woman preparing a large salad

When it comes to healthy diets, three important factors are balance (not consuming one nutrient at the expense of others), variety, and moderation (via Nestlé). But many consumers — and even health professionals — believe that certain so-called "superfoods" have almost magical health-promoting properties. According to the Harvard’s The Nutrition Source, "There’s no scientifically based or regulated definition for superfood, but generally, a food is promoted to superfood status when it offers high levels of desirable nutrients, is linked to the prevention of a disease, or is believed to offer several simultaneous health benefits beyond its nutritional value." The concept of "superfoods," however, began in the early 20th century when United Fruit Company created a marketing gimmick to promote bananas.

Although the hype around superfoods is more about sales than science, it’s true that some foods are nutritional powerhouses. And if eating a particular food is good for you, wouldn’t eating more of it be even better? Unfortunately, the answer isn’t always yes. Sometimes, overindulging in healthy foods can be disastrous for your health. In some cases, you may get too much of particular vitamins and minerals. In others, you may expose yourself to toxic substances, dangerous pathogens, or damaging "anti-nutrients."

Too much soy could lead to brittle bones

collection of soy products

Soy is a staple in many vegan and vegetarian diets, and for good reason. It’s packed with protein and compounds known as isoflavones. According to one 2008 study published in Inflammopharmacology, "The potential health benefits of isoflavones may include protection against age-related diseases including cardiovascular disease, osteoporosis, hormone-dependent cancer, and loss of cognitive function."

However, soy can also contain high levels of cadmium, according to the Agency for Toxic Substances and Disease Registry. Cadmium is a heavy metal found both naturally in the Earth’s crust and as a part of many fertilizers. Somewhere between 1 percent and 10 percent of the cadmium in the food you eat will enter your body, and chronic, low-level exposure can cause kidney damage and brittle bones.

A 2011 study in Science of the Total Environment found that soy plants take in a significant amount of cadmium from the soil, and the beans can contain three to four times the maximum limit of cadmium set by the Codex Alimentarius Commission. Another study published in the same journal observed that tofu, tempeh, and products such as tofu hot dogs, soy burgers, and tofu cheese had the highest concentrations of cadmium of all the foods researchers tested.

Too many cruciferous vegetables can be dangerous for certain people

broccoli crowns

Cruciferous vegetables are touted as some of the healthiest foods out there (via Cleveland Clinic). This large group, known as the Brassica family, includes arugula, broccoli, Brussels sprouts, cabbage, cauliflower, chard, collard greens, kale, turnips, and many others. They’re packed with important nutrients like soluble and insoluble fiber, folate, vitamin C, potassium, selenium, and phytochemicals that act as powerful antioxidants. They also contain glucosinolates, enzymes, and other compounds that may prevent damage to DNA and protect against cancer.

But, according to the Kresser Institute, some foods, including cruciferous vegetables, are also goitrogens, meaning they interfere with the thyroid’s ability to take up the iodine it needs to make the hormones that control metabolism. This interference may be particularly problematic for individuals who have a preexisting thyroid disorder.

The institute noted, "Crucifers are the biggest goitrogenic offenders, with certain varieties of kale, collard greens, and Brussels sprouts at the top of the list." Luckily, you don’t have to swear off this group of healthy foods entirely; you may just need to change how you prepare them. "Steaming crucifers until fully cooked reduces goitrogens by two-thirds," according to the institute. Boiling crucifers for 30 minutes destroys 90 percent of the goitrogens."

In excess, tea can wreak havoc on your body

tea kettle and cups

A daily cup of tea offers numerous benefits. Of course, the health effects may differ according to the variety of tea you drink (via WebMD). For example, white tea contains the most antioxidants, while green tea boasts the highest levels of the anti-inflammatory compound EGCG (epigallocatechin gallate).

In general, tea may reduce your risk for chronic conditions such as type 2 diabetes and heart disease, promote weight loss, and offer protection against certain types of cancer. But tea may also contain high levels of fluoride. It’s true that fluoride does wonders for protecting your teeth (which is why it’s likely in your tap water and toothpaste), but too much of it can be a bad thing (via Healthline). According to Medical News Today, excess fluoride consumption can cause a variety of problems, including tooth discoloration, bone and joint pain, neurological issues, and reproductive dysfunction.

