The week before my period is a bit of a drag, especially at the gym. The moment I start retaining water, my workouts get tougher. Going for a run feels like trudging through mud, my heart rate spikes, and weights feel heavier.

It turns out rising hormones not only affect a woman’s reproductive organs, but they influence metabolism, heat tolerance, mood, and fluid retention, too, writes Stacy Sims, an environmental exercise physiologist and nutrition scientist, in her book “Roar: How to match your food and fitness to your unique female physiology.”

All this makes exercise feel harder and your clothes a little tighter. Here’s how adding a little sodium to your water can help you hydrate better.

How salt can help
We’re told to drink more water to reduce bloating, but plain water isn’t really effective in hydration, says Lynn Rogers, research assistant professor of physical medicine and rehabilitation at Northwestern University. We need sodium — and a little glucose — to appropriately absorb water into the blood supply.

And if you’re a menstruating woman, your sodium and water needs change throughout your cycle, says Rogers, who’s also the director of the Neural Plasticity Laboratory at the Shirley Ryan AbilityLab in Chicago, where her group is investigating how female hormones affect the neuromuscular system.

Estrogen and progesterone peak during the luteal phase — around a week before the period starts. Estrogen shifts fluid from the blood to other parts of the body, hence the snug pants. At the same time, progesterone decreases sodium stores, which makes it harder to hold water in the blood, she says.

So, does losing fluid and sodium cause bloating? “It seems counterintuitive, right?” says Rogers. This reduction in fluid — about 8% in plasma volume — takes the displaced water and stores it in bodily tissue, resulting in puffiness, she says.

Why exercise is harder
Exercise is recommended for relieving PMS, but with less plasma volume, the heart has to work a little harder to pump blood through the system. This makes it more difficult to get blood, oxygen, and nutrients to the muscles, says Rogers. Progesterone also raises our core body temperature, which makes it harder to sweat and cool off.

There are few studies that address the menstrual cycle. But research shows performance is negatively affected when active women exercise in the heat during their luteal phase. A study by Sims and her team shows that adding sodium before exercise increased plasma volume and aided endurance in female athletes during their high hormone phase.

What about when you’re on hormonal birth control? Well, you’re technically always in your high hormone phase, explains Rogers.

Scientists have often avoided studying the menstrual cycle because “it is complex,” says Rogers. She says that this has created a void in research and a potentially incorrect assessment of why a woman is retaining water and how she can relieve it.

“We’ve studied men forever and, in many cases, specifically avoided women because (researchers) understood that hormones were going to interact and get in the way,” says Rogers. “It was too complicated for the question they were looking at.”

How to work with your cycle
Whether you’re hitting the gym or not, adding a little sodium throughout the day can be beneficial for hydration during the luteal phase, says Rogers. Salt can help move fluid where it needs to go so that the body can pump blood more efficiently, instead of just storing extra water in the tissue.

If you’re going to exercise, Rogers suggests eating some chicken or miso soup the night before. Don’t worry about getting a particular percentage of sodium, but don’t overdo it. “Just overloading with extra amounts is not the best way to go,” says Rogers.

Since I’m not a professional athlete, I just add some salt to my oatmeal and throw in a banana for some glucose. If I’m going for a moderate workout, I’ll eat a higher sodium Clif Blok with 8 ounces of water beforehand.

Stay active
Even if your period makes you feel sluggish, exercise can help mediate some of the hormonal changes. The more fit you are, the less impact the circulating hormones have on your body, says Sims, who helped create OSMO hydration for women.

Since estrogen reduces your carb-burning ability — instead pulling from your fat stores — Sims suggests trying lower intensity endurance exercises like longer slower runs or bike rides. Do some (not heated) yoga and don’t try to hit any personal records.

Should I worry about adding sodium?

If you’re sweating on a regular basis and you don’t have a history or diagnosis of hypertension or kidney disorder, then there is no reason to limit your sodium, says Sims. And since women in the high hormone phase kick out total body sodium, adding salt is not harmful, she adds.

Keri Wiginton is a writer and photographer focusing on issues related to women's health, mental well-being, and feminism. Her work has appeared in the Chicago Tribune, Washington Post, Austin-American Statesman, Tampa Bay Times and Houston Chronicle. Follow her work at www.keriwiginton.com or on Twitter at @keriphoto.