What is the ingredient found in diet sodas that causes metabolic syndrome?

Sweeteners. Artificial sweetners (stevia included), trick several neuro hormones by tasting very sweet. One of these is leptin, and there are several others. Some antaogize Insulin sensitivity, and others simply cause overproduction. Studies have shown a link, but they are not definite because overweight people are prone to metabolic syndrome. That having been said, i think everything is made too sweet!
I love your question. I had my eye on this one for 20 years since we found out that diet soda causes weight gain. Now we know that diet soda causes weight gain by increasing Insulin levels, causing metabolic syndrome. We do not know why but it is independent of the sweetener. Other sweets, unless you gain weight, do not cause diabetes, soda does. One soda a week causes it and it is dose dependent, more is worse.