The tea bush, Camellia sinensis, from which white, green, oolong, and black teas are made, is very good at taking up fluoride from the soil. In fact, research has shown that some teas contain as much as 6.5 parts per million (ppm) of fluoride, which exceeds the Food and Drug Administration’s restrictions (2.4 ppm) as well as the Environmental Protection Agency’s restrictions (4 ppm).

Eat too many almonds and you risk mineral deficiencies

almonds

Nuts make an excellent snack, and almonds are one of the healthiest options (via Healthline). A single ounce is packed with 6 grams of protein, 3.5 grams of fiber, and respectable amounts of vitamin E, magnesium, manganese, copper, riboflavin (vitamin B2), and phosphorous. Research suggests that these and other compounds in almonds may lower blood pressure and cholesterol, improve blood sugar control, and fight free radical damage. Even though they’re calorically dense, almonds may actually promote weight loss.

The bad news for nut lovers is that almonds contain the anti-nutrient phytate. According to Healthline, phytate (phytic acid) is a substance found in the seeds of plants. It provides a stored source of the mineral phosphorous, which the seed will need when it begins to grow. When eaten by humans, however, phytate can impair the absorption of minerals such as iron, zinc, and calcium and may lead to mineral deficiencies.

Healthline noted that almonds are very high in this anti-nutrient, containing between 0.4 and 9.4 percent phytate by dry weight. Rather than giving up almonds entirely, try sprouting them by soaking them in water. This starts the germination process, according to HuffPost, and removes a significant amount of the phytate.

Too much brown rice can lead to disease

bowl of brown rice

Health professionals recommend opting for the "whole" version of grains rather than their refined counterparts, and rice is no exception. Healthline noted that unlike white rice, brown rice contains the outer bran and germ, which is where most of the nutrition resides. A cup of brown rice contains 5 grams of protein, 3.5 grams of fiber, 88 percent of your daily manganese needs, and respectable amounts of many other vitamins and minerals. It’s a great choice for those following a gluten-free diet and may help control blood sugar levels and safeguard heart health.

Brown rice, however, can contain high levels of inorganic arsenic. A report released by the Food and Drug Administration in 2016 found that brown rice contained 154 parts per billion (ppb) of arsenic, compared to 92 ppb in white rice. The reason for the discrepancy? When the rice plant takes up water from the soil, any arsenic in the water becomes concentrated in the outermost layers of the rice grain, which are left intact in brown rice. According to the World Health Organization, "Long-term exposure to arsenic … can cause cancer and skin lesions. It has also been associated with cardiovascular disease and diabetes."

Kidney stones can result from eating too much spinach

raw spinach in bowl

Spinach is packed with vitamins and minerals, including carotenoids, vitamin C, vitamin K, folic acid, iron, and calcium (via Healthline). It also contains powerful antioxidants such as lutein, zeaxanthin, and quercetin. But despite its many pros, spinach has one big con: oxalate. According to Healthline, oxalate (oxalic acid) is an organic compound found in many plant foods. Oxalate binds to minerals — usually calcium — to create crystals, Urology of Virginia explained, and these crystals leave the body through both stool and urine. In some individuals, however, these crystals collect in the kidneys and become kidney stones.

While several substances can concentrate in the kidneys and become stones, the National Kidney Foundation noted that calcium oxalate stones are the most common. Kidney stones can be extremely painful to pass and may require surgical removal. They’re also much more common than you might think: Roughly 7 percent of women and 13 percent of men will get kidney stones at least once in their lives, and having one kidney stone puts you at much higher risk for additional stones.

The University of Chicago identified spinach, both cooked and raw, as the food with the highest oxalate content per serving. For those who need to follow a low-oxalate diet, they recommend avoiding spinach entirely.

Too much grilled chicken breast can lead to chronic disease

grilled chicken breast

In addition to being a rich source of protein, grilled chicken offers vitamin B12, choline, iron, zinc, and copper (via WebMD). Even though chicken is a staple of many health-conscious diets, grilling it may not be doing you any favors. When cooked at high temperatures, chicken contains advanced glycation end products (AGEs), Healthline explained. AGEs form when protein or fat combine with sugar in the bloodstream.

According to research published in the Journal of the American Dietetic Association, average AGE consumption is about 15,000 kilounits (kU) daily. If you regularly consume significantly more than that, you may risk setting yourself up for problems. A serving of chicken breast grilled for only four minutes contains approximately 4,364 kU of AGEs, so you can see how it adds up.

