Here's Why You Can Actually Gain Weight If You Don't Eat Enough (2024)

Whether you deal with constant bloating or you tend to feel heavier during your period, seeing seemingly unexplainable weight gain on the scale can be frustrating. However, weight fluctuation is a natural part of life, and factors like age, decreased activity levels, loss of muscle mass, and a slower metabolism can all contribute to unexplained weight gain.

In some cases, there may be a connection between your weight gain and insulin. “Weight gain around the waistline in adults—outside of pregnancy—is often a sign of insulin resistance or insulin hypersecretion,” says Naomi Parrella, MD, a family physician and obesity medicine specialist at Rush University Medical Group. Although it’s less common, Parrella adds that weight gain may also be a sign of excess cortisol, a condition like Cushing’s syndrome, or in more serious cases, an illness like ovarian cancer.

If a few extra pounds are starting to impact your mental health or way of life, it may be time to consider an appointment with your dietitian or doctor—especially if you're noticing joint pain in the hips and knees, reduced daily activities and movement because of pain, or a worsening mood.

Before you decide to book a visit to your doctor’s office, 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 based in Los Angeles. Perhaps you are eating more calories throughout the day or you’re sitting at your desk more often than usual due to a heavier workload. Even a busy schedule involving being away from home with lots of travel can impact unexplained weight gain.

As for a general benchmark, Dr. Parrella says it is common to see a weight increase of one to two pounds per year from the age of 20 to 65. However, if your weight gain is greater than that and you can't seem to pinpoint the underlying reason, here are some potential conditions that may be at play.

Meet the experts: Naomi Parrella, MD, is a family physician and obesity medicine specialist at Rush University Medical Group. She is also the medical director for Rush University’s Center for Weight Loss and Lifestyle Medicine. Melina Jampolis, MD, is an internist based in Los Angeles and the host of the Practically Healthy podcast. Rocío Salas-Whalen, MD, is an endocrinologist at the Medical Offices of Manhattan.

1. You have 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 your metabolism, and if you’ve got an underactive thyroid (called hypothyroidism), your metabolism may slow down, triggering weight gain.

Women with hypothyroidism may also suffer from low energy levels or fatigue, dry skin, hair loss, muscle cramps, or constipation, says Dr. Jampolis. Notice any of these signs and you should book a chat with your doc, who can check on your thyroid with a simple blood test if necessary.

2. You have 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.

3. You are dealing with 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.

4. You're not sleeping well or enough.

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. People who slept just one hour more per week lost more fat than those who slept an hour less, a 2018 study in the journal Sleep found. 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.

5. You have small intestinal bacterial overgrowth (SIBO).

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. Making changes to your diet can also help.

6. You're going through 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.

7. You're taking certain medications.

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 pounds. “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.

8. You have 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, an assistant professor of diabetes, endocrinology, and bone disease at the Icahn School of Medicine at Mount Sinai.

It can sometimes be caused by certain medications like steroids used to treat asthma and autoimmune disorders.

“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.

Cushing’s disease can be managed with medication, radiation, or surgery, adds Dr. Parrella. You'll also want to tamp down your stress levels, which means taking really good care of yourself and finding ways to recharge on a regular basis.

9. You're dehydrated.

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.

Most of us aren’t drinking nearly enough water, says Kristen Neilan, RD, of University of Florida Health. 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.

Make sure to drink plenty of water, especially at higher altitudes (like air travel), on hot days, with fevers and infections, or with extra physical exertion. Choose options with the least amount of sugar—coconut water, clear broth, and of course pure H2O are all fair game.

If your pee looks darker than usual or you're not using the bathroom as often, that's a sign to begin drinking more fluids, notes Dr. Parrella. Other warning signs include fatigue, muscle cramps, and lightheadedness when standing up.

10. You have ovarian cancer.

In rare cases, an expanding belly is the result of an ovarian tumor and the fluid buildup associated with it, says Sanaz Memarzadeh, MD, PhD, a gynecologic cancer surgeon at UCLA Health. “Patients come in with abdominal bloating, and their usual pants are not fitting,” she says. “Sometimes the tumor is so large it can cause distention of the abdomen."

