Deductions & credits

I'm saying that I sold off a bond ETF after making a small capital gain. But, the ENTIRE proceeds of the sale, including original principle ALL counted as 'adjustable gross income'. This greatly reduced my Covered California health care benefits. I would have been better off just keeping this money in cash the whole time. This had me wondering if a maturing CD would be treated differently. Would JUST the interest count as 'adjustable gross income' or would the entire amount of original principal ALSO count as 'adjustable gross income'?