A 2017 paper found that AGEs accelerate the aging process on a cellular level. AGEs are a biological waste product that builds up throughout the body and causes a "loss of protein function and impaired elasticity of tissues," according to the paper. AGEs have been linked to chronic diseases like Alzheimer’s disease and diabetes, and the authors concluded that preventing "AGE formation and accumulation in tissues can lead to an increase in lifespan."

Excessive lemon water can damage your teeth

pitcher of lemon water

Is there anything that screams "health" louder than a glass of lemon water first thing in the morning? According to Medical News Today, lemon water is a great way to stay hydrated. Plus, it provides a substantial amount of vitamin C. The popular drink includes antioxidants that protect cells from free radical damage, and its high citric acid levels may prevent kidney stones from forming.

Devotees of lemon water claim it offers many benefits for your skin. It may fight inflammation and the hydration it provides keeps skin looking young and vibrant. The vitamin C in the lemon can stimulate collagen growth, and the citric acid may help fade brown age spots, dermatologists told Insider.

However, lemon water isn’t necessarily good for your teeth. In an interview with Cosmopolitan, Dr. Luke Thorley, a dentist, explained that "citrus fruits like lemon and lime are very acidic and can erode teeth enamel." He noted that the eroded enamel makes it easier to see through to the layer of yellow dentin underneath the tooth’s surface, giving teeth a yellowed appearance. The American Dental Association cautioned that citrus fruits like lemon can also irritate mouth sores.

Too many oysters can make you sick

person carrying a platter of oysters

According to the National Institutes of Health, the mineral zinc plays many important roles in the body. In addition to being a necessary component of about 100 enzymes, zinc plays a part in immune function, protein and DNA creation, wound healing, and cell division. It’s particularly critical during times of rapid growth, such as pregnancy, childhood, and adolescence. Adult men and pregnant women should aim for 11 milligrams daily, while non-pregnant women need 8 milligrams and lactating mothers need 12 milligrams.

Oysters are far and away the best dietary source of zinc. Just 3 ounces of cooked oysters contains 74 milligrams, more than six times the recommended daily allowance. Although this is significantly higher than the 40 milligrams" tolerable upper intake limit" (UL) for zinc, the main danger of over-consuming oysters — especially raw ones — comes from pathogens and pollutants (via Healthline). As filter feeders, oysters may pick up harmful Vibrio bacteria, which can cause diarrhea, vomiting, and fever. In some cases, these infections may be more serious, even fatal. Oysters may carry norovirus, the virus most often responsible for the so-called "stomach flu." These mollusks can also contain high levels of heavy metals such as mercury, lead, and cadmium.

Too many servings of albacore tuna can cause mercury poisoning

cans of tuna

With a milder flavor than some other types of fish, canned tuna is a cheap and delicious source of protein and omega-3 fats. Experts recommend getting at least 250 to 500 milligrams daily for optimal health, and albacore tuna has 500 to 1,000 milligrams per 3-ounce serving (via Healthline).

Unfortunately, albacore tuna also contains significant levels of mercury. According to the Environmental Defense Fund (EDF), albacore tuna has 0.32 parts per million (ppm) of mercury, while "light" (skipjack) canned tuna contains only 0.12 ppm. Because of its higher mercury levels, albacore tuna shouldn’t be eaten very often. Children under six should only consume it once per month, while children aged six to 12 can have it twice a month. Teens and adults can up their intake to three times per month (via EDF). Although light tuna contains significantly less mercury, it also offers a more modest amount of omega-3s: just 200 to 500 milligrams per 3-ounce serving.

The Environmental Protection Agency noted that exposure to mercury can cause a number of adverse effects. Methylmercury (the form found in fish) is a powerful neurotoxin that can lead to loss of peripheral vision, lack of coordination, pins-and-needles sensations in the limbs, problems with speech and hearing, and muscle weakness.

Brazil nuts in excess can be toxic

Brazil nuts

Selenium is a trace mineral you’ve probably never heard of, but it’s essential for good health. According to the National Institutes of Health, selenium is "a constituent of more than two dozen selenoproteins that play critical roles in reproduction, thyroid hormone metabolism, DNA synthesis, and protection from oxidative damage and infection."

