The body breaks carbohydrates from food into glucose, the primary fuel of muscles and the mind. When muscles work hard, they produce lactate as a byproduct. Though recent studies have shown that muscles can re-use the lactate as fuel, conventional wisdom holds that it can also build up in the form of lactic acid, reducing performance and causing pain.
The new research shows that the brain can switch to lactate during strenuous exercise, using the muscles' byproduct as backup fuel.
Note: this has nothing to do with lactose metabolism. There’s actually a substantial literature on the role of lactate in brain metabolism – it seems to be very important for times when the brain must recover from hypoxic conditions, so there has been a lot of research. So there probably is a deeper story to be written here – the “backup energy source” hypothesis is a little simplistic.