problems were not entirely their own fault 鈥� that the fast-food companies share the blame. Perhaps, for example, they should have labelled the food to inform customers of its high calorific value.

Any hint that the food is addictive, though, would make Banzhaf鈥檚 job a great deal easier. And he knows it. Banzhaf already says he believes that fast food has 鈥渁ddictive-like鈥� properties. 鈥淲e might even discover that it鈥檚 possible to become addicted to the all-American meal of burgers and fries,鈥� he says.

But how can something you need for survival be addictive? The answer could be in the food itself. The difference between a fast-food meal and a home-cooked one is the sheer quantity of calories and fat it delivers in one go. The US Department of Agriculture鈥檚 recommended daily intake for a normal adult male is 2800 kilocalories (11,723 kilojoules) and a maximum of 93 grams of fat. A meal at a fast-food outlet 鈥� burger, fries, drink and dessert 鈥� can deliver almost all of that in a single sitting (see Diagram). Biologists are now starting to realise that a binge of these proportions can trigger physiological changes which mute the hormonal signals that normally tell you to put down the fork.

In the past decade, researchers have discovered myriad hormones that play a role in regulating appetite. Under normal conditions these hormones control eating and help maintain a stable body weight. Leptin, for example, is continuously secreted by fat cells and its level in the bloodstream indicates the status of the body鈥檚 fat reserves. This signal is read by the hypothalamus, the brain region that coordinates eating behaviour, and taken as a guideline for keeping reserves stable.

The problem is, people who gain weight develop resistance to leptin鈥檚 power, explains Michael Schwartz, an endocrinologist at the University of Washington in Seattle. 鈥淭heir brain loses its ability to respond to these hormones as body fat increases,鈥� he says. The fatter they get, and the more leptin they make, the more insensitive the hypothalamus becomes. Eventually the hypothalamus interprets the elevated level as normal 鈥� and forever after misreads the drops in leptin caused by weight loss as a starvation warning.

But you don鈥檛 need to become overweight to perturb your leptin system. The latest research suggests that it only takes a few fatty meals. In a study published in December, physiologist Luciano Rossetti of the Albert Einstein College of Medicine in New York City fed rats a high-fat diet and found that after just 72 hours the animals had already lost almost all of their ability to respond to leptin (Diabetes, vol 50, p 2786). The good news, says Rossetti, is that these changes are reversible. 鈥淏ut the fatter a person becomes the more resistant they will be to the effects of leptin and the harder it is to reverse those effects.鈥�

Sarah Leibowitz, a neurobiologist at Rockefeller University in New York City, has more evidence that eating fast food is self-reinforcing. Her experiments show that exposure to fatty foods may quickly reconfigure the body鈥檚 hormonal system to want yet more fat. She has shown that levels of galanin, a brain peptide that stimulates eating and slows down energy expenditure, increase in rats when they eat a high-fat diet.

In fact, Leibowitz has found that it only takes one high-fat meal to stimulate galanin expression in the hypothalamus. When the effects of galanin are blocked, the animals eat much less fat. 鈥淭he peptide is itself responsive to the consumption of fat, which then creates the basis for a vicious cycle,鈥� she says.

What鈥檚 more, early exposure to fatty food could reconfigure children鈥檚 bodies so that they always choose fatty foods. Leibowitz found that when she fed young rats a high-fat diet, they invariably became obese later in life. She is still investigating what鈥檚 going on, but her theory is that an elevated level of fats called triglycerides in the bloodstream turns on genes for neuropeptides such as galanin that promote overeating. This suggests that children fed kids鈥� meals at fast-food restaurants are more likely to grow up to be burger-scoffing adults.

Rossetti鈥檚 most recent studies have also found a connection between triglycerides and food intake. Using a catheter implanted in the brain, Rossetti delivered lipids directly into the arcuate nucleus 鈥� a region of the hypothalamus 鈥� to either normally fed rats or overfed rats, and then measured their food intake for three days. In the normally fed group the excess fats curbed food intake by up to 60 per cent. But the overfed rats just carried on scoffing. What鈥檚 more, Rossetti discovered that this effect is not dependent on the composition of the diet, whether high-fat or high-sugar, but instead depends on the total amount of calories.

Hormonal changes may remove some element of free will, but on its own that hardly means that fast food is addictive. However, there is another strand of research that suggests gorging on fat and sugar causes brain changes normally associated with addictive drugs such as heroin.

