Robert H. Lustig, MD, MSL, is a Professor of Pediatrics, and member of the Institute for Health Policy Studies at the University of California, San Francisco. He is also the author of Fat Chance: Beating the Odds against Sugar, Processed Food, Obesity, and Disease, and has written extensively about how his research indicates that much of our food supply contributes to metabolic syndrome, which in turn contributes to the increasing worldwide obesity epidemic. In between his many media appearances, Rob just completed a law degree so he can have a direct impact on legislation that regulates the food industry, and the way we treat metabolic syndrome.
Dr. Shulman: One of your primary arguments is that it is bad science to say that “a calorie is a calorie.”
Dr. Lustig: Yes, that is one interpretation of the First Law of Thermodynamics, which says that all energy cannot be destroyed, only reconfigured. But it is the wrong interpretation. Obesity is about energy deposition, not energy balance. Not everyone agrees with me, but I don’t think all calories can be treated as equal because not all calories are metabolized the same way. Isocaloric does not mean isometabolic. Demanding that our bodies process many calories from fructose, which happens when we eat food spiked with all kinds of sugars, has potentially harmful consequences.
Dr. Shulman: I’ll get back to the “spiked with sugar” comment in a moment, but why are sugary calories riskier than other calories?
Dr. Lustig: What happens if you give a 5-year-old a cookie? They bounce off the walls, right? It’s a sugar high. That’s because their levels of the hormone leptin increase, which signals to the hypothalamus that the body has enough energy to engage in “expensive” metabolic processes; the sympathetic nervous system activates the muscles and the kid gets rambunctious. But all that activity keeps the kid in relative energy balance. In obese kids, however, if you give them a cookie, they don’t bounce off the walls, they go looking for another cookie. That is because something prevents their leptin from communicating with the hypothalamus — that something is usually insulin.Insulin normally commands fat cells to take energy from the blood and keep it for storage. Meanwhile, it tells the brain, “Cut back! I don’t need any more; I’m already metabolizing here.” Insulin also extinguishes reward through dopamine clearance, so you can reach satiety. This all works well if the brain is insulin-sensitive.Leptin, when it is working, reduces your appetite and tells your brain to limit energy intake, and that’s good. But if leptin is prevented from communicating with your brain, then the brain orders more sucrose consumption because it thinks it is starving. Sometimes the disruption is caused by brain tumors that affect the hypothalamus, either because of the tumor itself, the neurosurgery to remove it, or the radiation or chemotherapy used in attempts to kill it. But other times, if insulin levels are chronically high, as happens when we consume extreme amounts of sugar, then we become insulin resistant. The high insulin impairs leptin signaling, leading to “brain starvation”.Although insulin and leptin bind to separate receptors in the brain and have their own separate pathways of action, they share the same signaling molecules. So in insulin resistance, leptin signaling in the brain is diminished. This is why insulin resistance, hyperinsulinemia, and leptin resistance often go hand-in-hand.
Dr. Shulman: So, you’re saying that junk and fast foods make the body always think it’s hungry?
Dr. Lustig: Exactly. But there is more to it. Obese people generally don’t like to exercise. The reason is that when their insulin is high and their leptin isn’t being seen by their brain, the brain thinks it’s starving, so it orders the body to go into conservation mode. Obese people don’t want to release energy, they want to store it. So, caloric expenditure will be low, especially voluntary expenditure.
Dr. Shulman: Back to your comment about sugar in our food. Why not just change what we eat?
Dr. Lustig: Because we don’t always know what we’re eating. The food industry has 56 different names for sugar. How do you reduce your intake of something if you don’t even know you’re eating it? And, of the more than 600,000 items in the American food supply, 80% of them have been spiked with added sugar. Not for any reason other than manufacturers know that if they add it, you will eat it, and your body will then crave it, and so you will eat more.
Dr. Shulman: So are you saying that this is like an addiction?
Dr. Lustig: What I would say is that chronic hyperinsulinemia drives energy into adipose tissue, interfering with the leptin signaling, and giving the brain the idea that the body is starving. This extinguishes the dopamine clearance by insulin in the synapses of the brain and thus inhibits the normal blunting of reward of further food intake after a meal. This cycle promotes both obesity and addiction.