Unintentional weight gain occurs when you put on weight without increasing your consumption of food or liquid and without decreasing your activity. This occurs when you’re not trying to gain weight. It’s often due to fluid retention, abnormal growths, constipation, or pregnancy.
Reminder: It’s totally normal to gain weight as you age. After all, thanks to something called responsibilities, you probably aren’t moving around the same way you were younger (read, running up and down a playground all day). Less activity plus the natural loss of muscle mass can both contribute to a slower metabolism as you get older.
But if you’re experiencing sudden weight gain that just doesn’t seem to make sense to you (and it doesn’t seem to have to do with normal life changes), then it may be a sign that something odd is going on with your bod. Think: out of whack hormones or other sneaky health conditions that are causing your metabolism to misfire.
Ultimately, you’ll need to work with your doc to get a clear explanation for your unexplained weight gain. Before visiting the doc, keep a log of everything you eat as well as your exercise habits (including activity outside of your workouts) for at least a few days if not a week or two, says Melina Jampolis, MD, an internist and physician nutrition specialist based in Los Angeles. Maybe you’re eating more calories now, or you’ve spent weeks sitting down more than usual thanks to a heavier workload.
Your doc can help you get to the bottom of whether any lingering health issues might be causing the weight gain. And there are tons of reasons for unexplained weight gain that have nothing to do with eating more and moving less.
Ahead, a list of conditions that could be causing your sudden weight gain, and how to tell if it’s time to see a doctor.
- Hypothyroidism –
When a young woman walks into a doc’s office with unexplained weight gain, the thyroid is the first place most physicians will investigate, says Dr. Jampolis. And for good reason: A whopping one in eight women will develop a thyroid disorder in her life, according to the American Thyroid Association.
That butterfly-shaped gland in the neck is responsible for secreting a hormone that regulates the metabolism, and if you’ve got an under-active thyroid (called hypothyroidism) the metabolism may slow down, triggering weight gain.
Women with hypothyroidism may also suffer from low energy levels or fatigue, dry skin, hair loss, hoarseness, or constipation, says Dr. Jampolis. Notice any of them and you should book a chat with your doc who can check on your thyroid with a simple blood test if necessary.
- Polycystic ovary syndrome –
Research shows that as many as one in five women have polycystic ovary syndrome (PCOS)—an endocrine disorder that throws off the balance of reproductive hormones estrogen and testosterone, and can trigger a number of unpleasant symptoms like wacky periods, facial hair growth, and migraines.
PCOS can also muck up the way your body uses insulin (the hormone that helps turn sugars and starches into energy), which means (womp, womp) unexplained weight gain around the mid-section is common, says Dr. Jampolis.
If your menstrual cycles are off, a gyno will likely take a peek at your hormones to diagnose this one.
- Depression or anxiety –
When you’re stressed, you’re thrown into fight-or-flight mode and get a surge of adrenaline, along with a heavy dose of the hormone cortisol, which is supposed to help you restore energy reserves and store fat. Because, hello, you just sprinted three miles from a tiger (okay, dramatic example)—you’re starving.
The problem? Lots of us get chronically stressed sitting at our desk all day or just living a crazed life, says Dr. Jampolis. When your cortisol levels stay elevated for a prolonged period, then your body continues storing fat, which can lead to weight gain.
If you’ve persistently felt down in the dumps or anxious, have trouble sleeping, feel fatigued, or you’ve lost interest in the stuff that used to make you tick, talk with an MD or mental-health pro who can make suggestions for getting back on track if stress seems to be the culprit behind your sudden weight gain.
- Insomnia –
There’s nothing like a busted night of sleep to make a girl crave sugar and fat (anything to survive at work the next day, right?). That’s because missed shuteye does a number on your hunger hormones and metabolism: Sleeping too little raises ghrelin, the hormone that signals it’s time to eat, while lowering your levels of leptin, the hormone that conveys the “I’m full” feeling, says Dr. Jampolis. The result: a totally unsatisfying chow-fest the next day.
