As endurance athletes we know it's a part of our event to carry as little extra weight as possible. The goal is to have maximum strength to weight ratio -- getting as light as possible while still maintaining 100% strength.
A challenging part of triathlon in particular is the balance of three different sports that require different body compositions and then getting putting all three together. Complicating that further is the weather at your race and the distance you’ll be racing.
Swimming: in general requires you to carry a little more weight and in particular a lot of upper body weight purely by muscle development. If you are too lean your swimming is negatively impacted.
Cycling: as a sport puts a lot on maximum strength to weight ratio. If you’re too heavy, you’ll need to lug that up and down hills; too light and you lose too much power to cycle well. Most can get away with a few extra pounds on the bike if it keeps their power higher overall. Since cycling isn’t a weight-bearing sport (the bike carries you), you are better to be a touch heavy and maintain strength in cycling than be too light and weak.
Running: is about being as little and light as possible. As triathletes we are already at a disadvantage with our developed upper bodies from swimming and our developed quads, glutes and calves from cycling. Making ourselves as lean as possible with this extra muscle mass (that a run only person wouldn't have) is important to running well. We will only truly reach our pure run potential when we are as lean as possible.
Weather :It’s best to be a little leaner for hot races and have a little extra on you for cold races.
Training volume and recovery: The volume of training and recovery demands that are required for your distance of racing is important. Athletes who are too lean will start to find it difficult to avoid illness, injury and recover enough. You start to run into trouble from losing strength that you need to endure such long training -- the balance gets trickier because you still need to be light enough to maximize your long day and be as efficient as possible.
Putting it all together :Most of us are type A and get great pride in showing off our uber lean bodies. I've never seen a sport where the participants get nearly as much enjoyment over the vanity as the actual performance. Most are always targeting ultra lean no matter if that is what will be the best for performance -- so you'll need to decide if you are in it for vanity or performance.
There are ways to figure out your best weight to maintain strength for cycling and swimming, still have the recovery capability to maintain an immune system and endure the training required and be lean enough to reach run goals.
I recommend getting a body composition test done so you're not chasing a number on a scale made up in your head. Just because you are heavy doesn't mean you aren't lean -- chasing a number your mate reached is not going to help you.
Once you establish your goal body composition, create a sustainable daily routine. We are creatures of habit so chose a routine and habits you can maintain all year. Make small changes for health and performance gradually over time and stay healthy and happy. It should be a lifestyle, not a sacrifice.
Having a good body composition for your event requires the same discipline and planning as your training. Treat it as part of the entire package to reach your goals.
Stay strong and fast