It is already well established that food and addiction are closely linked. Many addiction researchers believe that addictive drugs such as cocaine and nicotine exert their irresistible pull by hijacking 鈥渞eward鈥� circuits in the brain. These circuits evolved to motivate humans to seek healthy rewards such as food and sex. Eating energy-dense food, for example, triggers the release of endorphins and enkephalins, the brain鈥檚 natural opioids, which stimulate a squirt of dopamine into a structure called the nucleus accumbens, a tiny cluster of cells in the midbrain. Exactly how this generates a feeling of reward isn鈥檛 understood, but it is clear that addictive substances provide a short cut to it 鈥� they all seem to increase levels of dopamine in the nucleus accumbens. Repeated use of addictive substances is thought to alter the circuitry in as yet unknown ways.

Sugar junkies

Most of this research has been done with the aim of understanding drug addiction. But a few researchers are now asking whether the brain鈥檚 reward circuits can also be hot-wired by mega-doses of fat and sugar. John Hoebel, a psychologist at Princeton University in New Jersey, is interested in whether it is possible to become dependent on the natural opioids released when you eat a large amount of sugar. Along with a team of physiologists from the University of the Andes in M茅rida, Venezuela, Hoebel recently showed that rats fed a diet containing 25 per cent sugar are thrown into a state of anxiety when the sugar is removed. Their symptoms included chattering teeth and the shakes 鈥� similar, he says, to those seen in people withdrawing from nicotine or morphine. What鈥檚 more, when Hoebel gave the rats naloxone, a drug that blocks opioid receptors, he saw a drop in dopamine levels in the nucleus accumbens, plus an increase in acetylcholine release. This is the same neurochemical pattern shown by heroin addicts as they go into opioid withdrawal (Obesity Research, vol 10, p 478). 鈥淭he implication is that some animals 鈥� and by extension some people 鈥� can become overly dependent on sweet food,鈥� says Hoebel. 鈥淭he brain is getting addicted to its own opioids as it would morphine or heroin. Drugs give a bigger effect, but it鈥檚 essentially the same process.鈥�

As yet no one knows how a big hit of fat and sugar compares with a dose of, say, heroin. But Hoebel says: 鈥淗ighly palatable foods and highly potent sexual stimuli are the only stimuli capable of activating the dopamine system with anywhere near the potency of addictive drugs.鈥�

Ann Kelley, a neuroscientist at the University of Wisconsin Medical School in Madison, has uncovered more evidence that the release of opioids in the nucleus accumbens tells your brain to keep eating. She found that if rats鈥� opioid receptors are overstimulated with a synthetic enkephalin, the rats eat up to six times the amount of fat they normally consume. They also raise their intake of sweet, salty and alcohol-containing solutions, even when they are not hungry.

Kelley has also discovered that rats that overindulge in tasty foods show marked, long-lasting changes in their brain chemistry similar to those caused by extended use of morphine or heroin. When she looked at the brains of rats that received highly palatable food for two weeks, she saw a decrease in gene expression for enkephalin in the nucleus accumbens. 鈥淭his says that mere exposure to pleasurable, tasty foods is enough to change gene expression, and that suggests that you could be addicted to food,鈥� says Kelley.

However, the idea that food is addictive is far from mainstream. And while many nutritionists think it is a plausible idea that deserves more research, others are sceptical. Michael Jacobson, executive director of the Center for Science in the Public Interest, a Washington DC lobby group that focuses on nutrition, doesn鈥檛 think the argument will fly. So far, the CSPI has not seen any evidence that fast food is addictive.鈥滳onsidering the paucity of evidence, I think the burden is on advocates of the addiction argument to provide evidence of addictiveness,鈥� Jacobson says.

Some practitioners also dispute the idea. There is no reliable evidence that addiction can account for bingeing and obesity, says Jeanne Randolph, a psychiatrist at the University of Toronto who specialises exclusively in treating obese patients. Randolph admits that the behaviour of many of her patients is remarkably similar to drug cravings: at predictable times of day, in predictable circumstances, they describe an increasingly intense drive to obtain their preferred sugary snack or junk food, and afterwards feel immediate relief and calm. But, she says, you can explain this without invoking addiction. Fast food, sweets and snacks in which simple sugars predominate can set up a cycle of instant satiation followed by a plunge in blood sugar, which leads to a natural desire for another snack.鈥滻t鈥檚 a set-up for a late-afternoon binge rather than an addiction.鈥�

The argument has a long way to go. But chances are it won鈥檛 get the chance to mature naturally. Some time soon the allegation that fast food is addictive will be made in court, and once that happens the terms of the debate are out of the scientists鈥� hands. It won鈥檛 make for a scholarly discussion. But it is still a debate worth having.