The researchers then pushed further. They repeated the experiments in mice with type 2 diabetes and in mice lacking the receptor needed to respond to GLP-1. They blocked glucagon or GLP-1 signaling with drugs, surgically cut the vagus nerves below the diaphragm on both sides, and tested whether other dietary fats produced similar results.

What did eating fat first actually do?

The timing changed everything. When olive oil arrived before glucose, the initial blood sugar rise was smaller than when the oil came afterward. Fat-first eating also slowed gastric emptying — a term researchers use for how quickly food moves from the stomach into the small intestine — so glucose reached the intestine later.

Eating fat first also stimulated an early surge of several hormones: glucagon, GIP, and GLP-1.

Previous clinical studies had already shown that eating vegetables, protein, or fat before carbohydrates can limit post-meal blood sugar increases in people. Scientists have generally credited two mechanisms: slower gastric emptying and greater release of incretins such as GLP-1. What remained unclear was how gut and pancreatic hormones interact with the nervous system. This study begins to fill that gap.

Why is glucagon the surprise?

Glucagon is famous for the opposite of blood sugar control — it raises glucose by signaling the liver. Yet when the researchers blocked glucagon signaling alone, the benefit of fat-first eating survived. Blocking GLP-1 signaling alone also failed to erase it.

Only when the team blocked both pathways at the same time did the effect diminish. That result suggests glucagon and GLP-1 work as partially redundant teammates in a system that moderates blood sugar after fat precedes carbohydrates. Despite its usual role in raising glucose, glucagon can contribute to a response that ultimately limits the post-meal rise.

The vagus nerve experiments added a third piece. Cutting those nerve pathways suggested that communication between the gut and the brain also helps shape the body's response to nutrient order.

"Our findings suggest that the order in which nutrients reach the gut can engage a coordinated hormone-nerve response that changes how quickly the stomach delivers glucose to the intestine," Yabe says.

Could meal order help manage diabetes?

The findings give a clearer biological explanation for why eating fat before carbohydrates may blunt blood sugar spikes. No single hormone or digestive process carries the effect. Instead, digestive hormones, pancreatic signals, and the nervous system appear to coordinate to control how quickly glucose enters the intestine.

That has practical appeal. Reordering food within a meal is simple and may be easier to sustain than diets that ban entire food groups.

But the caveats are real. All of these experiments were conducted in mice, and this research has not established the same mechanisms in humans. Additional clinical studies will be needed to determine whether the hormone-and-nerve interplay identified here can improve blood sugar regulation in people with or without diabetes.

For now, the takeaway is modest but concrete: in mice, a small dose of olive oil 15 minutes before sugar changed how the body handled that sugar — through a surprisingly cooperative system of hormones and nerves rather than one single switch.