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 fat. This suggests that obese people are not leptin deficient but rather leptin resistant.8 Their hypothalami can’t see their leptin, so their brains think they’re starving, and will therefore try to increase energy storage (gluttony) and conserve energy usage (sloth).
In 1999, Steven Heymsfield, then at Columbia University, gave daily injections of leptin at varying doses to obese adults for six months. All these people had high leptin levels to start. The degree of weight loss, even with the highest dosage of leptin, was underwhelming.9 Clearly these obese people were leptin resistant. They couldn’t respond to their own leptin, and no amount of extra leptin was going to make a difference. Heymsfield’s study was the end of the promise of leptin as a stand-alone therapy for obesity and the end of Amgen’s interest.
Hypothalamic Obesity: Behavior or Biochemistry?
This is where I enter the story. In 1995, I arrived in Memphis to start work at St. Jude Children’s Research Hospital as a pediatric neuroendocrinologist. My training is in taking care of kids with brain tumors, and St. Jude had a large population of survivors, Many of these children develop hormonal deficiencies because of damage to the hypothalamus—due to the tumor itself, the neurosurgery to remove it, or the radiation and chemotherapy they receive to try to kill it. The good news is that we endocrinologists can treat these children by replacing most of the hormones that are missing—we can affect their growth, energy metabolism, and cognitive status; induce puberty when the children are age appropriate; and improve their overall health.
However, a relatively small number of children like Marie (and adults) who survive their brain tumors become massively obese after their tumor therapy is complete. Their hypothalamus is damaged, and their weight skyrockets. Their appetites aren’t that different from those of other obese children, but their energy expenditure is markedly decreased. (Marie didn’t move.) Those affected sit on the couch, watch TV, eat, poop, sleep, and generally lose interest in the world around them. As one parent stated, “It’s double jeopardy. To think you might lose your kid to a cancer, and survive it, but then to lose your kid to a complication instead.” Patients with this form of obesity, called hypothalamic obesity, can’t lose weight. Even if these kids eat only 500 calories a day, they gain weight.10 The neurons in the hypothalamus, which sense the leptin signal, are all dead. The “servo-mechanism” for energy balance had been short-circuited. This is leptin resistance at its worst—an anatomic leptin resistance. Rodent studies dating back to the early 1950s show that when you damage the VMH, the animal will become massively obese, and not even food restriction will reverse that. The VMH-lesioned rats ate more than they needed and burned less than they should have. Unlike the leptin-deficient mice, no amount of leptin would fix the problem. These animals had anatomic leptin resistance.11 The leptin had no place to act.
The obese children I saw at St. Jude were similar to these VMH-lesioned rats. There was no fixing them because there was no way to regrow those neurons. Those kids were stuck forever in bodies that just kept storing energy instead of burning it,12 with brains that constantly thought the bodies were starving. They would forever get fatter on fewer calories, never feel good, and would lose interest in everything around them. If this isn’t hell on earth for parent and child, I don’t know what is.
Worst yet, there was no treatment. Diet and exercise is notoriously ineffective in these children. Weight loss drugs also didn’t work. In 1995, I was faced with a clinic full of patients with hypothalamic obesity following their brain tumor therapy. How to help them? I couldn’t give them leptin, because the block at the hypothalamus would not allow leptin to work. If any therapy were to be successful, it would have to work downstream of the leptin neuron, somewhere between the brain and the fat cell.
Insulin: The “Leptinator”
Normally, the amount of insulin released in response to a meal is yoked to the blood sugar rise. But there are a few things that force the pancreas to make extra insulin, the vagus nerve being chief among them. When the brain can’t see the leptin signal, as in children such as Marie, it interprets starvation. The vagus nerve goes into overdrive to store more energy, and kick-starts the pancreas to make extra insulin—even more than the