When you eat carbohydrates, digestion begins in the mouth. Your salivary glands secrete a hormone called salivary amylase that starts breaking down starches and other large carbohydrates into smaller carbohydrates and glucose.
Recent research has shown that different people contain different levels of this salivary amylase, and the levels are both genetic and environmental. People who are descended from populations that traditionally ate a lot of carbohydrates have a lot more salivary amylase; up to half the protein in their saliva can be amylase, whereas other people have hardly any at all.
Environmental influences include the amount of carbohydrate in your diet now, and stress.
Recently two scientists from Rutgers University in New Jersey and the Monell Chemical Senses Center in Philadelphia wondered if people with high levels of amylase (which they termed HA) would digest starches faster and hence have higher blood glucose (BG) levels than people with low levels of amylase (which they called LA).
The two researchers, Abigail L. Mandel and Paul A. S. Breslin, predicted that the answer would be yes. I would have predicted the same thing. Usually rapidly digested carbohydrates have higher glycemic indexes (GIs) than more slowly digested carbohydrates, and this is the rationale for suggesting that people with diabetes eat low-GI diets.
To their surprise, when they tested the hypothesis, they found that the opposite was true. People in the HA group had lower BG increases after eating starch than people in the LA group.
This reminds me of the old "truth" that large molecules of starch were digested more slowly than small "simple sugars" like sucrose. For years, no one thought to test the idea. They just kept telling people to eat starch instead of sugar. Then some scientists actually tested the response to various carbohydrates and discovered the opposite: starch has a higher GI than sucrose.
Why, you might wonder, does rapid digestion of starch in the mouth result in lower BG levels? The HA group showed slightly higher insulin levels in the early period after eating the starch. This early insulin secretion would result in a "head start" on bringing BG levels down.
We know that in healthy people, just thinking about food or smelling food starts the process of digestion. Our mouth "waters," meaning we secrete saliva, and our stomach rumbles and other enzymes and hormones start gearing up for the anticipated food. This is called the cephalic phase of digestion.
Then, in healthy people, when the food is actually eaten, we produce a fast spurt of insulin called the first-phase insulin response. It is this first-phase response that is lacking in people with type 2 diabetes, and that's one reason our BG levels soar when we eat carbohydrate foods.
But type 2 diabetes is an example of what happens when you don't get a rapid initial secretion of insulin. This study was done in healthy lean volunteers with no known illnesses. The results from the people with low amylase were not as severe as those in people with type 2 diabetes who lack a first-phase response, but they were in the same direction.
It's possible that people with genetically low amylase levels who were eating a high-carbohydrate diet would slowly kill off enough beta cells by glucotoxicity that they'd develop type 2 diabetes.
We all know that different people with type 2 diabetes can tolerate different levels of carbohydrates, sometimes different types of carbohydrates, and it would be interesting to compare the amylase levels of such different groups.
What this research does show is that there's one explanation for the fact that some people can eat very high carbohydrate diets, remain thin, and never develop diabetes when others eat the same diet and become obese and diabetic.
Of course there are other factors too, both genetic and environmental, that contribute to type 2 diabetes, but every small piece of the puzzle helps.