Why Am I Gaining Weight? Common Causes Of Unexpected Weight Gain

A lot of exercisers work out and watch what they eat for weight management purposes. This can mean trying to lose weight or weight maintenance. Hopefully, and if your diet and workouts are going well, you should see the results you want every time you hop on the scales.

Unfortunately, things may not always go as planned, and you could actually gain weight unexpectedly. This can be frustrating and could even undermine your motivation. After all, you’re doing your best, but your best doesn’t seem to be good enough. Maddening!

But, before you throw out your scales, quit your diet, and cancel your gym membership, take a moment to explore the most common causes of unexpected weight gain. In a lot of cases, weight gain is actually quite explainable and can often be remedied, too.

In this article, we reveal the most common causes of short and long-term weight gain.

Common Causes of Short-Term Weight Gain

When talking about short-term weight gain, we mean over a few days and up to a week. For example, if you weigh 135 lb. on Monday, but on Thursday, your weight is closer to 140 lbs., that’s short-term weight gain.

In most cases, short-term weight gain is not extra muscle, and it’s probably not body fat either. Muscle and fat weight tend to increase more gradually and less noticeably. Instead, short-term weight gain is more likely to be something more temporary, and that means it’s easy to fix if it needs fixing at all.

The most common causes of short-term weight are:

1. Hydration and Water Retention

Your body is made up of about 60% water. There is water in your muscles, blood, digestive system, cerebrospinal fluid, and even your eyeballs! Your body uses water as a medium for chemical reactions, as a lubricant, transporting substrates, and regulating your body temperature.

Your hydration levels change constantly. For example, if you drink a large glass of water or do a sweaty workout for an hour.

Water is heavy, weighing in at one kilogram per liter or about one pound per pint. So, as your water levels fluctuate, so too will your weight.

If you weigh yourself when slightly dehydrated, such as after a workout, and then a day or two later, when you are more hydrated, your weight will probably increase. Your body fat and muscle levels haven’t changed; you’re just storing a bit more water than before.

Related: Truth or Myth: Does Sweating Help You Lose Weight?

In some situations, your body will retain more water than usual, leading to weight gain. Causes of water retention include:

  • Consuming more sodium – increases in dietary sodium or salt can cause your body to retain water. Suppose you’ve been eating a lot of salty food lately, such as potato chips or take-outs. In that case, your water weight may temporarily increase.
  • Consuming more carbs – your body stores carbs in the form of glycogen, which is glucose chemically bound to water. If you’ve been eating more carbs than usual, especially after a low-carb diet, your body will increase your glycogen stores and retain more water. For every gram of glycogen, your body stores three grams of water.
  • Using creatine – creatine monohydrate is a hydrophilic supplement which means it LOVES and attracts water. If you’ve recently started using creatine, your body will retain extra water, especially within your muscle cells. One way around this is to use creatine HCL instead of monohydrate. HCL does not cause water retention. (Find your daily creatine intake here
  • You haven’t been drinking enough water – weirdly, if you are in the habit of not drinking enough water, your body tends to store water in more significant amounts. This is a sort of survival mechanism designed to keep you alive when water is in short supply. Drinking more water will tell your body that it’s okay to release its excess water stores.
  • Certain medications – some medications have side effects, and one common side effect is water retention. If you’ve just started taking a new medication, such as steroids, and noticed you are gaining weight, that could be the cause. Read the information leaflet that came with your medicine, or ask your doctor, and you may discover the reason for your short-term weight gain.

2. Increases in Muscle and Liver Glycogen

As mentioned above, glycogen is glucose bound to water. Your body stores glycogen in your muscles and your liver. Muscle glycogen is used to fuel the muscles where it is stored. The glycogen in your quads is used by your quads, and the glycogen in your biceps is used by your biceps. These are known as localized glycogen stores.

In contrast, the glycogen in your liver is released when your blood glucose levels start to fall. Liver glycogen ensures that your brain always has a constant supply of glucose, its preferred source of energy.

Intense workouts, long workouts, and not eating enough carbohydrates will deplete your glycogen stores. As your body uses glycogen, it releases the water, leading to the water weight fluctuations outlined above. Glycogen depletion will also contribute to weight loss.

Eating carbohydrates replenishes your depleted glycogen stores and, if you consume more carbs than you’ve used for energy, your glycogen stores will be bigger than they were before. Combined with the additional water retention that accompanies glycogen storage, this can cause a short-term increase in weight.

Interestingly, one of the effects of regular workouts is larger than normal glycogen stores. This means you could gain even more glycogen weight if you are a habitual exerciser (1). Eating more carbs than usual, perhaps because you are carb cycling, or just had a cheat meal on your diet, will also increase your glycogen stores.

