Body weight and body composition can be a big issue

Over the last 3 months I gradually lost weight

DXA scan of a study participant to measure body composition

Is body weight important?
Many athletes are concerned about their weight or are even obsessed with it. There is clear evidence that excess weight will reduce performance especially in weight bearing activities such as running. For swimming body weight is less of an issue and for cycling it only really becomes an issue when there are hills involved. For distance running body weight is undoubtedly an important factor. However, it must be kept in mind that it is not simply weight that matters; it is the power-to-weight ratio! When trying to lose weight athletes often also lose lean body mass and power is reduced meaning that the power to weight ratio does not actually changes or even declines. So the trick is to lose weight slowly without affecting lean body mass.

My body weight

The figure above shows my body weight in the lead up to Hawaii. Starting at just over 72 kg in May, I gradually lost body weight to around 69.5 kg when I am about to board the plane to Hawaii. The decrease is steady and linear. Of course there is quite a bit of daily variation but by plotting the weight over time and collecting the information almost on a daily basis a very clear picture emerges. During my travels I was not able to measure my body mass on the same scales and therefore there are some gaps in the line.

Power to weight ratio
Over the same time my power output (measured in the lab and measured by SRM on the bike) stayed the same or even increased a little, meaning that my power to weight ratio improved quite a bit. This is further backed up by measurement of body composition using DXA. This shows that my lean body mass increased a little and my percentage body fat decreased from 11.8% to a little over 9%. A DXA scan also provides information of the area where fat is deposited and the comparison of the scans showed that body fat was lost in legs, arms as well as trunk. My lean body mass stayed more or less the same but I lost 2.5 kg of pure fat from May to September.

Variation in body weight
The variation in body weight from day to day is interesting. Often my weight is a bit lower after a hard training session. Presumably because my muscle glycogen stores have not fully recovered. Each gram of glycogen is stores with almost 3 grams of water so the difference between depleted and fully restored glycogen stored may represent 2-3kg of weight. The days after a hard training session, especially if there is muscle damage involved (long hard runs), my weight usually increases above normal for a couple fo days. This also happens after an Ironman race, my weight suddenly increases to 2 kg above my glycogen loaded pre-weight. This is mostly fluid retention and oedema as a result of muscle damage. It takes a few days for the weight to come back to normal. Because there are so many factors that influence body weight and because there is considerable variation, it is important to collect information consistently and over longer periods of time. The best time to do this is of course in the morning. Measure nude body mass after your first toilet stop and write the weight down. Use scales that are accurate and measure within 0.2 kg and make sure they are calibrated and reliable.