Monitoring Weight & Body Composition

Most people who embark upon a weight-loss regime have a goal weight they wish to achieve and will use a set of standard weight scales to measure and track their progress. In this article, we’ll show you the importance of using Body Composition as a better measure of your progress, especially if you aim to increase your lean muscle mass.

Body Composition

When you stand on a scale, the weight of your total body mass is the only information you can obtain. Many people are concerned when the number they see doesn’t conform to the “healthy” weight range figures we typically see in height/weight tables. However, your total body mass includes the weight of fat, muscle, water, and bone. Whilst we cannot change our genetics, we can change the composition of some aspects that contribute to our overall body mass. In particular, to achieve a leaner look we can increase the proportion of lean muscle mass in our total body mass by reducing our body fat. For some people this physical improvement to overall appearance may not coincide with an actual loss of total body mass on a weight scale, however the results in both sporting performance, cardiovascular fitness, appearance and perhaps even strength may be well worth the effort.

Body Mass Index (BMI)

Many people are aware that BMI is regarded as a good benchmark to understand how far you are either side of the "healthy weight range". However, BMI usually overestimates fat in heavily muscled body types and for some ethnic groups the ranges might need to be modified. This is because the standard BMI calculator only takes into account your total body mass and height. For active people, this is therefore not the ideal guide to understanding your body composition, and in fact, a more detailed knowledge of the composition of your total body mass could improve your triathlon performance. So, which aspect is most important?


Take this link to our BMI Calculator (located in the tools section).

Do muscles really weigh more than fat?

Your total body water will typically fall in the range from 45%-75% of total body weight. The range is due to differences across people’s body composition. Muscle tissue is about 70%-80% percent water, while fat tissue is about 10% water. This is why people often say “muscle weighs more than fat” – the science behind this is that muscle retains more water than fat – and that’s a very good thing for triathletes and endurance athletes.

This simple fact also explains how two people of the same height and general appearance can vary in weight, even by as much as 10kg! The person with a higher lean muscle mass will simply weigh more than another person of the same height with a lower proportionate muscle mass and is why BMI values are not the ideal comparison tool.

Another interesting fact is that trained muscles will hold more water than non-trained muscles. Through training adaptations, the muscles can retain the necessary water to maintain proper hydration and electrolyte levels. What’s more, a hard workout will increase the metabolism, often resulting in an increase (although small) in total body water. This simple fact can often surprise people who think that after a hard workout they should see a weight loss.

One final, but important word about water in muscles is that weight loss can occur rapidly through water loss, which is the basis for some of the most popular fad diets such as the Atkins Diet, Ketogenic Diet, the Zone Diet, the Scarsdale Diet. The quick water-weight loss on these diets is mistaken for fat loss because the muscles dehydrate giving a more defined shaped, but the water content quickly returns and so does the weight.

How to lose weight by losing body fat

Firstly, to look leaner, you need to change your body composition. As we’ve just outlined above, losing body water is not the solution for safe and long-term weight loss. Your bone weight is generally constant in healthy adults so the only remaining factors are body fat and muscle mass - both of these can be modified. Many diets are bad as you’ll tend to lose muscle mass and is the reason some people look more flabby after losing weight than they did whilst weighing more. The ideal weight loss scenario is to INCREASE your lean muscle mass, through a LOSS of body fat. This in itself may not result in weight loss, but the enormous advantages of an increase in lean mass will improve many body functions, including metabolism, that when combined with a sensible exercise routine will result in long term healthy weight loss coupled with improved muscle tone.

To achieve this you'll need to understand how to avoid muscle loss through correct nutrition both pre and post workout and improve your recovery techniques, whilst ensuring you maintain a calorie deficit of between 300 - 500 calories per day. However, for active people like triathletes who easily achieve the calorie deficit through training, there are many that are baffled why they still do not lose weight. The answer lies in monitoring your total body composition, aiming to increase the ratio of muscle mass whilst losing weight to ensure you are losing fat - not water or muscle. We recommend you take some time to do further reading. See our article Weight Management and Fat Loss. We also recommend the book Weight Management for Triathletes, available in our online shop. Full of practical tips and strategies to help you solve the weight-loss/training riddle that afflicts so many triathletes.

Finding your body fat % with bioimpedence

Body fat is often expressed as a percentage and indicates the proportion of total body mass that comes from the weight of your body fat.

A bioimpedence monitor is now a common (triathlete) household item. Available from sports stores for around $200 - $400 you can get a complete body composition profile simply by standing on it with bare feet. I personally use the Tanita Ironman Body Composition monitor, which I purchased from a Rebel Sports Store. Initially, you setup your personal information (age, gender, height, activity level). Most models can store profiles for 4 people, plus allow for a guest. The monitor will display weight, body water %, body fat %, muscle mass, bone mass, Basal Metabolic Rate (BMR), Visceral Fat Rating, Metabolic age, and a physique rating.

Basal Metabolic Rate tells you the absolute minimum number of kilojoules you need to consume to maintain YOUR basic daily body functions “before factoring in exercise”. You can then calculate how much exercise you are doing, and add this to your BMR to calculate how much food you should be consuming to meet whatever goals you have. I find when I have been inactive, this number drops. When my metabolism is up due to lots of training, this number also increases slightly.

The two features I really like are the Body Fat Range icons. This indicates where you stack up for someone of your age/gender. An icon to indicate if you fall into the range for underfat, healthy, overfat, obese is shown on the display together with your bodyfat %. One thing I’ve noticed is that this number is usually at its HIGHEST % first thing in the morning. The instruction manual suggest this is most likely due to a bit of dehydration from sleeping which means the scale is not able to correctly analyse your mass. The manual suggests taking your measurements during the mid afternoon – ideally 2 hours after eating lunch, and before eating your evening meal. I’m happy with that, because sure enough, that’s when the numbers are most kind!!

For me – Female age 40-59 my body fat % fluctuates between 18% - 25% on the scale (above 23% most mornings but always under 20% in the afternoons). The healthy range is 23%-34% and underfat range is 0 – 23% .

The second feature I also like is the Physique Ratings. There are 9 ratings (numbered 1 – 9) that correspond to a physique range (see instruction manual for explanation) or here I found the scale online on Tanita’s website - Tanita Website

This is a more meaningful rating scale than the standard BMI as this factors in what your mass is made of – muscle, bone and fat. Interestingly, on this scale I am either a 6 (when my body fat % is in the healthy range) or a 9 (at some times of day my body fat % is in the underfat range).

For comparison, using the standard BMI calculator, I have a BMI of 23. So, when I compare this to the results on the Tanita Body Composition scale I can understand that being muscular contributes to that number.

Overall, after using the impedance body composition scale for a few years, particularly to help with monitoring my weight loss, I can generally estimate what the scale is going to say. I think that’s probably the most important thing – developing your instinctive awareness of your body. It also helps when I need to build hydration levels pre-racing – it’s right there on the monitor telling me to drink more!

A reality check is always good – sometimes I think I look/feel fat when in fact I am in the healthy range. Interestingly enough, according to researchers, most women tend to believe they look their best at BMI values between 20 to 22, but if your BMI is between 23 and 25, you are not considered overweight by most people. Over 26 and you’ve got to take action – but you knew that already!


Find your ideal body fat % in Weight Management for Triathletes and discover how to achieve your goals.

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Created: February 2012
Revised: March 2024
Latest Feedback: September 2012