3. Undigested Food

Food contains calories, and unused calories are converted into fat. If you have consumed more calories than you need, it will lead to weight gain. However, this is more of a long-term issue.

But the food you eat can affect your weight in the short term, too. For example, if you weigh yourself when your stomach is empty and again after eating a big meal, that food will still be in your stomach and will contribute to your weight, even though your body fat levels will not have changed.

Ideally, your digestive system should process the food you eat pretty quickly, eliminating any waste when you go to the bathroom. But, if your diet is low in fiber, or you are just an irregular pooper, you may have waste material in your large intestine, leading to a short-term weight increase. A good bowel movement or two should eliminate most of this short-term weight gain.

Keep your digestive system working properly by eating plenty of high-fiber foods, such as whole grains, fruit, and vegetables.

4. Menstruation Cycle

Women’s monthly periods are usually accompanied by short-term weight increases, usually due to water retention. However, as many women also crave carbs during their periods, glycogen increases could also contribute to temporary weight gain.

Track your periods. You’ll probably be able to see cycles of monthly, short-term weight gains. This will make weight changes more predictable and easier to explain.

5. Inaccurate Weight Tracking

In a lot of cases, short-term weight gain is actually the result of not tracking your weight properly. For example, suppose you weigh yourself first thing in the morning with an empty stomach and after your morning pee. In that case, you’ll naturally be lighter than usual. Then, if you next weigh yourself in the evening with a stomach full of food, invariably, you’ll weigh more. This makes it looks like you have gained weight.

Avoid this problem by standardizing your weighing practices. Try to weigh yourself in the same physical state every time so that your readings are less likely to fluctuate. The best time to weigh yourself is on rising and after having had your first pee.

Summing up

Short-term weight gain is rarely cause for serious concern. Your weight fluctuates throughout the day and from one day to the next. As such, it’s entirely possible to think you are gaining weight when actually it’s just a blip.

Avoid this problem by weighing yourself at the same time of day and under the same circumstances. Also, weigh yourself every day so you can see longer-term trends. Over the course of a week, you should be able to identify your average weight, which is more meaningful than day-to-day fluctuations.

Finally, consider tracking your body composition or using girth measurements, e.g., waist, hips, or thighs, alongside your weight. Suppose your weight increases, but your body composition or your waist measurement remains unchanged. In that case, the chances are that something like fluid or glycogen retention is the cause.

Common Causes of Long-Term Weight Gain

By long-term, we mean weight gain over several weeks. For example, if you weigh 160 lbs. on the 1stof the month, 163 lbs. on the 12th, and 166 lbs. on the 20th, that’s long-term weight gain.

The most common causes of long-term weight gain are:

1. Fat Gain

The most common cause of long-term weight increases is gaining fat. This can be incredibly frustrating if you are dieting and exercising specifically for weight loss.

The causes of fat gain are usually easy to identify – you are consuming more calories than you are burning. This creates a calorie surplus, and your body converts those unused calories into fat and stores them for later. Fat is pretty heavy stuff, so fat gain is usually accompanied by weight gain. It’s estimated that one pound of fat contains about 3,500 calories.


A lot of dieters and exercisers are convinced that their calorie intake is equal to their calorie expenditure, or they are burning more than they consume. They’re genuinely baffled that they continue to gain weight.

Reasons for gaining weight rather than maintaining or losing weight include:

  • Overestimating how many calories your workout burns – a lot of workouts boast that they burn 1000 or more calories per hour. This is nothing but marketing hyperbole. You’ll be lucky to burn 500 calories per hour during most workouts! While exercise DOES burn calories, it’s usually a lot less than most people realize.
  • You’re more sedentary than you think – while exercise CAN help you control or lose your weight, workouts only contribute a relatively small amount to your weekly caloric expenditure. Even if you work out 3-5 times a week, there is still a good chance you are mostly sedentary. Being sedentary means you won’t burn as many calories per day, putting you into a calorie surplus and resulting in weight gain. Combine your workouts with plenty of NEPA (non-exercise physical activity) such as walking, gardening, DIY, playing games with your kids, washing your car, and anything else that gets you up and moving. All physical activity can help prevent weight gain and not just exercise.
  • Underestimating portion size – most people are terrible at judging portion sizes and usually underestimate how much food they’ve actually eaten. Even if you eat healthily, you could still end up with a calorie surplus and gain weight if your portions are too big. Avoid this problem by weighing and measuring the food you eat, so you don’t inadvertently consume more than you realize.
  • Not tracking your food intake calorie and food tracking are not always necessary, but if you are gaining weight and aren’t sure why it could help. Tracking your food intake could reveal a calorie surplus. As the saying goes, if you aren’t measuring it, you can’t manage it. You may be surprised to discover just how much food you’re eating per day.
  • Cheating on your diet more than you realize – the occasional treat or cheat probably won’t hurt your diet or lead to weight gain. But if what should be a small weekly indulgence has become a daily one, that could be why you are gaining weight. Avoid this problem by cutting back or eliminating cheats and treats from your diet.
  • Forgetting what you eat  –nutritionists often talk about something called the mouth-mind gap. This describes how many people eat mindlessly, forgetting what and how much they’re actually eaten. Causes of this phenomenon include eating while watching TV, eating on the go, and constant snacking. Eat more mindfully and remember to track every morsel of food you eat to avoid this all-too-common issue.
  • Drinking your calories – even if you eat healthily and sensibly, you could still gain weight if you don’t take the calorie value of beverages into account. For example, a small soda contains about 150 calories, a beer has 200, and a blended coffee drink could have 500 or more. It doesn’t matter where you get your calories from; they all count and contribute toward your intake. Make sure to count food AND fluid calories to get a more accurate idea of your total caloric intake.

