New triathlete = major weight GAIN???

With any abrupt change, my first suggestion would have to be to consult your physician and have some blood work done on your thyroid/parathyroid.

Barring any medical issue, this seems to be a pretty common phenomenon.  I think that many of us tend to overestimate the amount of calories that we actually burn during exercise and underestimate the amount that we eat.  Many folks still go by the 100 cals per mile approximation for running-- and I used to do this as well.  It was a real eye opener when I finally got a heart rate monitor that included a program to calculate calorie consumption based on heart rate (and not some arbitrary height/weight formula).  I do not burn anywhere near 100 cals per mile-- it's more like 80.

As endurance athletes, our bodies become very efficient at what we do.   This is a good thing!  In addition, we can't train long and not eat during a workout, so this means we need to eat as cleanly as possible when we aren't working out.

I understand what you are going through.  I gained 5 pounds while training for my first 1/2 ironman last year.  I pretty much went from 127 pounds (5'5") when I was just doing sprints distances, to 132 pounds at the 1/2 iron distance.  This past December I decided that I was going to completely revamp how I was eating and no longer think of food as a "reward" for a 4 or 5 hour training day.  It came off slowly, but within about 4 months, I lost over 10 pounds.  I'm now a steady 120-121.

Also-- it's not about age.  A registered dietician once told me that the difference in calorie requirements between myself as a 33-year-old (very soon to be 34-year-old) and a 23-yer-old is only about 100 cals per day (or a very small apple).

Your story is actually very similar to mine, minus the broken foot.  The weight that I maintain when I am inactive can be anywhere from 10 - 20 lbs heavier than my active weight.  My running-only weight is 5 - 10 lbs lighter than my tri-ing weight.  I was doing tris by the time I got pregnant with DS1. I am 5' 1.5" so small gains are very noticeable.  I was significantly overweight after my first son was born.  I couldn't lose it even though I was tri-ing again.  I switched back to mostly running (5 x per week, 1 hr per run for 4 runs and one longer run - on average) and got back to my tri-ing range.  I then got mono.  I lost a huge amount of weight in a 2 month period.  By the time I got pregnant again, I was still under my first pre-pregnancy weight.  After my second son was born, I breast fed for almost a year.  During this time I could eat what I wanted and didn't gain weight.  Then my son's ped thought he was allergic to dairy so I cut out all dairy.  I couldn't maintain weight.  Once I stopped breast-feeding, the weight went back on and I am currently on the upper-limit of what I consider to be my tri-ing weight which is the lower-limit of my sedentary weight.  I know I eat too much - but that has always been my problem.  I started training for a marathon while I was still breastfeeding and did the marathon a few months after finishing breastfeeding.  I was heavier on race day than when I started my training.

I did not lose weight when I trained for my 1/2-iron back before I had kids.  Once I added the biking and swimming to my weekly routine, my weight went up.  For me, running burns a lot more calories than cycling and swimming.  At 32, your age shouldn't be playing in to this IMHO.

Really can't tell w/o seeing your diet.  I'm always a proponent of getting your bf% professionally tested, figuring out your maitenance cals, and then tracking your diet for a while.  The culprit is almost always diet.

the last bit of this^

some rough numbers to work with (again ROUGH NUMBERS)

swim is approx 250 cal per 1000m
bike for most people is approx 500 an hour at a steady pace, a bit more for bigger people/less for those that are fairly light (this is from a 145-165 pound person range).
run is almost exactly 100 per mile for someone in that size range, a tiny bit over for those bigger/tiny bit under for those smaller.

be very very warry of HR monitors that have cal burned on them. a power meter is one of the only good ways to actually see cal burned during a workout as it's showing actual energy output vs making guesses based on HR.

that said, it is almost always a diet thing.

think about it like this. say you have whatever cal balance you need in non-trianing life to stay at one weight, not going up, nor down.

you add in 35 miles a week of running. to lose approx +/- 1 pound a week, you need to not ad ANY food (or just a tiny bit after workouts).

at 100 cal per mile, 35 miles is 3500 cal. to burn off a pound of weight its going to require you to brun off 3500 extra cal in a week.

how many people to you know add that sort of run volume without eating more/a LOT more?

i'm currently at just under 150 pounds, pretty low BF (so not much room to screw up nutrition), training approx 23-25 hours a week right now, and eating right around 2200-2400 cal most days. (i'm 24/male). this has me not gaining or losing weight right now.

i would put a lot of money on the fact that most people eat WAY more than than when working out, and wonder why they are not losing weight.

WOW!!!!  That sounds insanely low for the amount you are working out!  Are you sure you are not depriving yourself?

OP- I agree with others.  It's probably a case of eating more than you think you are.  Maybe try keeping a daily food log for a few days to get an idea of how many calories you are really consuming.

not losing weight/feeling too tired, so i'd say no.

on days with longer sessions (long rides over 3 hours) i'll eat approx 200 cal an hour. over 4 hours i'll take in 300 an hour during the workouts. and a bit more after, but thats pretty normal day for me

Some good insight on here. A lot of women do gain when when they start endurance sport - studies show that women are more likely to overcompensate for training than men, unforunately. For most people, training does not burn nearly as much as we tend to think. Also be very aware of the calories you take in, both in everyday life and in training. No one needs gatorade, gels and a cliff bar to go on a 90 min ride. Also, small things can add up - a starbucks latte and bluberry muffin to treat yourself after a tough workout is as many/more calories as your dinner should be. Livestrong.com has a great food log that lets you input the calories of everyday food, restaurant meals etc.

more to diet, per the bmi (so far the only thing i'm relying on with regard to weight loss), i'm 5 lbs away from being overweight (5.2" 24 yo woman here.. my meds are a contributor to my weight gain, which is another long story). a friend of mine, a gym buff and an instructor himself, taught me how to play with calories. he then computed that if i want to maintain my weight, i should stick with 3513 calories per day. before i took my meds, my fittest weight was at 102lbs therefore if i need to lose weight i should stick with 2937 calories per day with the assumption that i continuously run, swim, cycle and hike. at first it was difficult. i got hungry a lot because i was just cutting the daily servings on my regular diet; thus, resulting to more cravings and buying quick snacks whenever i see a 7-11, so i told my friend about my "suffering". he said that i should not cut the servings but instead look for substitutes, which i can eat more but with less calories and high protein. he further said that if i starve myself, my body would just store in more fats. i followed his advise of diet overhaul and log what i eat. coincidentally i bumped into this website so it helped a lot i am eating quite a lot lately but i lost a pound, which is nice.

Biggest thing I discovered with tri is this: swimming and biking are intensity driven. Running is not. 

If you run 10K you burn 600-700 cals no matter how you do it (even walking). You can't cheat unless someone is pulling you in a little red wagon.

If you ride 20 miles in an hour at a decent power you will burn calories (for me it's about 20cal/km). If you just noodle about at 10 mph you will burn almost nothing. But you'll still eat like you did. Same with swimming - you can slack and think you did a "workout" but in reality you maybe did an extra 100 or 200 cals over just sitting on the couch.

The other thing people do is go for a nice fatty coffee and monster muffin after a weekend long run and eat more than they burnt. They still get the fitness of the run, but not the cal deficit.