*** many thanks to Finn “Dataman” Zwager for this blog post ***

Dataman gets the skinny on food and weight measuring

If you are attempting to reach your own ideal racing weight for your summer triathlon, or are interested in creating some ‘weight gain buffer’ before the holiday kilos stack on, keep reading.

Most of us triathletes get a kick out of, and normally aim, to achieve quite specific goals. When my Swiss Half IM race got cancelled due to a mud slide blocking the road just two kilometers into the race, I couldn’t reach my specific goal and was gutted. Of course there is always a next race, I told myself, but as mine was too far away I needed another shorter term fix. I set myself a one month weight loss goal that tied into an existing longer term plan of getting lean and lowering my fat percentage through careful, scientifically-based measurement. In between races, this weight management plan satisfied my hunger for the next goal. This blog is about how I went about it.

Why be lean?

While it may be completely obvious, to help motivation it’s helpful to review the influence that being lean may have on your (long distance) triathlon.

For swimming there may be not much advantage in being lean. Fat floats and provided you have the strength, fitness, technique and flexibility, you can swim really well even with a relatively high fat percentage. Top swimmers have fat percentages of 10-12 percent for men and 19-21 percent for woman. A bit of fat is even an advantage in cold water.

Tip: Even though it burns relatively few calories - partly because you are not supporting your own weight against gravity - you are hungriest after swimming. This is your body’s natural reaction to cold water – it’s looking for insulation in the form of food - even if the water is hardly cold in Dubai!  If you are aiming to lose weight/fat it may help to go for a jog straight after a swim before you have something to eat to keep these hunger pangs at bay. The increased body heat from the run is thought to lessen your craving for food. Combining running with swimming as we do in Dubai sea swims may also do the trick.

When it comes to cycling, weight may be a bit more complicated. Taking away aerodynamic drag which is determined by the size of your body, the shape of your bike and your frontal position on the bike, your bike speed will mainly depend on how much power you can put into your pedals. That in turn is largely determined by your strength and is expressed in Watts. The more Watts you produce the faster you go. A large Über-biker will go really fast, for a while at least, until he meets a hill. Going uphill the larger biker will discover the limits of power-to-weight ratio, expressed in cycling at Watt per Kilogram. Climbers by contrast are tiny and skinny and produce by far the most Watts per kilogram of body weight. Depending on the cyclists’ specialization, fat percentage of the pros may vary between 6-11 percent for men and 12-16 percent for women. I am not sure if that is measured with or without doping. Oh no, I forgot, the sport is clean now…

Have you ever seen fast runners who are fat? Neither have I. Every ounce you don’t have to cart around for 5, 10, 21 or 42 kilometers helps. You have to lift every extra gram against gravity with every step you take. Each extra kilo costs more effort and creates increased loads on your muscles and joints. That’s why top runners have fat percentages of around 7 for men and 12 for women.

Where does that leave us triathletes? You guessed it, somewhere in the middle. Professional male triathletes will have a fat percentage of between 6 to 10 percent and women 12 to 16. What works for you may need some experimenting. While overall the leaner the better, being too skinny may make you a bit more vulnerable to sickness, and often makes you pretty grumpy too!

Calculate your fat percentage

There are various ways to calculate your fat percentage. A very accurate test can be done by submersing your body in water and, along the lines of Archimedes, using some formulas to figure out your exact body composition. This is normally done in a testing facility. Check out www.getdunked.com for example.

For home or gym popular methods are scales and hand held devices that send a weak electric current through your body. Because water and muscle have a different electrical resistance to fat, the machines can estimate your body fat percentage.

Another method is to measure waist, hip and neck circumference and combine this with your height in a formula. There are many sources on line to do this, such as www.calculator.net/body-fat-calculator.html

Yet another method is to measure the size of skin folds, which would normally be done by a doctor or dietician.

Because all these methods work with averages and rely on humans and machines they do have margins of error. As usual, it doesn’t matter too much what method you use, as long as you use one consistently throughout your program, possible calibrating it from time to time by using one of the other methods. For instance, my electronic scales tell me my fat percentage is around 12 percent, while the formula method and my own estimate put it at 9 percent. I now use my scales for daily measurements; keeping in mind a likely over estimate of around 3 percent.

Determine your daily calorie use

Now that you have worked out your current fat percentage and decided on a percentage goal to aim for, you need to figure out how many calories you use or burn per day.


You first need to know the amount of calories you burn in rest, called your Basal Metabolic Rate (BMR); in other words the number of calories you'd burn if you stayed in bed all day.

The most accurate method available in the UAE to determine this is a breath analysis using aeroscan from Aerofit. Breath analysis is based on measuring the differences between inhaled and exhaled carbon dioxide and oxygen concentrations during rest. This will give you your metabolic rate at rest (RMR) from which your BMR can accurately be determined.

[During the complete aeroscan this is then combined with heart rate and running speed (or bike power in Watts) providing you with an accurate calorie burn for each level of exercise intensity.]