2. Muscle Gain

If you are lifting weights and training hard, you may gain weight because you are building muscle. But, before you start celebrating going from 140 to 160 lbs. in four weeks, it’s essential to understand that increasing muscle size is a slow and laborious process. It will probably only amount to a pound per two per month (2).

To gain muscle, you need a calorie surplus, eat plenty of protein, and push yourself hard with consistent strength training workouts. You’ll also probably gain some fat with your hard-won muscle. Cardio and low-intensity workouts will not result in muscle gain.

So, while small weight gains could well be increased muscle mass, if you are training for fat loss or mostly do cardio, this is very unlikely. Even if you do gain muscle, it’ll probably only be 1-2 pounds per month.

3. Long-term Water Retention

While water retention can cause short-term weight gain, if ignored or ongoing, it can lead to long-term weight gain too. However, it’s worth noting that you can lose body fat while gaining or retaining water, so your scale weight increases while your body composition improves.

Water retention often goes in cycles, which is why it makes it onto our lists of short AND long-term causes of weight gain. Some people, especially women, are particularly prone to water retention.

While we don’t recommend trying to sweat off excess water, in some cases, using a mild diuretic, such as dandelion extract, can be helpful. That’s especially true if your water retention leaves you feeling bloated or looking puffy.

If you are concerned about long-term water retention, speak to your doctor for advice.

4. Pregnancy

While weight gain varies from person to person during pregnancy, it’s an unavoidable part of having a baby. Healthy weight gain during pregnancy is usually around 15-25 pounds per trimester.

But just because you expect to gain weight during pregnancy doesn’t mean you should indulge every craving and gain excessive amounts of weight. After all, you’ll probably want to lose that weight after you’ve given birth. Conversely, you should not try and avoid gaining weight during your pregnancy, as doing so could mean your baby lacks the nourishment needed for optimal development.

If you are a woman gaining weight unexpectedly, it may be worth getting a pregnancy test in case that is the reason.

5. Medical Reasons

There are various medical reasons you may be gaining weight in the long term. While most causes of weight gain are easy to explain, and outlined above, very occasionally, it’s something more serious. So, suppose you have eliminated all the other possibilities. In that case, you should speak to your doctor if you are gaining weight and don’t know why.

Possible medical causes of weight gain include:

  • Aging
  • Congestive heart failure
  • Crushing’s syndrome
  • Depression
  • Diabetes and metabolic syndrome
  • Edema
  • Menopause
  • Polycystic ovary syndrome
  • Sleep apnea
  • Thyroid disorders

Summing Up 

The most common causes of long-term weight gain are overeating and under-exercising. This creates a calorie surplus, and your body will convert those excess calories into fat and store them. Eating less and exercising more should put an end to long-term weight gain.

Weight management requires a long-term approach, which means you need to make healthy eating and physical activity part of your daily life. There are no short-term fixes for long-term weight issues!

That said, if you are pregnant, have a medical condition, suffering from long-term water retention, or gaining muscle, you will also see your weight gradually increase.

Wrapping Up

There are few things more frustrating than gaining weight when all you want to do is shed those unwanted pounds. After all, that’s the absolute opposite of what you are trying to achieve.

Use the information in this article to identify and fix the causes of unexpected weight gain. In most cases, you should have no problem getting back on track and achieving or maintaining your ideal weight.

However, in a very small number of cases, unexpected weight gain can be caused by an underlying medical condition or pregnancy. So, if you are still concerned about unexplained weight gain, make sure you speak to your doctor.


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