I've now done a couple of rides using Nuun calorie-free isotonic tabs. Tentative conclusions - it makes it easier to take on liquid, and I don't need to take in as many calories as I think. Sunday I managed 68 miles (not flat) at a respectable speed on one sandwich, a handful of toasted corn, half a packet of peanuts, one muesli bar, a half-pint of OJ and soda and 1.5 litres of isotonic drink. I probably ate less than I usually do in a working day, and I felt better than I did after my previous long ride.