Fat Chance: The bitter truth about sugar. Dr. Lustig Robert

Чтение книги онлайн.

СКАЧАТЬ on the sympathetic nervous system (responsible for muscle activity and fat loss), and turns off the vagus nerve (responsible for appetite and fat gain); while orexigenesis does the opposite. However, high insulin blocks the leptin signal, mimicking “brain starvation” and driving orexigenesis, so that we feel hungry even when we have eaten.

      From there, the hypothalamus sends signals from the brain to the body via two components of the autonomic nervous system. The autonomic nervous system is that portion of your body that controls your heart rate, blood pressure, and energy metabolism without your conscious effort. It is composed of two parts: the sympathetic nervous system (responsible for the fight-or-flight response) and the parasympathetic nervous system (responsible for “vegetative” functions such as food absorption and energy storage). The vagus nerve is one of the key components of the parasympathetic nervous system. There is a delicate balance and feedback loop between the sympathetic and parasympathetic systems. When that balance changes, that’s when problems ensue.

      The vagus nerve is fascinating. It connects the brain to all the digestive organs in the abdomen: the liver, the intestine, the pancreas, and also to the fat cells. It performs many different functions but with one ultimate goal: to store energy. The vagus is your energy storage nerve. The vagus has two parts: the afferent part (organs to brain), and the efferent part (brain to organs). The afferent vagus communicates the sensation of hunger between the stomach and brain, and also communicates information on energy processing during a meal between the liver and brain. The VMH interprets all these afferent signals, which leads to one of two physiologic states: anorexigenesis (I don’t need any more food, I can burn energy as needed, and I feel good) or orexigenesis (I don’t have enough food, I don’t want to burn any energy, and I will feel lousy until I get some more).

      The anorexigenesis signal turns on the sympathetic nervous system (SNS), which promotes energy expenditure by telling the adipose (fat) tissue and the muscles to burn energy, thereby resulting in weight loss and a sense of well-being. Anorexigenesis also turns off the vagus nerve and, in so doing, reduces appetite. Conversely, orexigenesis stimulates the vagus nerve to promote energy storage by increasing appetite. It accomplishes this by sending multiple signals through the vagus nerve: to the gastrointestinal tract to digest and absorb the food; to the adipose tissue to store more energy (make more fat); and to the pancreas to increase the amount of insulin released (promoting more energy storage into adipose tissue).

       Leptin and the Elusive “Holy Grail” of Obesity

      When the hormone leptin (from the Greek Leptos, for “thin”) was discovered in 1994, for the first time, scientists thought that obesity might have a biochemical basis. Leptin has been a veritable godsend to scientists who study obesity. It provided the starting point to understanding the biochemistry of the brain pathways that control food intake and the impetus for scientists and the National Institutes of Health (NIH) to believe that there was a simple way out of this mess, one that could be easily treated with medicine and science. The U.S. government began, and continues today, to shovel money at obesity research, hoping for a treatment that works. Conversely, leptin has been the biggest disappointment to those who suffer from obesity. And woe to the pharmaceutical industry, which hoped to harness its potential for a cure and generate megabucks in the process. The pharmaceutical company Amgen was so enamored of leptin’s blockbuster marketing potential that it offered $30 million for the exclusive marketing rights to the hormone, even before a human experiment had been performed. Amgen has since become so disillusioned that it has farmed leptin out to another company, Amylin Pharmaceuticals, to see if it will have better luck.

      Leptin is a protein made and released by fat cells. It circulates in the bloodstream, goes to the hypothalamus, and signals the hypothalamus that you’ve got enough energy stored up in your fat.² The discovery of leptin closed the loop, providing a servomechanism (like your home’s thermostat) in which the body’s fat cells told the hypothalamus whether the animal was in energy surplus (obesity) or dearth (starvation). Obese animals and humans deficient in leptin respond immediately to leptin treatment with remarkable losses of fat and also with increased activity.³ Leptin replacement corrected both behaviors, the gluttony and the sloth. The thought was, if you’re obese, then your leptin doesn’t work—you must be deficient and you just need more. Problem solved, right? Unfortunately, for the obese population, this simple-minded explanation was just that.

       Defective Leptin Signaling: Brain Starvation

      The VMH is constantly looking for the leptin signal. In the short-term, hormonal inputs can govern the size or the quality of this meal or that, but long term it’s all about leptin. Leptin tells the VMH that you have enough energy on board to burn the excess, feel good, reduce your long-term food intake, and remain weight stable. When your leptin signal works, you’re in energy balance, burning energy at a normal rate and feeling good.⁴ Every human has a “personal leptin threshold” above which the brain interprets a state of energy sufficiency. Thus, the leptin-replete state is characterized by appropriate appetite, normal physical activity, and feelings of well-being. Woe to the 97-pound weakling who can’t bulk up and gain weight; his leptin threshold is set too low, and his leptin is telling his brain to burn off any excess.

      But what if leptin doesn’t work or the threshold is set too high? When the VMH can’t see the leptin signal, the brain interprets this as “starvation” and will direct the rest of the body to do whatever it can to increase its energy stores. The VMH relays messages to the sympathetic nervous system (SNS) to conserve energy and reduce activity. Energy expenditure is reduced by 20 percent, a great reason to feel like a sloth.⁵ Furthermore, the VMH wants the body to increase energy storage. It will increase the firing of the vagus nerve in order to amplify insulin release from the pancreas and shunt more energy into fat cells, with the ultimate goal of making more leptin. The vagus makes you hungry in order that you store more energy (gluttony). Simply put, defective leptin signaling in the VMH is what brain starvation is all about. This phenomenon occurs in two ways:

      Leptin deficiency. Dr. Jeff Friedman of Rockefeller University is credited with cloning the leptin gene from leptin-deficient mice,⁶ which are the rodent equivalents of a 400-pound couch potato. While normal weight at birth, these mice immediately eat like there’s no tomorrow and just sit there—the only time they ever get off their behinds is if you put food on the other side of the cage; then they’ll waddle over to it, devour it, and sit there instead. These mice are deficient in leptin due to a genetic mutation. Their behaviors of gluttony and sloth are genetically determined. Their brain can’t see their fat and in turn thinks the body is starving.

      Friedman’s lab also showed that giving these mice back the leptin they were missing by daily injection reduced their food intake and increased their physical activity back to normal. They lost the weight. Not only that, but all the physiological problems associated with their obesity—the diabetes, the lipid problems, and early death from heart disease—all disappeared. This made leptin look for all intents and purposes like the “holy grail” of obesity. If leptin deficiency was the cause of this pandemic, we could simply replace it, and all the unfortunate souls afflicted could be saved.

      Thus far, fourteen children with mutations of the leptin gene have been identified in the entire world. These children cannot make leptin no matter how big their fat cells are, and their brains are in constant starvation mode. Amazingly, with a shot of leptin every day, they lose weight rapidly, and it’s all fat (no muscle). They stop their ravenous behavior, start moving, and their puberty goes into gear.⁷ For these patients, leptin is hormone-replacement therapy; while not a cure, it’s the next best thing.

      Leptin resistance. This is the key to the obesity epidemic. With a few rare exceptions, the other 1.5 billion overweight or obese people on the planet suffer from this. Deciphering leptin resistance is the “holy grail” of obesity. These people have plenty of leptin, and each one’s blood leptin level correlates with his or her amount of body СКАЧАТЬ