I did try the protein shakes for a while but they didn't seem to do anything for me. If I'm going big on a session (say 50k on the bike) then I do use the SIS gels and rehydration tablets in my drink. The gels taste horrible though so they really are a bit of a last resort and then only as a boost, if I'm going along fine then I won't bother at all. A handful of jellybabies is quite often all I need.
Because of the timetable at my local pool I can only swim at 7:30am and I need to go in to the water with an empty stomach or I start to get stomach cramps after about 800 metres - by the time I've done my session I'm tight against the clock to get to work on time. If I'm lucky I will grab porridge on the way in and I always have a bag of trail mix on the go but I won't eat properly until lunch time so that shake does sound like a good plan. Is it an online product or are the city centre stockists?
Online is always cheaper mate, try bulk powders. Waxy maize is probably best for you as the sugar will be negligible. Protein shakes really won't make too much of a difference to an endurance cardio enthusiast energy or mass wise, you're better off having carbs for sure.
Off to the gym now as it happens!
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