Marathon training weight gain: why the scale goes up

Marathon training weight gain is one of the most confusing parts of a build. You're running more than you ever have, and the scale goes up. The usual explanation is boring and reassuring: a bigger training load means more stored carbohydrate, stored carbohydrate brings water with it, and long runs leave your legs holding extra fluid while they repair. On top of that, appetite rises, sometimes faster than the training does.

None of that means your training is failing. This post explains where the number comes from, why fueling matters more than the scale during a build, and when it's worth talking to a professional. If you also lift, a Runna-style plan that doesn't ignore the gym keeps the running and strength sides of the week in one place.

One note first. This is about understanding the scale, not changing your body. We're not going to promise weight loss, and nothing here is a personal nutrition prescription. If you have a medical condition, a history of disordered eating, or weight is causing you real worry, talk to a doctor or a registered dietitian.

Why weight gain during marathon training happens

A marathon build changes how much you train, how much you eat and how much fuel and water your body stores. All three move the scale. A typical build, like a marathon plan that keeps your strength work, adds weekly miles and a long run that grows to around 18 to 20 miles, so it would be odd if your body weight didn't respond somehow.

The whole training side of that is covered in how to train to run a marathon. Here's the scale side.

1. Glycogen and the water that comes with it

Your muscles and liver store carbohydrate as glycogen, and glycogen doesn't travel alone. A study measuring muscle biopsies during recovery from prolonged exercise found that each gram of glycogen was stored with at least 3 g of water (Fernández-Elías et al., 2015). An older review describes glycogen as stored in hydrated form, with three to four parts water (Kreitzman et al., 1992).

When you train more and eat enough carbohydrate to fuel it, you keep those stores topped up. More stored fuel, more stored water, more weight on the scale. You'll see the extreme version during carb loading: in one study, three days of a 12 g/kg/day carbohydrate diet after a glycogen-depleting session raised muscle glycogen and total body water, from 39.3 kg to 40.2 kg on average (Shiose et al., 2016).

That water is not fat, and it's not a problem. It's fuel you'll use.

2. Repair after long runs

Long runs damage muscle in small ways, and repair involves fluid. In a biopsy study of marathon runners, the first one to three days after the race showed swelling inside and around muscle fibers (Warhol et al., 1985). Training runs are smaller doses than a race, but a weigh-in on the morning after your longest run is not a neutral measurement.

3. Day-to-day fluid swings

Your weight moves a lot from day to day anyway. The ACSM, Academy of Nutrition and Dietetics and Dietitians of Canada position stand notes that during exercise, a loss of 1 kg of body weight represents roughly 1 L of sweat loss (Thomas, Erdman and Burke, 2016). Weigh yourself after a hot long run and again the next morning after rehydrating, and you can see a big swing that has nothing to do with body fat.

4. Appetite

Running more makes most people hungrier. How much people eat in response to exercise varies a lot between individuals, and a review on exercise and appetite describes this as compensatory increases in hunger and food intake, with wide individual variability (King et al., 2012).

The classic marathoner version: "I ran 18 miles, I deserve the whole pizza." Sometimes you do. Sometimes the 18 miles was on Saturday and the pizza tradition has expanded to Sunday through Wednesday. That isn't a moral failing. It's just worth noticing.

5. Strength work

If you're keeping one or two lifting sessions a week, which you should, some of the change on the scale may be muscle you're keeping or building. Strength training for marathoners isn't designed to add size, but it is designed to stop you losing strength over four months of running.

Why you shouldn't diet during a marathon build

This is the part that matters most. Cutting food to push the scale down mid-build is the most common way people turn marathon training weight gain into a worse problem.

Your fuel needs go up with your training. The position stand lists daily carbohydrate targets of 5 to 7 g/kg for a moderate program of around an hour a day, and 6 to 10 g/kg for endurance programs of one to three hours a day (Thomas, Erdman and Burke, 2016). That's a lot more than most people eat on a normal week.

The same position stand warns that chronically low energy availability, meaning not eating enough for the training you do, is associated with impairments in a variety of body functions (Thomas, Erdman and Burke, 2016). In plain terms: under-fuelling a marathon build costs you training quality and can cost you more than that.

Practical version:

• ›Fuel long runs and quality sessions properly. Before, during and after.
• ›Don't skip carbs to "offset" a heavy scale reading. The reading is partly the carbs doing their job.
• ›Eat real meals on your biggest training days. Most of healthy eating for marathon training is simply eating enough, mostly from normal food.
• ›If eating or weight is stressing you out, talk to a registered dietitian. That's what they're for.

How to read the scale during marathon training

If you weigh yourself at all:

• ›Same time, same conditions. First thing in the morning, before eating, after the bathroom.
• ›Watch the trend, not the day. A weekly average tells you more than any single reading.
• ›Ignore the day after the long run. And carb-loading week. And the week after the race.
• ›Or stop weighing for the build. Your training log, energy and long-run quality are better feedback than a number that moves with your water intake.

Marathon training and weight gain by phase

The pattern usually follows the plan:

Taper weight is worth a special mention because it alarms people right before race day. You're running less, your glycogen stores are full, and race week finishes with carb loading. Of course the scale is up. That's what full tanks look like. Keep race week boring, with familiar food and maybe the shakeout run the day before, and leave the scale alone.

Timing matters too

Weight concerns are one more reason to give the build enough time. A rushed marathon squeezes a big jump in training into a few weeks, with appetite and fluid changes to match. How long does it take to train for a marathon? Usually 16 to 20 weeks for someone already running, and longer from scratch.

If your race is far off, six month marathon training gives you time to build gradually, so the training load, and your eating, ramp up together. If you're doing your first marathon, expect the body changes to feel newer and stranger than they will next time.

The opposite approach, attempting a marathon with no training, skips the gradual build entirely, along with the fueling practice that makes race day go well.

When to talk to someone

See a doctor or registered dietitian if:

• ›Weight is changing fast and you can't explain it with the training, fueling or fluid patterns above.
• ›You notice swelling that doesn't settle, or other symptoms that worry you.
• ›You find yourself restricting food, skipping fuel on long runs or feeling anxious about eating.
• ›Your periods change or stop during training.

None of those are things to troubleshoot from a blog post.

How exrsz handles marathon training

exrsz builds a periodized marathon plan from your goal, schedule, level and equipment, with running and lifting balanced in one plan. It adapts each week to how you actually trained and reshuffles missed sessions so the build lands on race day. It doesn't do your shopping, and it won't tell you what to weigh.

FAQ

Is it normal to gain weight during marathon training?

Yes, it's common. More training means more stored glycogen, and each gram of glycogen is stored with at least 3 g of water (Fernández-Elías et al., 2015). Appetite and fluid shifts after long runs add to it.

Why do I gain weight marathon training if I'm running so much?

Mostly fuel and water. Your body stores more carbohydrate to support the training, and that carbohydrate holds water. Appetite also tends to rise, and how much varies a lot between people (King et al., 2012).

Should I try to lose weight during marathon training?

Not during the build. Fuel needs rise with training, and the ACSM, Academy of Nutrition and Dietetics and Dietitians of Canada position stand links chronically low energy availability to impaired body functions (Thomas, Erdman and Burke, 2016). If weight is a concern, talk to a registered dietitian.

Why do I gain weight during the marathon taper?

You're running less while your glycogen stores fill up, and carb loading adds more stored fuel and water at the end. It's expected, and it's fuel you'll use on race day.

Does weight gain during marathon training go away?

Much of it is water linked to stored glycogen and recovery, which shifts as your training changes after the race. Give it a couple of weeks before reading anything into the number.