Starch and Genetics
When we eat starchy foods, the first step in their digestion occurs in the mouth. The enzmye that catalyzes the digestion of the starch is called salivary amylase, and it’s produced by a gene called AMY1. There’s another enzyme produced by the pancreas called pancreatic amylase, and it does most of the work of breaking down starch in the intestine, but the job starts in the mouth.
Lately, researchers have found out some interesting things about the AMY1 gene. We can have more than one copy of this gene, from 1 to 15 copies, and they’ve found that people who are descended from populations that eat a lot of starch have lots of copies, and people who are descended from populations that ate less starch have fewer.
The amount of the AMY1 gene you have controls how much amylase is in your saliva, although environmental factors such as stress can also affect the saliva levels. In some people, up to 50% of their salivary protein can be amylase, and in others the levels are hardly detectable.
Recent reports say that people who have lots of salivary amylase (multiple copies of AMY1) are less apt to be obese.
Why would that be?
Logically, someone with lots of salivary amylase (which from now on I’ll just call amylase, as the pancreatic enzyme doesn’t come into this discussion) would break down some of the starch in the mouth into glucose, as starch consists of lots of glucose molecules linked together. So having a lot of amylase would effectively increase the glycemic index of the starch you ate, because eating starch would be like eating glucose, which has a high GI.
But wait! We’ve been told that high GI foods are unhealthy because they raise the blood glucose (BG) levels faster, and many people lose weight when they go on low-GI diets. So why would having more amylase reduce your risk of obesity?
A study published in 2010 showed that people with higher AMY1 copy numbers, and hence higher amylase levels, sensed that starch solutions were getting less viscous (thinner) in the mouth when they ate them. The researchers noted that amylase not only reduces the “creaminess” of starchy foods but also hastens the release of flavor from the food.
So would people with less amylase (the ones more apt to be overweight) eat less starchy food because it wasn’t as flavorful? Or would they eat more, trying to make up with quantity what the food seemed to be lacking in quality?
I favor the latter possibility. I think one problem with cheap American food is that it’s not very flavorful (just salty, sugary, and fatty), so people don’t feel satisfied and keep eating until they’re stuffed. But is this the only reason low AMY1 copy numbers are associated with obesity? I suspect not.
Another study done in 2012 showed that people with high amylase levels had better BG control. Insulin levels didn’t vary significantly between high-amylase and low-amylase subjects when they looked at the levels for 2 hours. So why did the high-amylase subjects have better BG control (after eating starch, they peaked at 136 mg/dL instead of 172 in the low-amylase group)?
When researchers looked closely, they discovered that the high-amylase group had significantly higher insulin levels for the first 9 minutes after eating, and apparently this early surge of insulin was enough to keep the BGs lower at later times.
In healthy people, insulin is released in two phases. The first phase is very short, and the second phase lasts for a long time. People with type 2 diabetes lack the first-phase insulin release. When I was in a clinical study at the Joslin Diabetes Center in Boston, the study showed that I had zero first-phase insulin response.
Both the high- and the low-amylase groups were able to produce this fast first-phase response when they were given glucose. In other words, the people with low amylase levels were not diabetic. They could produce insulin when they ate glucose; they just couldn’t do it as well when they ate starch.
The authors couldn’t say for sure what caused the fast insulin release. They suggested that perhaps sweet taste receptors in the mouth trigger the release of hormones or incretins that help to stimulate insulin release. Those with a lot of amylase might release just enough glucose to trigger those hormones, although not enough for the participants to taste the sweetness without chewing for a long time.
But they noted that that this research suggests that the glycemic index may differ from person to person depending on their AMY1 copy numbers, even in young healthy people like the ones in this study.
Most of us already know about YMMV (your mileage may vary). What works for me might not work for you, and these studies suggest that our genes may play a part in this phenomenon.
The AMY1 copy number variation may explain why some groups stay lean and healthy on high-starch diets and others don’t. Could it be that one cause of the obesity epidemic is that people who don’t have high AMY1 copy numbers have been advised to eat high-carbohydrate diets when in fact their metabolism is much better geared toward lower-carbohydrate, higher-fat diets?
If you think that might be relevant to your family, it’s certainly worth a try of a low-carb diet. Maybe it’s just what you need to get healthy again.