Running More But Gaining Weight? Here’s Why

Running more but the scale is going up? It’s frustrating, and it can make you question whether your training is actually working. The truth is, this is more common than most runners expect. And in many cases, it has nothing to do with gaining fat.

Here’s what’s really going on.

Can Running Cause Weight Gain?

Although it seems counterintuitive, running can sometimes cause weight gain. Fortunately, it’s often temporary and, in most cases, has nothing to do with gaining body fat.

When you start running regularly, increase your mileage, or change the intensity of your training, your body goes through a period of adaptation. During that time, the scale can be misleading.

Here are the most common reasons this happens.

1. Water Retention from Muscle Repair

Running places stress on your muscles — especially if you’re new to it, coming back after a break, or increasing volume or intensity. That stress causes small amounts of muscle damage, inflammation as your body repairs and strengthens, and temporary water retention.

This extra water can easily show up as 1–3 kg (or more) on the scale, even though it’s not fat gain.

This is especially common if:

• You’ve recently increased your weekly mileage (such as during a marathon training block)
• You’ve added speed work, hills, or treadmill incline
• You’ve just become more consistent with running

In other words, your body is under more training stress, getting stronger and adapting — not failing.

2. Running Increases Appetite (Often More than Expected)

Running doesn’t just burn calories — it also increases hunger hormones. If you’ve ever trained for a marathon or another long-distance race, you’ve probably experienced the never-ending hunger that tends to hit partway through a training block.

This can lead to larger portions (often without realizing it), extra snacks, and more liquid calories to satisfy hunger and fuel recovery.

The tricky part is that it’s very easy to eat back — or exceed — the calories you burn while running, especially if you’re tired or under-fuelled earlier in the day. This doesn’t mean you should eat less, but it does mean awareness matters. If fat loss is your goal, you may need to be more intentional with food choices, portion sizes, and how you fuel around your runs.

3. You May Be Gaining Muscle While Losing Fat

If you’re relatively new to running (or returning after time off), your body composition can change even if the scale doesn’t cooperate.

Muscle is denser than fat, so you can lose fat, gain muscle, and still weigh the same — or even slightly more.

Signs this might be happening include:

• Clothes fitting better
• Feeling stronger on runs
• Improved pace or endurance
• Faster recovery between workouts

The scale alone can’t tell the full story, so it’s often helpful to track progress using additional measures like body measurements, progress photos, and overall performance and energy levels.

4. Training Stress and Poor Recovery Can Work Against You

Something that often surprises runners is that more running doesn’t always mean better results — especially if recovery is lacking.

High training stress combined with poor sleep, insufficient calories, cold weather training, illness, or busy schedules can increase cortisol levels.

Elevated cortisol is associated with water retention, fatigue, and increased cravings, all of which can influence scale weight.

This is very common in runners who go “all in” quickly without building in enough recovery — or who prioritize sticking rigidly to a training plan no matter what (something many runners, myself included, are guilty of).

5. Running Alone Isn’t Always Enough for Weight Loss

Running is excellent for cardiovascular fitness and mental health, and it does burn a significant number of calories — but it isn’t magic on its own.

For many runners, especially once the body has adapted to regular running, weight loss often requires changes beyond simply adding more miles. In particular:

• Strength training helps improve body composition
• Adequate protein intake supports muscle repair and helps manage appetite
• Low-intensity cardio (like walking or swimming) can burn calories without raising cortisol
• Daily movement outside of runs (NEAT) matters more than most people realize

Weight loss — if that’s your goal — tends to happen when running is part of a balanced routine rather than a standalone tool.

What to Do if You’re Running and Gaining Weight

If this sounds familiar, there are a few practical steps you can take to stay on track with fat loss goals and help the scale move in the right direction again:

Track trends, not daily scale weight
Look at weekly or monthly patterns instead of day-to-day fluctuations.

Fuel your runs properly
Under-eating during training often leads to overeating later.

Prioritize protein
Especially post-run, to support muscle repair and satiety.

Support recovery
Sleep, rest days, and adequate nutrition matter more than people think.

Add light strength training
Even 1–2 short sessions per week can help improve body composition.

Replace some high-intensity sessions with low-intensity cardio
Easing off intensity can help lower cortisol and better support fat loss.

Be patient and give it time
Your body may need a few weeks to adapt before the scale reflects what’s actually happening.

The Bottom Line

If you’re running regularly and gaining weight, it doesn’t mean running is causing fat gain — or that it “isn’t working.”

In most cases, it means your body is adapting to the added stress of running by holding onto water, repairing muscle, and adjusting hormones and appetite.

If this is you, stay consistent, support recovery, and focus on more than just the scale. Running is usually doing exactly what it’s supposed to do — even if the number doesn’t show it yet.