Putting off sleep to watch just one more episode? That hour could be contributing to sudden weight gain. A 2018 study in the journal Sleep found that people who slept just one hour more per week lost more fat than those who slept an hour less. The people who slept less lost less—even though everyone in the study ate the same number of calories, proportionate to their weight at the start of the study.
- Small intestinal bacterial overgrowth (SICO) –
The gut relies on good bacteria to function well (probiotics, anyone?), but there’s also bad bacteria chilling in your digestive tract. When that balance of good to bad gets thrown off, small intestinal bacterial overgrowth (SIBO, for short) can take place, triggering extra gas in your GI tract along with bloating, abdominal pain, diarrhea, and—you guessed it—sudden weight gain. Docs aren’t entirely sure how SIBO may trigger those extra pounds, says Dr. Jampolis, but treatment for SIBO typically includes antibiotics to treat the bacterial overgrowth, according to the U.S. National Library of Medicine.
- Perimenopause –
The transition period to menopause (a.k.a. perimenopause, which can start in women as early as their mid-30s, but usually starts in your 40s) triggers hormones like estrogen to rise and fall unevenly, which can cue weight gain in some women, says Dr. Jampolis. (Other signs of perimenopause include irregular periods, hot flashes, mood swings, and a change in your libido—symptoms your doc can usually suss out with her eyes closed.)
Compound perimenopause with the other inevitable body changes that happen with age (like a loss of muscle mass and increase in body fat), and it may feel like the scale’s tipping fast. Talk to your doctor to manage “the change” in stride.
- Medication –
There’s a laundry list of both prescription and over-the-counter meds that can trigger sudden weight gain or water retention that shows up on the scale as extra poundage. “Antidepressants—most commonly the selective-serotonin reuptake inhibitors (SSRIs) such as Paxil, Lexapro and Prozac—may affect the appetite center in the brain,” says Rocío Salas-Whalen, MD, an endocrinologist at the Medical Offices of Manhattan.
Meanwhile, beta-blockers (meds that reduce blood pressure) can slow your metabolism, and certain steroids (like prednisone—an anti-inflammatory that causes water retention and an increase appetite) can add on pounds. Even OTC antihistamines like Benadryl, which can disrupt an enzyme in the brain that helps regulate food consumption, can trigger noticeable weight gain, says Dr. Salas-Whalen.
A word to the wise: Don’t stop taking any pills cold-turkey—chat with your doctor, who may be able to find a more waist-friendly substitute.
- Dehydration –
There’s a reason behind the bloat, and it may have just as much to do with the water you forgot to drink as the food that you ate.
Kristen Neilan, RD, a dietitian at University of Florida Health, says most of us aren’t drinking nearly enough water. That’s because many of us mistake the feeling of thirst for the feeling of hunger. “Confusion, tiredness, and lightheadedness are all signs of even mild dehydration,” she says. Sounds a lot like how we feel when we’re hankering for a snack.
Mixed signals aren’t the only only possible culprits behind your unexplained weight gain. “Adequate hydration increases mitochondrial function—what that basically means is that it increases your metabolism,” says Neilan. Without enough water, your cells can’t do their thing (namely, convert your food into energy) quickly and efficiently.
- Cushing’s Disease –
A super-rare condition called Cushing’s disease (only 10 to 15 people per million are affected, but 70 percent of those diagnosed are women) causes excess cortisol production and can trigger excessive weight gain just around the abdominal area (the legs and arms usually stay lean) and the back of the neck, says Reshmi Srinath, MD, assistant professor of diabetes, endocrinology, and bone disease at the Icahn School of Medicine at Mount Sinai.
“Cushing’s typically presents with significantly low energy and complications like diabetes, high blood pressure, and high cholesterol. But the telltale sign is very large, red stretch marks on their belly,” she says. If this sounds eerily familiar, talk to your doctor asap.