The recommended dietary allowance (RDA) has been set at 55 micrograms for adult men and non-pregnant women, 60 micrograms for pregnant women, and 70 micrograms for breastfeeding mothers. Brazil nuts are inordinately high in selenium: A single ounce contains 544 micrograms, almost 10 times the RDA. In fact, that’s more selenium than the upper tolerable intake level (UL), which is set at 400 micrograms for all adults.

Excess selenium can cause both acute and chronic toxicity (via San Francisco Chronicle). The first clue that you may have overdosed on selenium (a condition known as selenosis), is a garlic smell to your breath and a metallic taste in your mouth. Other signs include nail and hair loss or brittleness, neurological issues, and gastrointestinal problems like diarrhea and nausea. Individuals may also experience rashes, skin lesions, fatigue, and irritability. In the most severe cases, kidney damage, heart failure, and even death may occur.

You’ll consume too many vitamins if you eat too much beef liver

beef liver on plate

Despite having a taste and texture that turn many people off, there’s probably no food more packed with nutrients than liver. According to WebMD, a 3-ounce serving of cooked beef liver contains 23 grams of protein. Liver also contains an almost absurd amount of certain micronutrients (via Healthline). For example, 100 grams (3.5 ounces) of beef liver contains a whopping 860 to 1,100 percent of the RDA for vitamin A, 3,460 percent of your vitamin B12 needs, and 1,620 percent of your copper requirement. It also has significant amounts of iron, choline, riboflavin (vitamin B2), and folate (vitamin B9).

But sometimes you really can get too much of a good thing. If you eat liver regularly, you may be seriously overdoing it on both vitamin A and copper. (Although liver contains truly staggering levels of B12, this is a water-soluble vitamin, meaning any excess is excreted in urine.)

The National Institutes of Health (NIH) noted that "chronic intakes of excess vitamin A [can] lead to increased intracranial pressure (pseudotumor cerebri), dizziness, nausea, headaches, skin irritation, pain in joints and bones, coma, and even death." When it comes to copper, chronic overexposure can cause liver damage and gastrointestinal distress (via NIH). Although copper toxicity is rare in healthy individuals, certain genetic conditions can make it more likely.

Too much cinnamon can damage your liver

cinnamon in various forms

Cinnamon is one of those rare foods that’s both extremely healthy and has a taste most people love. According to WebMD, there are two types of cinnamon commonly sold in the United States: cassia and Ceylon ("true") cinnamon. A number of studies, mostly conducted on animals, suggest that cinnamon may help lower blood sugar and cholesterol levels in diabetic individuals. It may also assist with weight loss, irritable bowel syndrome, allergies, and infections. Cinnamon has both antioxidant and anti-inflammatory properties. The compound cinnamaldehyde is believed to be behind many of cinnamon’s supposed health benefits.

But cassia cinnamon — the cheaper, more widely available form of the spice — contains high amounts of coumarin, a compound that could have adverse health effects, per Healthline. And you don’t have to eat a lot of cinnamon to get too much coumarin. One teaspoon of ground cassia cinnamon contains 7 to 18 milligrams of coumarin. The tolerable upper intake limit (UL) for coumarin is 0.05 milligrams per pound of body weight, so the UL for a 130-pound woman would be only 6.5 milligrams, Healthline explained. Excess consumption of coumarin has been linked to liver damage, mouth sores, and an increased risk for certain types of cancer.

Drink too much water and you risk water poisoning

woman drinking a glass of water

Keeping well-hydrated has a number of benefits, including maintaining proper fluid balance, moving nutrients throughout the body, assisting the kidneys with filtering the blood, and keeping skin healthy and youthful (via WebMD).

The Institute of Medicine (IOM) recommends women aim for 91 ounces (2.7 liters) of fluids each day, while men should get 125 ounces (3.7 liters). The IOM noted, however, that not all fluid has to come from plain water, and everyone’s exact hydration requirements vary. Factors such as exercising, temperature, illness, and pregnancy can all impact fluid needs.

But even with something as essential as water, you can get too much of a good thing. Water intoxication, also known as water poisoning or hyponatremia (low sodium levels), occurs when someone drinks too much water too quickly, according to Medical News Today. This dilutes the concentration of important electrolytes such as sodium. Water intoxication can cause confusion, disorientation, nausea, and vomiting. Severe cases may lead to seizures, brain damage, coma, and even death. Drinking more than 3 or 4 liters in a short period of time could put you at risk for hyponatremia, revealed a study published in Annals of Pediatric Endocrinology & Metabolism.