Using savings is much desirable than using loan money in investing in bitcoin. This idea is becoming alive again because we're being near to the halving.
Alternatively, one can utilize funds that are freed from the necessities of daily living. If your savings are designated for everyday expenses, it is also not recommended.
That's also a possible thing, you can use spare funds instead of using borrowed money into investing. But as much as possible, you have to avoid getting a loan if the main purpose of it will be for buying bitcoin. If you can't keep up with daily expenses and your salary is just enough to your needs but you insist and you want to do something then you should increase the flow of your source of income.
However, it is even worse to use borrowed money for speculation. Taking double risks is not a wise course of action.
Interest and speculation, really not a good factors to be combined if you don't have that much money to play with a volatile market and unwilling to take on the interest payments.