Many things can cause depression, such as financial trouble, the loss of a loved one or an illness. In addition, there may not be a concrete reason for your depression.

However, experts have pinpointed a few surprising things that can cause depression – that most people would never suspect.

1. Not Getting Enough Omega-3

Low intake of omega-3 fatty acids, found in salmon and vegetable oils, may be associated with a greater risk of depression. A 2004 Finnish study found an association between eating less fish and depression in women, but not in men. These fatty acids regulate neurotransmitters like serotonin, which could explain the link. Fish oil supplements may work too; at least one study found they helped depression in people with bipolar disorder.

2. Not Sleeping Enough

It’s no surprise that sleep deprivation can lead to irritability, but it could also increase the risk of depression.

A 2007 study found that when healthy participants were deprived of sleep, they had greater brain activity after viewing upsetting images than their well-rested counterparts, which is similar to the reaction that depressed patients have, noted one of the study authors.

“If you don’t sleep, you don’t have time to replenish [brain cells], the brain stops functioning well, and one of the many factors that could lead to is depression,” says Matthew Edlund, MD, director of the Center for Circadian Medicine, in Sarasota, Fla., and author of The Power of Rest.

3. Taking Birth Control Pills

Like any medication, the Pill can have side effects. Oral contraceptives contain a synthetic version of progesterone, which studies suggest can lead to depression in some women. The reason is still unknown, says Hilda Hutcherson, MD, clinical professor of obstetrics and gynecology at Columbia University, in New York. “It doesn’t happen to everyone, but if women have a history of depression or are prone to depression, they have an increased chance of experiencing depression symptoms while taking birth control pills,” Dr. Hutcherson says. “Some women just can’t take the Pill; that’s when we start looking into alternative contraception, like a diaphragm, which doesn’t contain hormones.”

4. Smoking

Smoking has long been linked with depression, though it’s a chicken-or-egg scenario: People who are depression-prone may be more likely to take up the habit.

However, nicotine is known to affect neurotransmitter activity in the brain, resulting in higher levels of dopamine and serotonin (which is also the mechanism of action for antidepressant drugs).

This may explain the addictive nature of the drug, and the mood swings that come with withdrawal, as well as why depression is associated with smoking cessation. Avoiding cigarettes—and staying smoke free—could help balance your brain chemicals.

5. Having Thyroid Disease

When the thyroid, a butterfly-shaped gland in the neck, doesn’t produce enough thyroid hormone, it’s known as hypothyroidism, and depression is one of its symptoms. This hormone is multifunctional, but one of its main tasks is to act as a neurotransmitter and regulate serotonin levels. If you experience new depression symptoms—particularly along with cold sensitivity, constipation, and fatigue—a thyroid test couldn’t hurt. Hypothyroidism is treatable with medication.

6. Taking Prescription Drugs

Depression is a side effect of many medications. For example, Accutane and its generic version (isotretinoin) are prescribed to clear up severe acne, but depression and suicidal thoughts are a potential risk for some people. Depression is a possible side effect for anxiety and insomnia drugs, including Valium and Xanax; Lopressor, prescribed to treat high blood pressure; cholesterol-lowering drugs including Lipitor; and Premarin for menopausal symptoms. Read the potential side effects when you take a new medication, and always check with your doctor to see if you might be at risk.

7. Living In The City

You can endlessly debate whether city or country life is better. But research has found that people living in urban settings do have a 39% higher risk of mood disorders than those in rural regions. A 2011 study in the journal Nature offers an explanation for this trend: City dwellers have more activity in the part of the brain that regulates stress. And higher levels of stress could lead to psychotic disorders.

Depression rates also vary by country and state. Some states have higher rates of depression and affluent nations having higher rates than low-income nations. Even altitude may play a role, with suicide risk going up with altitude.

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