The desire to shed a few kilos is common, even among runners. Usually this has to do with either looking good or getting better race times. And here comes the concept of ideal racing weight. 

Ideal race weight is the weight at which you will race your fastest. What it means is that each individual has an ideal weight that you can strive for, and at which you would be able to perform well. There are charts for healthy weight or body mass index or fat percent, and while these can be a good guide for losing weight, it is important to note that athletic performance has requirements that may not coincide with such charts.

What is the science behind weight versus running pace?

Running is a form of propulsion that involves vertical and horizontal movement. Due to the need to lift your weight vertically, the lower your body weight, the easier it would be for you to move forward. A simple way to check this would be to strap on a rucksack with 5 kilos of sand on your back and check how this affects your pace. Obviously, as you get heavier you will slow down.

There is another measure, namely, VO2max. VO2max is a well-known measure of endurance and is measured in milliliters of oxygen per kilogram of body weight per minute. 

VO2max = ml O2 / Kg / min

As you can see, the mass or weight is in the denominator, and hence there is an inverse relationship between VO2max (which is your endurance capacity) and weight.

There is another advantage to losing weight. The heavier you are the more is the production of heat in the body when you run. Further, heat dissipation from the surface of the body is lower for a heavier person because heavier people have a lower surface area relative to their weight. This increase in heat will result in an increase in the core temperature of the body. Accordingly, the body has a mechanism by which it will divert blood away from the legs to the surface of the skin in order to cool the body. This will cause your pace to go down. Thus excess weight will affect performance, especially in hot weather.

While weight may not be the only determinant of your performance, it is one of the parameters that can influence your performance. 

How do you arrive at your ideal race weight?

The best way to lose weight is to lose body fat. A study assessing the factors predictive of personal best marathon race times in recreational male marathoners suggests that along with running speed during training, body fat percentage was an important factor for marathon performance time. Like with any other process, it helps to have some numbers that you can aim for — the guidelines for body fat percentage ranges are given below.

The ranges given here are a good starting point and motivator. However, it is important that you do not take them as definite goals. This is because the loss of weight may cause you to see other physiological changes such as insomnia, menstrual cycle disruption, increased stress, recurrent injuries, and fatigue. It may also lead to preoccupation with food, and in the worst case, can trigger eating disorders. 

For example, a woman who attempts to hit the fat percentage numbers in the given chart may find that there is a limit below which she will experience menstrual cycle changes. Excessive and sudden weight loss can also result in compromised bone density and can affect your immune system. Moreover, being too set on achieving a specific bodyweight may distract you from your training and understanding what is right for your body. 

Even while trying to lose weight, it is essential that you listen to your body. Observe any undesirable changes to your sleep cycle, energy levels, and your running performance. If as a recreational athlete you are obsessed with race times and focus solely on lowering your bodyweight, there is a risk that it may dip beyond the level and your performance may be impaired instead of improving. 

However, if despite losing weight your race times are suddenly going awry, in the absence of any other factors like chronic illnesses, it may be an indication that you have dipped below your lowest acceptable or ideal running weight.

It is essential that you approach your weight loss with the right mindset and with an effective and healthy plan. Your weight loss should supplement your training and not the other way around, as it can lead to a dip in your running performance.

Seeking professional help like consulting a sports nutritionist can help you achieve your goals in a structured and personalized manner without compromising your health and performance, for example, they can draft a personalized diet plan for weight loss without affecting your running routine.  

When is the best time to lose weight?

If you are not training for an event, anytime is good for you to work on your weight loss goal. However, if you are training for a race, the first phase in a periodized training program called base training is the best time to lose weight. This is a period during which the objective is to run easy miles and build endurance. There is no pressure on racing or speed training. The weeks during this phase can be used to target reduction in calorie intake. For example, if you are following a 24-week marathon plan, generally the initial 10-12 weeks will be your base training phase, during which you can plan to achieve your optimum body weight.

In the 10 weeks before a competition, it is best to drop any weight loss strategies, because if you are not fuelled up properly, your training could take a hit, and so will your race performance.

Everyone has a different ideal racing weight. Your optimal racing weight is a function of your optimal body fat percentage, which will vary from person to person. Train smart and with safe progressions along with a healthy eating lifestyle and you will not only get faster but also leaner. Listen to your body while making performance a priority and the rest will easily follow. 

Reference
1. Barandun U, Knechtle B, Knechtle P, et al. Running speed during training and percent body fat predict race time in recreational male marathoners. Open Access J Sports Med 2012; 3: 51-8.