A new medication that those of us who have type 2 diabetes take only once a week can bring our A1C levels way down. It can also help us to lose weight, something that almost everyone who has type 2 diabetes needs to do.

But predicting who will lose weight isn’t easy. Only one thing stood out in the clinical trials.

The new medication is Bydureon, which we have been able to get in our pharmacies for only the past four months. But Amylin Pharmaceuticals, which developed Bydureon, presented a study at the American Diabetes Association’s annual convention earlier this month that shows both glycemic and weight loss control data for people taking Bydureon for the past four years. Those were the people taking the drug in the clinical trials leading up to the U.S. Food and Drug Administration’s approval at the beginning of this year, as I wrote here in “Bydureon Approved Today.”

Bydureon is the latest in a new class of drugs called GLP-1 receptor agonists. Its generic name is exenatide, and it has the same active ingredient as Byetta, which we have been able to get since June 2005 and requires twice-daily shots.

I used Byetta from February 2006 to December 2007, reducing my A1C level from 6.2 to 5.3 and my weight from 312 pounds to 168. That kick-started my diabetes control, making me so happy that I bought 100 shares of Amylin stock. I also wrote a book about Byetta called Losing Weight with Your Diabetes Medication: How Byetta and Other Drugs Can Help You Lose More Weight than You Ever Thought Possible.

The study that Amylin researchers presented at the American Diabetes Association convention, “Exenatide Once Weekly Resulted in Sustained Improvement in Glycemic Control With Weight Loss through 4 Years,” showed that the 176 people who used Bydureon for four years typically had a clinically significant improvement in their A1C of  1.7 percentage points. That means, for example, going from an A1C level of 7.7 to 6.0.

They also lost an average of 5.5 pounds, the study says. While only drugs of this class lead to even that much weight loss, 5.5 pounds isn’t an impressive figure. It’s also close to the average weight loss that people taking Byetta had in its clinical trials.

So how is it that people can lose a lot more weight than that on Byetta or Bydureon? To get the answer to that question I interviewed David Maggs, M.D., Amylin’s Vice President of Medical Development.

One-fourth of the people in the clinicial trials lost “a very significant amount of weight,” he told me. One-half of them lost a significant amount. And one-fourth didn’t lose any weight at all.

I asked Dr. Maggs if they were able to figure out what was different about the people who lost weight and those who didn’t.

The only factor that stood out, he replied, was that people who were already taking metformin did better at losing weight than those who were taking any other drug.

I wondered if the metformin advantage was because people who were taking metformin were different or because metformin itself is different. Then, I remembered that metformin is indeed different.