Women are more likely to be diagnosed with ovarian cancer after menopause. But it’s important for women at every age to look out for this symptom, as well as feeling full too quickly, pain in the lower stomach area, and extra pressure on the bladder. See your doc if the bloating persists, especially if your family has a history of ovarian cancer.

11. You quit smoking.

Smoking can often act as an appetite suppressant, so when you quit, the cravings can hit you hard. Smoking can lead to a rise in dopamine, the neurotransmitter responsible for instant pleasure, says Pouya Shafipour, MD, a weight-loss specialist at Paloma Health. It's the same kind of pleasure you get when you eat a sweet snack, like ice cream.

Quitting smoking causes that dopamine level drop, but your craving for it still remains, and this craving for a dopamine hit can sometimes lead you to eat something satisfying, and more than usual. "When one quits smoking, the body still has cravings for dopamine and often people get this craving from excess intake of refined sugar and starch (i.e., candy and other starchy snacks) and gain weight," says Dr. Shafipour.

To counteract the lower levels of dopamine once you quit smoking, it's important to engage in other behaviors, like exercise or meditation, that help release feel-good endorphins and also provide a nice distraction and healthy new habit.

12. You have diabetes.

Type 1 and type 2 diabetes both require insulin management in order to keep blood sugar levels regulated. In people with type 1, the pancreas essentially isn't producing enough insulin, so those that have it need to regularly insert themselves with the hormone. Insulin allows the body to absorb glucose (or sugar) and use it for energy.

Generally, Type 2 diabetes is associated with insulin resistance from a poor diet, a sedentary lifestyle, and unhealthy eating behaviors. That can usually contribute to weight gain in itself, explains Dr. Shafipour. Type 2 diabetics have a higher baseline insulin level which by itself causes more weight gain, typically around the belly," he says.

But an increase in insulin from external hormone treatments can also lead to weight gain. Insulin lets glucose into your blood cells so that it can be stored for energy, but if you're eating more calories than your body needs, your cells will take what they need leaving the remaining glucose to be stored as fat.

To counteract the weight gain, it's important to closely monitor your diet and avoid eating too much fast food or foods high in refined sugar, Dr. Shafipour says.

13. You have other types of cancers.

Most cancers in their early stages will result in weight loss instead of weight gain—unless it's a cancer that causes the release of cortisol, like a tumor in the adrenal gland.

However, as cancer progresses, it can cause weight gain. "This weight gain can be due to growth of the size of the tumor itself or [if it spreads] to other organs like the liver, which can cause fluid buildup in the stomach or the stomach cavity," says Dr. Shafipour.

But don't be too alarmed, as this is usually a worst-case scenario. Most cancers will cause other symptoms that may cause you to see a doctor while it's still in an early stage.

When should I see a doctor for unexplained weight gain?

First, you should take a look at what your lifestyle's like. If your diet is poor, it's normal to gain between a half a pound to a pound of weight a week. Your menstrual cycle can also cause your weight to fluctuate between four and five pounds depending on what stage of your cycle you are.

But when is weight gain a cause for concern? If you're gaining one to two pounds or more a week, and you don't see the numbers going down, then it might be time to see a doctor. "If one notices that they're gaining weight rapidly, one to two pounds a week, and it's not related to menstrual cycle, poor sleep, anxiety or depression, or snacking or overeating, then they should probably see their primary care physician, who will do a thorough history and physical as well as some appropriate laboratory work-up to find the causes of weight gain," says Dr. Shafipour.

A doctor can work with you to determine whether an underlying condition is contributing to your weight gain, and find appropriate remedies to help you maintain a weight that makes you feel good.

Common Questions About Unexplained Weight Gain

Here are some of the most common questions doctors get asked about unexplained weight gain, according to Dr. Parrella—and what she typically advises her patients to do.

Why am I gaining weight even though I'm eating healthy and working out?

“There are many reasons this could be happening and the only way to understand it is to systematically investigate what is going on, like a detective solving a mystery,” says Dr. Parrella. “Weight gain as an adult, outside of pregnancy, is (most of the time) about the hormone balance.”

For many people, this means examining thyroid function. “Thyroid disorders and untreated sleep apnea can cause unexplained weight gain,” she says. “A few cancers can cause unexplained weight gain, but those are less common.” The most common cause of unexplained weight gain is insulin resistance, she says, which is associated with weight gain (especially around the waistline). However, this can be treated and managed in multiple ways, and you should always consult your doc to see what’s best for you.

