I live alone, which means I usually cook things I can eat in one sitting. But occasionally, I have leftovers. As convenient as it is to pan-fry one juicy chicken breast on a hectic weeknight, sometimes I want a casserole, a pasta dish, or soup–all meals that weren’t necessarily created with single folks in mind. And when I cook those dishes to enjoy for more than one meal, the microwave becomes my reliable companion throughout the week. Or, at least it used to be before I discovered that reheating certain items can be a health hazard.

Certain cooked ingredients, if reheated (particularly after being stored improperly), can actually make you physically ill. And I don’t say that to rile up panic, it’s simply important to be aware of the fact that heating foods changes their chemical structure, and for some ingredients, these changes that are spurred by temperature shifts can make the food incompatible with the human digestive system. So if you’re all about the leftovers, play it safe and avoid warming up these eight common foods a second time:

Spinach

Sautéed too much spinach for a quick and nutritious side dish at dinner? If you can’t eat it all right after it’s cooked, it’s best to just toss it or eat the leftovers cold (maybe stir them into a pasta salad). To avoid food waste in the future, aim to cook only what you need for the meal at hand. Spinach contains a high quantity of nitrates, which provide vital nutrients our bodies need to function. When we eat certain vegetables raw, something magical happens in the body that turns those good-for-you nitrates into nitrites. Nitrates don’t become a problem until the heating process activates them, prompting them to release poisonous carcinogenic effects when the body processes them. Every time you reheat spinach or other veggies that are rich in nitrates, they become increasingly toxic.

Potatoes

Here’s the deal on those spuds. When cooked potatoes are left out at room temperature or warmed up for a second time, they can take a toxic turn for the worst. Why? Warm temperatures promote the growth of the rare bacteria, botulism, that is commonly found in potatoes. If you can’t bear throwing leftovers away, the best solution is to refrigerate uneaten cooked potatoes immediately. As in, don’t pull them from the oven and let them stand for an hour or so until they reach room temp, and then pack them away. If you find yourself with quite a few leftovers, store the potatoes in multiple plastic containers and refrigerate promptly so that they cool down faster.

Celery and Carrots

The same rules outlined above for spinach likewise apply to celery and carrots. When possible, it’s safer to take celery and/or carrots out of a dish before reheating it.

Rice

Same deal as the potatoes here, don’t leave rice out at room temperature after it’s cooked. If stored incorrectly, cooked rice can develop bacterial spores that may produce poisons that cause intense physical illness. These spores multiply faster at room temperature than in the fridge. To avoid food poisoning or other digestive upset, make sure those fluffy grains are stored in the fridge in an airtight container right after cooking.

Mushrooms

Mushrooms are probably the most apt to make you ill of the items on this list, largely because of how vulnerable they are to microorganisms. When eating cooked mushrooms, it’s best to eat them immediately after they’re prepared. And if you plan to eat on them again the next day, make sure you eat them cold from the refrigerator because reheating mushrooms can be bad news for your belly.

Beets

We all know how good beets are, both in their flavor and nutritional benefits. But beets, like celery, spinach, and carrots, are rich in nitrates. Your safest bet for beets is to only cook what you think you’ll actually eat in one sitting, or plan to eat them cold (like on salads and such).

Eggs

A fantastic protein source for sure, cooked eggs can be a source of serious sickness when left at or re-exposed to higher temperatures. Whether boiled or scrambled, reheating eggs can be destructive to your digestive system. Not to mention… reheated rubbery eggs are kind of gross anyway. Just don’t.

Chicken

Another favorite protein source and dinner staple, chicken is kind of tricky when it comes to leftovers. The protein in chicken starts to deteriorate and causes digestive problems when it goes from cold to hot the second time around. A general rule of thumb if you want to enjoy leftover chicken warm is to reheat it in the microwave, a skillet, or the oven only one time after the original preparation. You also need to make sure it’s hot–as in completely hot through and through to the center of the piece of chicken, and eaten right away.

Now all of this is to say–don’t live in fear of leftovers, but do be careful and mindful of how you store them and enjoy them for a second time around. Leveraging leftovers is a time-saving, cost-effective strategy in the kitchen and I don’t plan on giving up on leaning on them anytime soon. The important takeaway here is that it never hurts to err towards the side of caution when it comes to what we’re putting into our bodies. And that includes being aware that some foods have a greater potential for toxicity when reheated than others. Does that mean that if you eat a reheated soup that contains celery and carrots or make a next-day hash using last night’s roasted potatoes that you’re guaranteed to find yourself with your head in the toilet (or worse)? Obviously not. I’m sure you’ve done so plenty of times without harmful side effects, but you may have experienced some digestive discomfort that you don’t even remember now–something mild that could have been worse under slightly different circumstances.

Point being, be cognizant of what you’re cooking and what parts of your dinner your packing for an office lunch the next day.

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