N=1, but: I read that carbs lower HDL, and I was eating a very carb-heavy diet.

So over about two months I switched to an informal low-carb diet. e.g. I stopped drinking milk, I ate Carl's Jr. six-dollar burgers as lettuce wraps for lunch, and sometimes just roast beef and mayo.

Over three months I lost something like 30 pounds, hitting a body fat percentage around 12% -- this wasn't the goal, but FWIW.

My HDL before was 17, after those three months, 25.

Then I added in various forms of exercise and got it up to about 55.

I've since engaged in various diets, and levels of exercise -- although I've never gone back to the original diet where whole meals consisted of a quart of ovaltine. My HDL has never dropped below 40, nor been higher than 65.