Some commonly used medications may also cause weight gain, but there may be alternatives that won't bring unwanted side effects. “If you are taking chronic medications, it is worth asking a pharmacist or your doctor if this applies to you,” says Dr. Parrella.

Apart from these reasons, your weight gain may be a sign to evaluate your nutrition, stress levels, physical activity, and sleep habits in detail. There may be certain behaviors (including those you believe are "healthy") that are contributing to your weight gain without you realizing it.

What is unexpected weight gain a symptom of? Could it be menopause, aging, or genetics?

Your weight can certainly be impacted by menopause, aging, genetics, and hormones like insulin, Dr. Parrella says. “How you eat, how you move, and how regularly you get rest can affect your hormones and the genes that are activated,” she explains.

But the good news is, you can help influence how these changes occur in your body, and seeing a provider can be a great first step. Dr. Parrella recommends working with a specialist who can help you uncover your body’s “signaling pattern” and learn how it may be impacting your weight gain. This ongoing process may involve ongoing investigation, self-tracking, and trying different options, she adds.

Weight gain can also be a sign that your body is experiencing chronic stress, Dr. Parella adds—especially if you’re a caregiver for a small child, someone with a chronic disability, or an aging parent, for example. “[If] your work demands interfere with your ability to relax and recharge regularly or to sleep well, you may gain weight even if your daily food intake is low,” she explains.

Can I just use medication to manage my weight, then stop meds later?

The short answer: It’s complicated. “If you use medications to lower your weight, with the medications currently available, you will likely need to stay on those medications to keep the weight down,” says Dr. Parrella. Everyone’s body is different, so always consult your doctor about starting and stopping meds, and what dosage and frequency is right for you.

You may find that medication is helpful for a while, but everyday lifestyle changes might benefit your body just as much. “If your doctor has ruled out obvious and less common medical issues like thyroid disorder or cancer, [the reason] could be the timing of when you eat, how often you eat, if you are drinking alcohol, what you are eating, medications, sleep, or chronic stress," Dr. Parrella says.

I'm barely eating, but I'm still gaining weight. Am I not getting enough calories? Should I eat breakfast?

“[Unexplained weight gain] is rarely about not eating enough calories or whether or not you eat breakfast,” says Dr. Parrella. Weight gain can occur if you’re not “eating the right nutrients, are eating the wrong nutrients for your body physiology, are not getting adequate sleep, or you have chronic stress with high cortisol,” she adds. It’s entirely possible to eat little and still have unexplained weight gain, and it can also happen if you eat or snack frequently throughout the night, she says.

The key, Dr. Parrella explains, is to eat the right nutrients for you and your body. For example, if you deal with insulin resistance, protein and fibrous vegetables can help keep your insulin at a lower level so that you can get health-promoting nutrients that allow your body to burn fat.

Rather than undereating, Dr. Parrella suggests that every 24 hours, you allow your body at least 12 continuous hours (usually overnight including your sleep hours) of not eating or drinking anything other than water, black coffee, or unsweetened tea. “It is good for your health to let your gut rest on a regular basis,” she says.

Moderating alcohol use may also help, she adds. Try eliminating alcohol for one month and seeing what happens with your weight, or alternatively, explore a sugar detox month without any sweets or sweetened beverages.

How can I stay motivated to eat less and exercise more?

You may have heard that “eating less and exercising more” is the solution for weight loss, but according to Dr. Parrella, weight gain is usually way more complicated than that.

“Motivation is not the problem for most people, and trying to eat less and exercise more is not the answer for most people,” she says. For many, weight gain is a sign of a hormonal imbalance. “Based on what you eat, how you move, how you socialize, sleep, and recharge, your hormones will vary,” she adds. “If your hormones are set up to cause you to grow, that is what you will do. If your hormones are set to burn, then you can burn fat [more easily].”

That said, if you are struggling with motivation to make lifestyle changes, consider your intentions behind wanting to manage your weight. Is it that you want to feel more energized during the day? Make it through your fun workout classes? Feel more confident taking cute selfies? Whatever your reason is, tapping into your internal motivation (and not just rushing to crash diet) can help you make sustainable, long-term changes.