Canned Tuna

This easy-to-prep, low-cal fish is a healthy salad or sandwich go-to. However, eating too much of it can result in dangerous mercury levels since tuna is higher in mercury than many other fish. Excessive mercury intake can lead to vision problems, impaired hearing and speech, lack of coordination, and muscle weakness. Make sure you don’t consume more than three to five cans of tuna a week, says Sam, and try swapping out tuna for lower-mercury fish like salmon, shrimp, and Pollock.

Water

While hydration is key to good health, excessive water consumption can cause water intoxication, says Alan R. Gaby, M.D., author of the textbook Nutritional Medicine. This happens when extreme water intake dilutes the sodium in the body, resulting in an abnormally low blood sodium level, which can lead to impaired brain function and even death. How much water would you have to drink for this to be an issue? It's typically only a problem for ultramarathoners and people who force themselves to over-drink. But to make sure you're not consuming too much water, check your urine: If it's always transparent, reduce your intake.

Soy
Tofu lovers, listen up! While soy can help control cholesterol levels and blood pressure when consumed in moderation, it also inhibits iron absorption, says Gaby—so excessive intake has the potential to cause iron-deficiency anemia. Also, because soy contains estrogen-like compounds (isoflavones), long-term consumption of large amounts has the potential to cause endometrial hyperplasia, a proliferation of the uterine lining that can lead to uterine cancer. While there's no exact cutoff for how much soy is safe, you probably want to stick to two servings per day or less.

Spinach

This versatile and delicious leafy green is packed with goodness: It’s an excellent source of protein, fiber, and various vitamins and minerals. Spinach is also high in lutein, a carotenoid that may help prevent age-related macular degeneration (a common cause of vision loss and blindness), says Gaby. However, spinach is also high in oxalate, a compound that can lead to the formation of kidney stones—so patients with calcium oxalate kidney stones should avoid overdoing it with this veggie.

Brazilian Nuts

This crunchy snack is a great source of protein, fiber, essential fatty acids, and selenium. Plus, eating nuts has been shown to lower serum cholesterol and blood pressure, says Gaby. Still, you shouldn’t nosh on these nuts every day since they're extremely rich in selenium and the nutrient can be toxic in high amounts, says Gaby. Taking in too much may result in hair loss, brittleness or loss of nails, dermatitis, neurological abnormalities, and in severe cases, death. Joel Fuhrman, M.D., a family physician, nutritional researcher, and author of The End of Dieting, says that shelled Brazil nuts maintain higher selenium levels—so you shouldn’t have more than 10 a day.

Lean Animal Protein

If you primarily rely on low-fat meats like chicken breasts or egg whites for your daily dose of protein, it might be time for a diet makeover. Fuhrman says that consuming too many animal proteins can be dangerous since it makes your body produce the hormone insulin-like growth factor 1 (IGF-1), which promotes aging and increases your risk of cancer (particularly breast cancer) when present in high levels. In fact, a study published in the journal Cell Metabolism found that there was a four-fold increase in cancer-related death risks and a 75 percent increase in overall mortality in people who got at least 20 percent of their calories from lean animal protein (as opposed to 15 percent or less). The simple fix? Try to get the majority of your protein from plants, such as beans, nuts, seeds, and whole grains, says Fuhrman.