David Ludwig, the Harvard endocrinologist, says that it's the direct effect of insulin on blood sugar that does the trick. He notes that when diabetics get too much insulin, their blood sugar drops and they get ravenously hungry. They gain weight because they eat more, and the insulin promotes fat deposition. The same happens with lab animals. This, he says, is effectively what happens when we eat carbohydrates -- in particular sugar and starches like potatoes and rice, or anything made from flour, like a slice of white bread. These are known in the jargon as high-glycemic-index carbohydrates, which means they are absorbed quickly into the blood. As a result, they cause a spike of blood sugar and a surge of insulin within minutes. The resulting rush of insulin stores the blood sugar away and a few hours later, your blood sugar is lower than it was before you ate. As Ludwig explains, your body effectively thinks it has run out of fuel, but the insulin is still high enough to prevent you from burning your own fat. The result is hunger and a craving for more carbohydrates. It's another vicious circle, and another situation ripe for obesity.
The glycemic-index concept and the idea that starches can be absorbed into the blood even faster than sugar emerged in the late 70's, but again had no influence on public health recommendations, because of the attendant controversies. To wit: if you bought the glycemic-index concept, then you had to accept that the starches we were supposed to be eating 6 to 11 times a day were, once swallowed, physiologically indistinguishable from sugars. This made them seem considerably less than wholesome. Rather than accept this possibility, the policy makers simply allowed sugar and corn syrup to elude the vilification that befell dietary fat. After all, they are fat-free.
Sugar and corn syrup from soft drinks, juices and the copious teas and sports drinks now supply more than 10 percent of our total calories; the 80's saw the introduction of Big Gulps and 32-ounce cups of Coca-Cola, blasted through with sugar, but 100 percent fat free. When it comes to insulin and blood sugar, these soft drinks and fruit juices -- what the scientists call ''wet carbohydrates'' -- might indeed be worst of all. (Diet soda accounts for less than a quarter of the soda market.)
The gist of the glycemic-index idea is that the longer it takes the carbohydrates to be digested, the lesser the impact on blood sugar and insulin and the healthier the food. Those foods with the highest rating on the glycemic index are some simple sugars, starches and anything made from flour. Green vegetables, beans and whole grains cause a much slower rise in blood sugar because they have fiber, a nondigestible carbohydrate, which slows down digestion and lowers the glycemic index. Protein and fat serve the same purpose, which implies that eating fat can be beneficial, a notion that is still unacceptable. And the glycemic-index concept implies that a primary cause of Syndrome X, heart disease, Type 2 diabetes and obesity is the long-term damage caused by the repeated surges of insulin that come from eating starches and refined carbohydrates. This suggests a kind of unified field theory for these chronic diseases, but not one that coexists easily with the low-fat doctrine.
At Ludwig's pediatric obesity clinic, he has been prescribing low-glycemic-index diets to children and adolescents for five years now. He does not recommend the Atkins diet because he says he believes such a very low carbohydrate approach is unnecessarily restrictive; instead, he tells his patients to effectively replace refined carbohydrates and starches with vegetables, legumes and fruit. This makes a low-glycemic-index diet consistent with dietary common sense, albeit in a higher-fat kind of way. His clinic now has a nine-month waiting list. Only recently has Ludwig managed to convince the N.I.H. that such diets are worthy of study. His first three grant proposals were summarily rejected, which may explain why much of the relevant research has been done in Canada and in Australia. In April, however, Ludwig received $1.2 million from the N.I.H. to test his low-glycemic-index diet against a traditional low-fat-low-calorie regime. That might help resolve some of the controversy over the role of insulin in obesity, although the redoubtable Robert Atkins might get there first.
The 71-year-old Atkins, a graduate of Cornell medical school, says he first tried a very low carbohydrate diet in 1963 after reading about one in the Journal of the American Medical Association. He lost weight effortlessly, had his epiphany and turned a fledgling Manhattan cardiology practice into a thriving obesity clinic. He then alienated the entire medical community by telling his readers to eat as much fat and protein as they wanted, as long as they ate little to no carbohydrates. They would lose weight, he said, because they would keep their insulin down; they wouldn't be hungry; and they would have less resistance to burning their own fat. Atkins also noted that starches and sugar were harmful in any event because they raised triglyceride levels and that this was a greater risk factor for heart disease than cholesterol.
