From your birth control to that new shirt, there are lots of reasons why your sweat might smell.

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If you think your roommate's lack of deodorant use is highly questionable, know this: Some people actually don't have smelly sweat. If you're not one of the lucky ones and are left scouring search results for answers to "why does my sweat smell," the good news is you have more control over your stench than you think.

Turns out, your armpit doesn't smell because of the sweat itself, but rather it's the bacteria that feast on your perspiration that causes the sweat to smell.

Here are nine possible answers to the age-old question of "why does my sweat smell?" Just remember this before you spend too much time on your brand scent: Sweat and smelly armpits are just a fact of life — even more so if you're an active human being. Go easy on your body; it's just doing its job.

Learn More: How Much Should You Really Sweat During a Workout?

Stress

If just imagining a blind date makes you sweat, you're not alone. A majority of people report breaking out into a nervous sweat when confronted with something stressful. Anxiety activates the hormone cortisol in your body, which in turn kicks your sweat glands into high gear. The sweat then gets trapped in your leggings or tank, combines with bacteria, and suddenly you have smelly sweat on your hands.

Certain Clothes

Natural fibers including cotton, wool, and linen soak up sweat from your skin and allow it to evaporate. But while synthetic fibers including Rayon, polyester, nylon, and even some natural fibers such as silk look pretty, their ability to repel water and sweat will make your sweat smell worse in the end. Instead of helping move the sweat away from your skin's bacteria, these fabrics keep it trapped on the surface. (Related: How to Shop for Sustainable Activewear)

Your Deodorant

A catch-22 of modern hygiene is that the stuff you buy to help keep your armpit smell away may actually be making the problem worse. Ingredients in some antiperspirants, deodorants, body washes, body sprays, and lotions can encourage smelly sweat by giving bacteria more food. Many doctors now advise patients with body odor problems to use plain water when washing and leave it at that, since sweat itself has no odor. If you're still left wondering why your sweat smells, doctors also suggest trying lemon juice diluted with water or apple cider vinegar — the acid will inhibit bacterial growth, the real source of the stench.

Medication

Still haven't found the answer to why your sweat smells? If it's sudden, it could be your prescription. Many medications have increased sweating or body odor as a known side effect. This includes over-the-counter pain relievers and allergy medications, as well as prescription meds including some anti-depressants, ADHD meds, and even birth control pills. (Related: The Mental Health Side Effects of Birth Control That No One's Talking About)

Nutrient Deficiencies

Eating tons of garlic can make you smell like an Italian restaurant at the gym the next day, but what you don't eat can also affect your body odor. Research has found that people who are deficient in magnesium, a vital nutrient found in leafy greens and nuts, had stronger body odor than people who got enough. Who knew that eating a sweet treat (dark chocolate is a great source of magnesium) could also make you smell sweet? (See: The Benefits of Magnesium and How to Get More of It In Your Diet)

Sweets

Bacteria love sweets, so eating too much sugar can cause an overgrowth of yeast on your skin,  which in turn converts those sugars into alcohols that cause you to smell anything but sweet, according to Edward Group, a certified clinical nutritionist.

Your Bathroom Habits

Holding in your waste will not only make you rush for the bathroom but can also make you smell like one. Research has found that in cases of severe constipation, toxins released by the digestive system may seep through the pores, giving you what's affectionately known as "fecal body odor." In addition, the "ammonia" smell from a urinary tract infection can become so concentrated that the odor comes out through your pores as well. Talk about making a smelly sweat worse!

Fried Food

Anyone who's done a long treadmill run the day after hitting their fave Mexican restaurant knows that foods such as onions, garlic, or other strong foods or spices can make your sweat pretty fragrant, but your French fry cravings may be equally to blame. Apparently, the fats and oils in fried and baked goods can become rancid in your body causing poor digestion and, consequently, body odor.