THERE IS NO ONE SIZE FITS ALL IN ANYTHING IN LIFE LET ALONE WHAT YOU WEAR IN A TRIATHLON! IF THERE IS ONE THING THAT I BEG OF YOU IT’S TO NOT LET ALL THE LYCRA YOU’LL SEE AT A TRIATHLON PUT YOU OFF ENTERING. IF YOU STOP LOOKING AT THE POINTY END OF THE FIELD YOU WILL SEE THAT YOU CAN WEAR ANYTHING ON THE DAY AS LONG AS YOU ARE COMFORTABLE IN IT.
To answer the question though first of all let’s start with the easy answer. In a triathlon the easiest thing to wear is a tri suit as this allows you to swim, ride and run in the one outfit. A tri suit is simply a lycra suit and it needs to be close fitting so that you don’t increase your resistance in the swim and bike (ie. making you swim and ride slower). Tri suits come in either a one or two piece and it is personal preference as to what you prefer.
If you are not comfortable with wearing a tri suit or unsure yet if you want to commit to the expense then use transitions to make changes to what you wear. I recommend swimming in just your togs for the first leg then putting top and shorts over top in the first transition. Remember you will be wet so trying to dress in transition can be a slow process so if you are concerned about time the first option is definitely best.
How blessed you are in this department will determine what bra you wear. If you are small enough you can get away with not wearing anything under your tri suit; some suits even have built in support. Otherwise you can add in a crop top or full underwire sports bra underneath. Again this is what makes you comfortable once you get onto the run leg and everyone is different. But I recommend wearing bras that you would normally run in, what does it matter if you have a bra strap showing?
As for undies, tri suits are just like cycling knicks and recommended that you don’t have any undies on underneath. You are definitely going to get wet so we want to reduce layers which will cause friction and chafing as much as possible.
SOCKS OR NO SOCKS?
How serious are you? Is your goal to get to the finish line? Then wear socks! Don’t risk getting blisters and being miserable for the run just to save 10 seconds. If however, you are hunting the podium or want to put everything you have into a PB then I recommend not using socks for all runs 10km and under. Build up to this distance though as you can virtually guarantee that your nice soft feet that aren’t used to it will break out in blisters.