Losing Weight Whilst Marathon Training

Losing Weight Whilst Marathon Training


Losing weight whilst marathon training can be a real battle. Eating enough calories to fuel your body is essential. At the same time, losing weight requires the body to be in a calorie deficit to lose weight. Losing 1 pound of weight requires a 3,500 calorie deficit. This deficit can come from diet alone, diet and exercise, or purely from exercise.

Training for a marathon is tough at the best of times, let alone when you are also trying to lose weight. How do you ensure that your body is fuelled adequately to train and run 26.2 miles without hindering your weight loss goals?

Work Out Your BMR / TDEE

Losing weight is as simple as creating a calorie deficit. As mentioned above, this can come from diet, exercise, or a combination of both. In order to work out how many calories you can consume, you need to understand your Total Daily Energy Expenditure (TDEE). Your TDEE takes into account your Basal Metabolic Rate (BMR); this is the amount of calories that your body requires in order to carry out its most basic functions (this amount would be required even if you were comatose). You can work out an estimated BMR here.

Your BMR is then multiplied by a factor depending on your daily activity levels in order to work out your TDEE.

  • Sedentary (no exercise) = x 1.2
  • Light Activity (1-3 days of exercise) = x 1.3
  • Moderate Activity (3-5 days of exercise) = x 1.55
  • Heavy Activity (6-7 days of exercise) = x 1.725
  • Very Heavy Activity (7 days / physical job along with exercise) = x 1.9

For example, using the website above would put my BMR at 1389. I would then multiply that by 1.55 to get 2152.95. To lose weight I would have to eat less than 2153 calories per day. So if I ate 1653 every day for a week that would be a total weekly deficit of 3500 calories and should result in 1 pound of weight loss.

If you follow your TDEE and decide to base your calorie deficit amounts from that, it is important to understand that this already takes exercise into account. That is to say that you don’t eat back any of your exercise calories as they are already included in the formula above.

A percentage of exercise calories can be consumed if you are using your BMR to calculate your calorie deficit. BMR only takes into account the calories required to live, and any extra exercise will have to be added separately. This method also doesn’t take into account walking, or if you have an active job.

Calorie Burn

The amount of calories burned whilst exercising is not an exact science. This figure is also less reliable if you are obtaining the number from a gym machine. Typical activity trackers or optical heart rate monitors are also not the most accurate for this calculation. Chest heart rate monitors are the most accurate, but as mentioned above, still aren’t an exact science.

For this reason it’s important to avoid eating all of your exercise calories back. If your fitness watch says that you have burned 500 calories, so you eat your daily calorie goal plus the 500 you burned, you may actually be in a calorie surplus. A general rule of thumb would be to eat at your maintenance calorie allowance (the minimum your body needs, excluding exercise), and then eat back around 50% of any additional calories burned. This way, even if you overestimate the amount of calories burned, you are more likely to remain in a deficit.

Log Your Food

It can be easy to over eat without even realising. Underestimating serving sizes can lead to almost double the expected calories being consumed. Keeping a log ensures that you can keep track of what you are eating. MyFitnessPal is a great, free option for keeping a food log.

Writing down what you have eaten also makes you more accountable. Having a log also makes it easier to eradicate junk food. Seeing your daily food intake in writing allows you to make healthier choices. For example, realizing how many calories a chocolate bar contains may make you more likely to opt for an apple.

Watch Your Macros

Losing weight whilst marathon training requires commitment not only to the training, but also to proper fuelling and nutrition. Correct fuelling isn’t just about calories though, and not all calories are created equally. A balanced diet is important to ensure that your body gets enough nutrients to recover from exercise and perform at its best. As such, it is important to keep an eye on your macros. Macros refer to the intake of carbohydrates, fat and protein. Carbohydrates help to fuel the body and top up glycogen stores (hence many runners carb-load before a big race). Fat is essential and is used by the body as fuel for longer distance events. Protein helps the body to recover and is essential for repairing tissue; it is important for every cell in the body.

Carbohydrates should make up between 40-55% of your daily calorie intake. Complex carbs should be a priority. White rice, pasta, and white potatoes are all example of simple carbs. They cause blood sugar levels in the body to spike and then crash. This crash is what causes sugar or carb cravings. Complex carbs are refined carbs such as sweet potato, brown rice, and wholemeal bread. These types of carbs have a higher fibre content and take longer to digest. As such, you will feel fuller for longer.

Fat should make up 20-30% of you daily calories. When it comes to losing weight a lot of people seem to label fat as “bad” and try to completely cut it out. This is a fundamental error! The body needs fat, and there are also healthy fats which should be incorporated into your diet. It’s important to understand that a healthy and balanced diet means getting enough of each nutrient, including fat. Part of having a balanced diet means that nothing should be off limits, but instead focus on moderation.

Protein should make up 25-35% of your calories. Protein is important for helping to repair and build muscle.

For tips on making healthy food swaps, check out this post.

Stay Hydrated

Staying hydrated is important, but even more so when training. Dehydration can cause muscle cramps and fatigue. The symptoms of dehydration can also feel like hunger. This sensation can cause people to eat instead of opting for a drink. Consuming more water, especially between meals, can also help to keep you feeling full.

Ditch The Scales

This may seem like a strange thing to include in a post about losing weight whilst marathon training, but stick with me! When you train, especially in the early stages, you will also be gaining muscle. This is why it’s important to also measure yourself using a body tape measure. Muscle is leaner than fat per pound of body-weight, so the scales may not have moved but you could have lost inches from your waistline.


Losing weight whilst marathon training is entirely possible. It may be difficult at first as you figure out what works for you. You need to understand your body’s calorie requirements and then also factor in exercise to create a sufficient but safe deficit. Try to train with a heart rate monitor to get a more accurate calorie burn calculation, especially if you are not using the TDEE method above. You may think that you have burnt 500 calories from a speedy 5k, when actually that figure may be under 300.

Some people find that instead of losing weight whilst marathon training they actually put weight on! This usually happens as people tend to feel a lot hungrier after running longer distances. To avoid this it is important to eat a balanced diet and ensure that your body is fuelled correctly. Increasing your protein intake and switching to complex carbohydrates can help to keep you feeling fuller for longer. Staying hydrated is also important and dehydration can make you feel like you are hungry when your body actually needs more fluids.

Remember though that the scales don’t always tell the full story! Take regular body measurements to see if you are losing fat but gaining muscle. It’s important to listen to your body and be patient. You didn’t put the weight on overnight so you won’t lose it overnight either! Make simple changes to your lifestyle and lose the weight safely and slowly.