Running and Gaining Weight? Why the Scale Goes Up and What to Do About It

If you have started running more, or running harder, and the scale has gone up instead of down, you are far from alone, and it is rarely what it looks like. This page covers why your weight can rise in the first weeks of a harder training block, how to build a weekly mix of runs, how to judge the scale fairly, how to check your calories in, and what to do when a strict diet works against you. Two real readers appear along the way as worked examples: Erica, who put on 5–6 pounds after switching to intervals, and Leah, who gained weight while dieting and running hard.

On this page:

1. Why the Scale Can Jump After Harder Running
2. Build a Weekly Mix, Not One Perfect Workout
3. Judge the Scale Fairly
4. Check Your Calories In
5. When a Strict Diet Works Against You

1. Why the Scale Can Jump After Harder Running

A jump of 5–6 lb in two weeks is rarely fat. One pound (0.45 kg) of fat needs roughly 3,500 calories more than your body uses. Gaining 5–6 lb (2.3–2.7 kg) of fat in 14 days would mean eating about 1,250–1,500 calories a day too much, every day, without noticing. That is very unlikely when you are training regularly.

Here is what is more likely. Harder running with incline makes your muscles store extra glycogen — carbohydrate kept in the muscle as fuel — and each gram of glycogen holds about 3 grams of water. That alone can add 1–2 kg. Unfamiliar fast running also causes tiny tears in the muscle, and your body holds fluid around them while it repairs. Add two weeks of broken sleep, a bit of stress, salt in your food or the time of your cycle, and you have your 5–6 lb.

One more possibility: harder sessions often make you hungrier. Many runners eat an extra 100–300 calories without realising, which can wipe out the extra 100–150 calories a harder session burns.

2. Build a Weekly Mix, Not One Perfect Workout

Steve Moneghetti, one of Australia's best long distance runners, once said that staying as lean as he is comes down to two simple rules.

Rule 1. Eat absolutely everything you want.
Rule 2. Run at least 100 km per week.

Meant as a joke — but the point stands. The more miles you cover, the more calories you burn. Most of us can't run 100 km (62 miles) a week around work and family, so variety has to do the work for you.

Intensity does raise your oxygen use during and after a session. This afterburn effect is real, but small — tens of calories, not enough to make 15 hard minutes equal 45 easy ones. It is also often confused with the fat-burning zone myth — worth untangling, since interval training doesn't fit the slow, low-intensity picture that myth assumes.

A good base is 40–45 minutes easy with a few bursts. What it often lacks is variety. Here is a mix for a runner doing five runs a week:

• Two easy runs of 40–45 minutes at your usual easy pace, with a few short sprints if you like. Erica's easy pace was 6.0 mph (9.7 km/h) and her sprints 9.0–9.5 mph (14.5–15.3 km/h). Also see base training.
• One interval session, shorter but with many more repeats. After a 5–10 minute warm-up, do 30 seconds on, 30 seconds off — or 1 minute on, 1 minute off — until you can't go anymore. Also see Interval Running.
• One faster, steady run at around 7 mph (11.3 km/h) for a longer stretch. That is Tempo Running — a threshold effort you can hold, but only just.
• One longer run, and build up slowly — a few minutes longer each week.

Keep the two hard days (intervals and tempo) apart, with easy running between them, and make every fourth week a consolidation week with easier sessions. This mix will support your weight loss goals far better than any single workout will. Also see Running for Weight Loss.

3. Judge the Scale Fairly

Two weeks is far too short to judge a new training block. Give any change 3–4 weeks before you decide it has failed.

Weigh yourself the same way each time: first thing in the morning, after the bathroom, before your run. Then use the weekly average — not the daily number. Your weight can swing 1–2 lb (0.5–1 kg) from one day to the next without any change in fat.

Also check how your clothes fit, and measure your waist once a week. Are your jeans tighter, or do they feel looser? Your body may be replacing fat with muscle. Your fat stores will diminish as you run, and running will help you build muscle. Muscle is denser than fat, so it takes up less space per kilo, which means the scale can stay flat or creep up even as you get leaner. If you also lift weights, as Erica did five days a week alongside hard running, that is quite possible. This is not a bad sign. This is progress. If your waist and clothes say you are fine, the scale is lying to you.

4. Check Your Calories In

Calories burned is one part of the equation — calories in is the other. Weight loss comes mostly from the gap between them, and running is the tool that widens it. Even if you are conscious about what you eat, keep a food journal for a week and add up your calories. You may be taking in a little too much without realising it, especially after harder sessions. Even when you are running and dieting, it is easy to eat back your deficit without noticing, because harder running makes you hungrier.

Even if your total is fine, you may need to rebalance towards protein. Many runners eat more carbs — and yes, carbs are important for energy. But you need protein to repair your muscles. To put it simply: too many carbs and not enough protein means the excess carbs get stored, while your muscles keep asking for more food because they still need repairing. Try swapping some carbs for protein at breakfast and after your run.

If none of this gets you anywhere, I'd recommend getting help from a nutritionist.

5. When a Strict Diet Works Against You

If you have been dieting hard and running hard and the scale still won't move, or has crept up, another cause is a diet so strict that your body has put the brakes on hard and is holding on to its energy reserves. This is your metabolism responding to a severe calorie cut. The fix is not to eat less — it is to eat more sensibly, with enough fuel to support your running.

Do not give up. If you stop now, you will not get any improvement at all. Keep your running going, steadily build up your distances and time on your feet, and combine that with a healthy — not punishing — diet.

In Short

A scale that goes up when you run harder or diet harder is rarely the failure it looks like. Give any change three to four weeks, mix your week with easy runs, one interval session, one tempo run and one longer run, and check your calories in before you cut further. Your healthy diet and regular running deliver benefits that go well beyond losing weight, even if the number on the scale is slow to move. If the scale stays stubborn for longer than a few more weeks despite all of this, read weight-loss plateau troubleshooting for exactly what to check next. If your weight keeps climbing despite the changes above, or you gain weight quickly for no clear reason, check in with your doctor.

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