Other methods rely on formulas based on gender, height, weight and sometimes fat percentage. For instance:

Women: BMR = 655 + ( 9.6 x weight in kilos ) + ( 1.8 x height in cm ) - ( 4.7 x age in years )

Men: BMR = 66 + ( 13.7 x weight in kilos ) + ( 5 x height in cm ) - ( 6.8 x age in years )

As you can see from the formula above, when you get older your BMR goes down. Yes, every year, it becomes harder to eat whatever you want and stay lean.

The act of dieting itself may also decreases your BMR. That’s why it is always so easy to lose the first few percentages of fat, but increasingly difficult to lose the last ones.

BMR correction formula

Now pay careful attention for the next step. To determine your total calorie use per day you can EITHER:

A. Correct your BMR using the Harris Benedict (or similar) formula below;


B. Measure the calories used during all your activities and add those to your BMR.

The last method involves more work, but should be more accurate.

The Harris Benedict used in method A is as follows:

  • If you are sedentary: BMR x 1.2
  • If you are lightly active: BMR x 1.375
  • If you are moderately active: BMR x 1.55
  • If you are very active: BMR x 1.725
  • If you are extra active: BMR x 1.9

Calories used during exercise

If you are NOT using a BMR correction factor, then you must add the calories burned during exercises and activity (walking, shopping, putting the kids to bed) to your BMR to determine the total calories used per day.

The calories used during exercise are most likely to come from your heart rate watch, provided you have entered your (updated!) weight, gender and age. But, there are many on-line calculators available, which won’t be very accurate but may do the job in most cases, for example:  www.bhf.org.uk/heart-health/prevention/calorie-calculator.aspx

Remember that for running and cycling you can get very accurate figures using your aeroscan results.

Determine your daily calorie intake

Now that you have figured out what goes out you must measure what goes in.

For limited periods at a time – if I keep it up too long it drives my family crazy – I keep very accurate records by measuring everything I eat or drink. At home the method is a scale and a note book – see photo.

I then enter the exact amounts into my Training Peaks account. It is a bit of work in the beginning, but once you have entered and saved all your favourite/standard foods, it becomes very easy to record it all – 10 minutes per day.

Besides, the calorie, fat, protein and other content of most foods are stored in Trainingpeaks already; you can just drag and drop:

Not only does my Training Peaks account display my total calorie intake, it also gives me the breakdown of macro and micro nutrients.

If your fat loss is not going anywhere despite consuming less calories than you burn, taking a look at what you eat rather than just a straight calorie count might be a good idea. If that still doesn’t work, double check your daily calorie burn – you likely over-estimated them.

Weight management – fat loss

Unfortunately there is no magic here. Fat loss will only occur if you have a calorie deficit: burn more calories than you consume. Each Aeroscan report states: “As a general guideline, a calorie deficit of approximately 500 Kcals/day will enable you to lose approximately 0.5 kilograms per week.

If your daily calorie deficit is too extreme on a consistent basis then your body could go into ‘starvation mode’ in which it retains fat and burns carbohydrate (since carbohydrate is the easiest form of energy for your body). This is one of the good reasons to take a long gradual approach to fat loss.

The only little bit of magic is food timing: Eat around and during exercise and more in the morning and less in the evening.

Exercise zones/intensity

All exercise burns calories, and exercising harder burns more calories. Exercising at a high intensity increases the calories you use per hour and there is some positive calorie burn ‘after effect’. However, at high intensity your body uses mainly the most readily available form of stored energy, carbohydrates. That is still fine for weight loss, provided you retain a calorie deficit for the day.

Exercising at a low intensity you burn less calories per hour, but your body is better able to convert stored body fat into useable energy. You can train your body to convert fat to energy at increasing intensity levels, so your training becomes more ‘fat burn’ efficient.

Tip: A great ‘fat-burning’ exercise is an early morning (very) slow half hour run on an empty stomach or even better after a cup of black coffee. Add this to your normal exercise program once or twice a week and watch the results.

Keeping track of results

In my case my scale sends daily weight and fat percentage data directly to my Trainingpeaks account where I have everything in one place, as shown on this dash board picture showing seven days of triathlon life:

Sleep and weight management:

I had to work from 1 am to 6 am and hence only had three hours sleep. This day I felt very hungry and ‘fell off the wagon’, which is a known body reaction to a lack of sleep. That’s why people that work irregularly or in night shift tend to have more weight management issues.

You can of course keep track in many other ways too. Just remember that the number on the scale is never the whole story for triathletes, as we are interested in race performance, not just losing weight and fat!



Did I achieve my goal? I set a new ‘PB’ of 79.7 KG, down from 87 KG at IM South Africa in April 2012. Will this lower weight make me faster? I am not totally sure yet, but the more important drop in fat percentage by about 3 percentage should make a difference. I’ll likely gain a bit of weight and fat in the next few months, but at least I am not starting on the back foot. Also, I now know what I can achieve and in what time frame by using a measured approach.

You could of course achieve the same results and not measure a thing. But, I find keeping close track of weight management data helps me stay motivated. Motivation is key because as we all know eating wisely and managing fat percentages is not always fun. Perhaps my grandfather had it right when he said “A healthy diet doesn’t make you live longer, it just feels like you do.”

Yours in data,