There are many reasons behind sudden weight gain, and Rebecca Jane Stokes from YourTango [1] is here to explain them.

It's not just in your head.

For many men and women, dealing with weight gain is a normal occurrence. Human beings aren't made of stone — we're made of water, muscle, tissue, fat, bone, and more.

To maintain a weight of their choosing, most people exercise regularly and eat in a mindful way. Crash diets aren't good for your body [2] or effective in the long-term, and difficult as it may be to change our lives, if weight loss is a goal, it's the only way to achieving it for keeps.

Still, even when you're doing everything right, sometimes the numbers on the scale beneath your feet can leave you feeling disheartened and taken aback. You're counting calories, working out, drinking loads of water, getting lots of sleep, but the numbers are still creeping up!

You keep asking yourself, "Why am I gaining weight?" Could there be some other factor at play? It turns out that the answer is yes! Here are the most common reasons for sudden weight gain, according to the experts below.

1. Slow Thyroid

Your thyroid (found beneath your Adam's apple) is responsible for regulating your metabolism. If you're noticing sudden weight gain, your thyroid could be slow or sluggish, which can make your metabolism slow down in a major way.

"If you've suddenly put on weight for no apparent reason, I suggest you see a doctor so a medical professional can decide whether it is a thyroid issue or another cause," says The Biggest Loser dietitian Cheryl Forberg [3].

2. Water Intake

No, I'm not about to tell you that you've got to start drinking more water, but I will tell you that drinking two glasses before you eat will work wonders, according to new research [4]. If you aren't drinking enough water, your body can start to hold it in (water retention) and that can appear as added pounds on your scale!

3. Depression

If you're a depressed person and you're on a medication to treat that depression, it can mean serious sudden weight gain. Alissa Rumsey [5], MS, RD, CDN, and Spokesperson for the Academy of Nutrition and Dietetics, said, "Some medications may cause food cravings, especially for carbs, and some find that their medication increases their appetite. The drugs may affect metabolism negatively as well."

Alternatively, sometimes depressed people lose their desire to eat food, and when their meds kick in, they suddenly have an appetite again.

4. High-Intensity Exercise

Yes, all that Spinning and CrossFit you've been doing could actually be causing you way more harm than good. It turns out that people who take these classes regularly tend to increase the number of calories they eat per day, which is often unnecessary.

These added calories are going away when you hop on the bike in Spin class, because if you're doing it regularly, your body adjusts to the new routine.

5. Shopping Habits

This one is just plain bizarre, and frankly, I'd expect the opposite, but it's apparently true. If you shop at a grocery store with self-checkout, you should use it. Studies have shown [6] that impulse candy purchasing dips dramatically when people scan their own goods. Plus, you're getting an arm workout with all that scanning!

If you're gaining weight and still having a cashier check you out, this could be a factor.

6. Staying Up Late

If you're a night owl like me, this one is a serious bummer, but the rumours are true: missing out on sleep will make you gain weight.

"Lack of sleep can lead to increased levels of the hunger hormone ghrelin, and decreased levels of leptin, the satiety hormone," said Alissa. "Research also shows [7] that when we're sleep-deprived, our brains respond more strongly to junk food and have less of an ability to practice portion control."

7. Making Healthy Choices

You're eating out with friends but trying to eat in a healthy manner, so you pick what looks like the healthiest dish on the menu, but you still gain three pounds. What gives?

Well, when your mind recognises that you are eating something "healthy," it actually secretes more of that hunger hormone ghrelin, making you feel less full and more hungry, which, in turn, will lead you to overeat. Better to eat out less frequently, order what you'd like, and take half home to go.

8. Job Stress

Everybody gets stressed out at work, but did you know that too much stress in the workplace can make you feel tremendously ill and it can make you gain weight? That's right, bring that to your next review and tell them to pay for your gym membership.

"The hormone cortisol is released when our body is under stress that causes triglycerides to be relocated to visceral fat cells, increasing storage of belly fat," Alissa said. "Elevated cortisol levels also cause an increase in blood glucose while suppressing the effects of insulin, leading to constant feelings of hunger and can lead to overeating. To make matters worse, all of that unused blood glucose is eventually stored as body fat."

9. Sodium

All of your dieting, exercising and water drinking will be for naught if you aren't being aware of how much sodium you're eating on a daily basis.

Salt is delicious and necessary to keep our brains functioning, but salt also is responsible for making you retain water in your guts. This doesn't just hamper good digestion, it makes you bloated and heavier than you really are due to water weight.

10. Lack of Protein

If you're eating well and still gaining weight, you might want to look at what you're eating. Missing out on protein can actually make you much more susceptible to weight gain.

"If you aren't consuming enough protein to keep your muscles and cells healthy, the body ends up breaking down muscle to access the nutrients it needs and this spells trouble. Less muscle mass means a slower metabolism, which over time, can cause weight gain," Alissa said.

11. No Sweets

Sometimes your diet can be too rigid. If you always opt for the diet treats instead of splurging on the real deal, chances are you're on your way to gaining weight and you don't even know it.

The sugar replacements and other additives in these treats can actually leave you feeling less satisfied, and thus lead you to eating more. Better to stick to the real deal in moderation.

12. Medication

It isn't just antidepressants that can cause weight gain; there are plenty of other medications that do so as well. "Some drugs stimulate the appetite or slow the body's metabolism. Others cause fluid retention or enough drowsiness to reduce physical activity, which can trigger weight gain," advised Alissa.

13. Sore Body

If you're feeling sore or have existing muscular issues, this can lead to weight gain too. "Musculoskeletal conditions that cause pain can lead to a decrease in physical activity, which can cause weight gain over time—especially if you are eating the same amount that you were eating when you were more active," warns Rumsey.

14. Using a Shopping Basket

If you are lugging a shopping basket around at the grocery store and not using a cart, you wind up speeding through your shopping just to get out the door before the load is too heavy for you to carry. This means that you aren't shopping in a mindful way.

Using a cart, even if it's just a quick trip, can help you take the time you need to buy the items that will be the best for you.

15. Age

Yup. Sad, but true. Once you hit 30, your metabolism begins to slow down. You're not the person who can eat a pint of ice cream consequence-free anymore. Adjusting your diet to suit your current body and current lifestyle will be a helpful solution to this problem.

Rebecca Jane Stokes [8] is a writer living in Brooklyn, New York with her cat, Batman. Her work focuses on relationships, pop culture, and news. For more of her work, check out her Tumblr [9].

Check out more great stories from YourTango:

• 10 No-Good, Very Bad Habits That Are *Secretly* Making You Fat [10]
• What Your Body Type Can Reveal About Your Personality & Approach to Weight Loss [11]
• The Sneaky Psychological Reason Your Diets Never Seem to Work — and What to Do Instead [12]