Atkins's diet is both the ultimate manifestation of the alternative hypothesis as well as the battleground on which the fat-versus-carbohydrates controversy is likely to be fought scientifically over the next few years. After insisting Atkins was a quack for three decades, obesity experts are now finding it difficult to ignore the copious anecdotal evidence that his diet does just what he has claimed. Take Albert Stunkard, for instance. Stunkard has been trying to treat obesity for half a century, but he told me he had his epiphany about Atkins and maybe about obesity as well just recently when he discovered that the chief of radiology in his hospital had lost 60 pounds on Atkins's diet. ''Well, apparently all the young guys in the hospital are doing it,'' he said. ''So we decided to do a study.'' When I asked Stunkard if he or any of his colleagues considered testing Atkins's diet 30 years ago, he said they hadn't because they thought Atkins was ''a jerk'' who was just out to make money: this ''turned people off, and so nobody took him seriously enough to do what we're finally doing.''
In fact, when the American Medical Association released its scathing critique of Atkins's diet in March 1973, it acknowledged that the diet probably worked, but expressed little interest in why. Through the 60's, this had been a subject of considerable research, with the conclusion that Atkins-like diets were low-calorie diets in disguise; that when you cut out pasta, bread and potatoes, you'll have a hard time eating enough meat, vegetables and cheese to replace the calories.
That, however, raised the question of why such a low-calorie regimen would also suppress hunger, which Atkins insisted was the signature characteristic of the diet. One possibility was Endocrinology 101: that fat and protein make you sated and, lacking carbohydrates and the ensuing swings of blood sugar and insulin, you stay sated. The other possibility arose from the fact that Atkins's diet is ''ketogenic.'' This means that insulin falls so low that you enter a state called ketosis, which is what happens during fasting and starvation. Your muscles and tissues burn body fat for energy, as does your brain in the form of fat molecules produced by the liver called ketones. Atkins saw ketosis as the obvious way to kick-start weight loss. He also liked to say that ketosis was so energizing that it was better than sex, which set him up for some ridicule. An inevitable criticism of Atkins's diet has been that ketosis is dangerous and to be avoided at all costs.
When I interviewed ketosis experts, however, they universally sided with Atkins, and suggested that maybe the medical community and the media confuse ketosis with ketoacidosis, a variant of ketosis that occurs in untreated diabetics and can be fatal. ''Doctors are scared of ketosis,'' says Richard Veech, an N.I.H. researcher who studied medicine at Harvard and then got his doctorate at Oxford University with the Nobel Laureate Hans Krebs. ''They're always worried about diabetic ketoacidosis. But ketosis is a normal physiologic state. I would argue it is the normal state of man. It's not normal to have McDonald's and a delicatessen around every corner. It's normal to starve.''
Simply put, ketosis is evolution's answer to the thrifty gene. We may have evolved to efficiently store fat for times of famine, says Veech, but we also evolved ketosis to efficiently live off that fat when necessary. Rather than being poison, which is how the press often refers to ketones, they make the body run more efficiently and provide a backup fuel source for the brain. Veech calls ketones ''magic'' and has shown that both the heart and brain run 25 percent more efficiently on ketones than on blood sugar.
The bottom line is that for the better part of 30 years Atkins insisted his diet worked and was safe, Americans apparently tried it by the tens of millions, while nutritionists, physicians, public- health authorities and anyone concerned with heart disease insisted it could kill them, and expressed little or no desire to find out who was right. During that period, only two groups of U.S. researchers tested the diet, or at least published their results. In the early 70's, J.P. Flatt and Harvard's George Blackburn pioneered the ''protein-sparing modified fast'' to treat postsurgical patients, and they tested it on obese volunteers. Blackburn, who later became president of the American Society of Clinical Nutrition, describes his regime as ''an Atkins diet without excess fat'' and says he had to give it a fancy name or nobody would take him seriously. The diet was ''lean meat, fish and fowl'' supplemented by vitamins and minerals. ''People loved it,'' Blackburn recalls. ''Great weight loss. We couldn't run them off with a baseball bat.'' Blackburn successfully treated hundreds of obese patients over the next decade and published a series of papers that were ignored. When obese New Englanders turned to appetite-control drugs in the mid-80's, he says, he let it drop. He then applied to the N.I.H. for a grant to do a clinical trial of popular diets but was rejected.
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