Steve Watts

In 1953 a Harvard nutritionist named Jean Mayer proposed that hunger is not driven by how much glucose is in the blood, but by how much glucose the brain is able to use. He called it the glucostatic theory. The idea was simple. Specialized cells in the hypothalamus act as glucose sensors. When those cells are taking up and using plenty of glucose, a person feels full. When glucose use falls, a person feels hungry and starts looking for food. Mayer's key point was that the absolute level of blood glucose is not the signal. The signal is glucose utilization. He estimated it from the arteriovenous difference, which is the gap between glucose in the arteries going into the tissues and glucose in the veins coming out of them. This explained something that bothered physicians at the time. People with poorly controlled diabetes often have very high blood glucose and are still constantly hungry. Without enough insulin, their cells cannot take the glucose in. The blood is full of fuel, but the tissues are using very little of it. Mayer published the idea in the New England Journal of Medicine in 1953. In 1955 he expanded it, and he placed it next to a second idea, the lipostatic hypothesis. That hypothesis held that the body also tracks how much fat it carries over the long term. That pairing turned out to be important. Over the following decades the glucostatic theory was shown to be incomplete: - Hunger is controlled by many signals, not one. - Gut hormones, stomach stretch, learned meal times, and reward all play a role. - The discovery of leptin in the 1990s confirmed that fat tissue signals the brain about long-term energy stores, which is much closer to Mayer's lipostatic idea. But the glucose part of the story never went away. Modern measurement methods brought it back. Campfield and colleagues, 1996: - 18 healthy adults were isolated from food and time cues, with blood glucose monitored continuously. - In 83% of them, rising hunger and spoken meal requests were preceded by a brief drop in blood glucose. - The average low point was about 10% below baseline, reached around 27 minutes into the drop. - In a small preliminary arm, insulin was infused to mimic these drops, and hunger ratings rose. When glucose stayed stable, hunger did not change. Melanson and colleagues, 1999: - Time-blinded men who drank a high-fat or a sugar drink requested their next meal during the decline in glucose that followed the post-drink peak. - After the fat drink, the next meal request took about twice as long (126 vs 65 minutes). Lennerz and colleagues, 2013: - 12 men ate a high-glycemic or low-glycemic meal matched for calories, macronutrients, and taste. - The high-glycemic meal produced 2.4 times more glucose in the first 2 hours. - 4 hours later, glucose was lower after the high-glycemic meal (4.7 vs 5.3 mmol/L) and hunger was higher. - At that point, brain imaging showed greater activity in the nucleus accumbens, a region tied to reward and craving. Wyatt and colleagues, 2021: - 1,070 people wore continuous glucose monitors across 8,624 standardized meals. - The size of the glucose dip 2 to 3 hours after a meal predicted hunger better than the size of the glucose peak. - Bigger dips were linked to a shorter time until the next meal, more energy eaten 3 to 4 hours later, and more energy eaten over 24 hours. These studies show the same pattern. The rise in glucose is not what drives the next round of hunger. The fall is. That is where insulin comes in. A large glucose spike brings a large insulin response. Insulin moves glucose out of the blood and into tissues, and when the response is large it can push glucose below where it started. That drop is the dip. The insulin infusions in Campfield's work produced the same kind of drop and the same rise in hunger. Mayer had the right instinct. The brain pays close attention to glucose, and the timing and direction of glucose changes after a meal shape when people eat next and how much. He was wrong only in thinking it was the whole story. The hormone that controls those changes is insulin. If the goal is to control hunger after a meal, it's clear that meals which avoid a large glucose spike, and the large insulin response that follows it, are the more reliable way to avoid the dip that drives a person to eat again.