Shoe Clinic are proud to have... Marnie Oberer as an Expert columnist.

Marnie is a nutritionist and athlete and television presenter. She is a trained dietitian and has a degree and two postgraduate diplomas from the University of Otago.

By 28 Marnie had set up her own business consultancy, advising high performance athletes and teams. Marnie started competitive aerobics and caught the ‘marathon bug’ running in Australia, UK and USA.

Column 43, August 2011

Will losing weight help me run a faster half marathon?

Becoming a better distance runner, unlike many other sports, is highly dependant on endurance. Training and technique definitely matter, but to be fast and efficient over distance a light weight is advantageous. On the flip side of the coin, too little body fat can hinder performance and lead to injury and illness.

Runners wanting to improve their Personal Bests (PB’s) therefore need to find a balance between a light weight, healthy body fat percentage and peak physiological functioning.

Over and under-weight in running

Excess weight puts pressure on the legs, requires more effort to move, and reduces the bodies’ ability to regulate temperature. It also places extra stress on the heart and lungs. The result therefore is reduced speed, stamina and an increased risk of injury.

Being ‘Underweight’ (see BMI calculation*) increases the risk of becoming weaker, slower and more prone to illness and injury.

Based on changes in maximal aerobic capacity, the following table provides a rough estimate of performance increases with weight loss. This is provided you still maintain a healthy body fat and are above a BMI of 18.5.

Table 1: Weight loss and performance prediction calculator

Weight Lost	5K	10K	Half Marathon	Marathon
1kg	14s	28s	57s	01:56:00s
2.5kg	34s	01:08:00s	02:24:00s	04:48:00s
5kg	01:08:00s	02:16:00s	04:48:00s	09:36:00s
10kg	02:16:00s	04:33:00s	09:36:00s	19:13:00s

Body fat, health and performance

Studies of healthy competitive distance runners have shown males to range between 5-8%, and females to have an average of 10-14% body fat. Based on the American Council on Exercise classification of body fat, this would put them between ‘Essential’ and ‘Athlete’ status (see table below).

Note that ‘essential’ body fat is the minimum necessary level to maintain health and body functioning, with a higher level required in women to maintain a normal menstrual cycle. Cessation of the menstrual cycle increases the risk of osteoporosis and stress fractures. Also note that ‘Average’ does not imply healthy.

Table 2: The American Council on Exercise body fat classification

Classification	Male (body fat)	Female (body fat)
Essential	2-5%	10-14%
Athlete	6-13%	14-20%
Fitness	14-17%	21-24%
Average	18-25%	24-31%
Obese	26.00%	32.00%

For the regular runner it would be difficult and unnecessary to attain the levels of body fat seen in elite distance runners. It would be more appropriate to aim for the ‘fitness’ range initially to improve both health and performance, then gradually work towards the ‘athlete’ range in order to become more competitive.

Interestingly, in a recent treadmill study of thousands at The Cooper Center, aerobic fitness was found to be a powerful indicator of longevity, and in fact in some cases being fit but ‘fat’ was better than being lean and unfit. Therefore wherever you fit on the body fat scale above, improving fitness through running will improve health even before body fat is lost.

Other considerations

Muscle mass
There is debate as to whether muscle is a help or hindrance to distance runners. Muscle weighs more than fat, but it is ‘functional’ rather than ‘dead’ weight.

The type of muscle may be important here as slow-twitch fibres have more mitochondria - the energy producing part of the cell - and are resistant to fatigue. They are also smaller, adding less bulk/weight than fast-twitch fibres. Weight loss aimed at reducing muscle mass should be questioned.

Speed of weight loss
If weight loss is achieved too rapidly ie greater than 0.5-1kg/week, performance is likely to be negatively affected.

Consistency of measurements
If you want to more accurately gauge body fat levels and change I would suggest finding an accredited anthropometric assessor and sticking with them. As there will still be a margin of error, try not to reassess more frequently than once a month.

Psychological issues
Distance runners are amongst the ‘higher risk’ athletes when it comes to developing an eating disorder. Females in particular need to be aware not only of the physiological effects of maintaining an unhealthy weight and body fat, but also the psychological impact.

Summary

Weight loss can improve performance times provided loss is gradual, and body fat/weight end points are healthy for the particular individual. There is no one magical formula. You may be the same height, age and stature as your friend, but this does not mean their ideal racing weight should be yours.

Knowledge, experimentation and data collection will assist you in finding your ideal running weight.

Here’s to your next PB!

Marnie Oberer

* BMI is calculated by

weight (kg)

height (m2)

BMI	Classification
<18.5	Underweight/malnourished
18.5-24.9	Normal Weight
25-29.9	Overweight
>30.0	Obese