The study used shrinking of the gel medium as a proxy for pig heart valve cell growth (the "valvular interstitial cell"), and used varying amounts of glucose in the medium. (And other things, see the abstract. The more the gel shrunk, the livelier the valve cells must have been growing.
The abstract suggests they only used 3 concentrations: 1g/L, 2g/L, and 4.5g/L. The 2g/L was the Goldilocks concentration; the others showed less growth.
And let's see: 1g/L is 100mg/dL--which is considered normal for humans, and 2g/L is 200mg/L which is bad. That made me go google for pig diabetes, and the first thing I found asked Why Don't Pigs Get Diabetes? ("we bred them not" to is the proposed answer). Another site discusses how to manage pig diabetes. "And even in a normal laboratory pig, blood sugar can vary from under 40 to over 100; their control is not as tight as healthy human beings have naturally." Sounds like pigs are OK for a comparison, so what's the difference?
Maybe the levels seen in vivo differ from those in the culture medium; the cells are organized in tissues in real life. In any event, this might be a mechanism for inducing the cascading problems that harass us.