Does a "two-fold decrease" result in a 'quantity - 200%' or a 'quantity/2'? [closed]

If I say that something had a "two-fold decrease", does it mean it was halved or that it was reduced by 200% (which can bring the value to a negative)?

In the scientific papers mentioned in comment, the researchers have already established a normal level or a level of x in healthy subjects. Let us say it is ten per cent of something or other.

A fivefold increase would then be an increase up to fifty per cent of x.

A 'fivefold decrease' is their way of saying a fifth. They mean a reduction from, say 10% to 2%.

Patients with anorexia nervosa had a fivefold decrease in subcutaneous fat and only a twofold decrease in intraabdominal fat compared with the values for the volunteers. Patients with Cushing syndrome had less than a twofold increase in subcutaneous fat and greater than a fivefold increase in intraabdominal fat compared with values for the healthy subjects.

Body Fat Distribution

It is not, really, what a fivefold decrease means.

They are extrapolating back - it seems to me. If something drops from 10% to 2%, it would need a fivefold increase to get it back to where it was. So they are calling it a fivefold decrease, initially.