When healthy foods turn toxic

By Liz Meszaros, MDLinx
Published July 3, 2019

Key Takeaways

The old adage of "everything in moderation" doesn't always just apply to delicious things that are bad for you in excess, like alcohol, a perfectly grilled piece of steak, or double mint stuffed chocolate chip cookies. Surprisingly, it can also apply to eating excessive amounts of some very healthy foods that—despite a plethora of health benefits proven by scientific research—can be very unhealthy in large amounts.

MDLinx searched out a few (there are more) very healthy food items that have a limit when it comes to how much of them you can eat, and you may be surprised at what we found.

Cruciferous vegetables

This family of vegetables includes broccoli, Brussels sprouts, kale, cabbage, and collard greens—and all are linked with numerous health benefits, including reduced risks of both cancer and heart disease.

Green smoothies and various vegetable juices rely heavily on cruciferous vegetables as ingredients. But beware: cruciferous vegetables contain thiocyanates and/or goitrins. In excess amounts, both of these naturally occurring chemicals can disrupt thyroid function and inhibit the body's ability to absorb iodine.

Goitrins can actually block the incorporation of iodine into thyroxine (T4) and triiodothyronine (T3). In addition, they can slow the release of thyroid hormones by the thyroid gland and disrupt peripheral conversion of T4 (storage hormone) into T3 (active hormone). Thus, excessive amounts of both thiocyanates and goitrins may contribute to the development of hypothyroidism.

In particular, those with a family history of thyroid problems, or those who are predisposed to it, should avoid eating too many cruciferous vegetables.

Omega-3 fatty acids and fish oils

The many benefits of omega-3 fatty acids include fighting inflammation, promoting brain development and function, and reducing the risk of cardiovascular disease. Omega-3 fatty acids can be found in fish and other seafood, especially cold-water, fatty fish—including salmon, mackerel, tuna, herring, and sardines—as well as in nuts and seeds (think flaxseed, chia seeds, and walnuts). They are also present in plant oils, like flaxseed oil, soybean oil, and canola oil.

Since most people do not get enough omega-3, supplements have become popular, and include omega-3 produced from fish, fish liver, and algae.

A normal dose of omega-3 is 1-6 g/d. Too much (13-14 g/d) may have antithrombotic effects, which can be risky in those prone to bleeding, and those taking anticoagulant medications. In addition, excessive amounts of omega-3 fatty acids produced from fish-liver oil may cause an excessive intake of vitamin A. This could lead to vitamin A toxicity, and is especially unsafe in children and pregnant women.


The praises of coffee as a pick-me-up have been sung since the 16th century and perhaps even before. Coffee culture is a worldwide phenomenon, and drinking the aromatic brew has become a daily ritual for millions over the centuries. Recently, modern researchers have documented the benefits of coffee in assuaging a number of health conditions, including cirrhosis, type 2 diabetes, and neurodegenerative disorders.

Caffeine is the active ingredient in coffee, and each cup holds roughly 80-120 mg. Experts consider a safe dose of caffeine to be about 400 mg/d. Go over 500-600 mg/d, and you have crossed over into "overdose" territory.

Too much caffeine can cause a number of harmful side effects, including insomnia, nervousness, irritability, stomach cramps, and muscle tremors. Excess caffeine can also overwhelm the nervous system and even cause heart palpitations.

Unfortunately, the amount of caffeine needed to qualify as "excessive" varies from person to person. Some people can drink coffee all day and even into the evening. Others experience symptoms of excess even with small doses.


Cinnamon is an ancient spice made from the inner bark of the Cinnamomum tree that dates as far back as 2,800 BC, according to Chinese writings. Native to Sri Lanka, this delicious spice was used by the ancient Egyptians in the embalming process, and even by medieval physicians for coughs, hoarseness, and sore throats.

In fact, cinnamon was so valuable that it was once used as currency and purportedly instigated a war. Today, it is used to add rich notes to breakfasts, desserts, coffees, and even in some meat dishes.

Cinnamon contains large amounts of antioxidants, and has been shown to reduce inflammation and serum sugar levels, as well as reduce the risks of cardiovascular disease, diabetes, cancer, and neurodegenerative diseases.

But cinnamon contains large amounts of a flavoring compound called coumarin, for which the daily intake is set at 0.1 mg/kg of body weight. There are two types of cinnamon: Cassia (regular cinnamon), which contains high doses of coumarin, and Ceylon (true cinnamon), which contains much less. Eating more than 0.5 to 2 g of Cassia cinnamon per day and over 5 g (1 tsp) of Ceylon cinnamon is not recommended. Anything more could cause not only liver toxicity, but cancer as well.

Eating cinnamon occasionally, in small amounts, is perfectly alright—but don't consume larger quantities frequently.

Brazil nuts

Brazil nuts are high in selenium, an essential trace element. In fact, just one Brazil nut contains 69-91 μg of selenium. Since the recommended daily allowance of selenium for adults is 50-70 μg, just one Brazil nut a day will suffice.

Selenium is an antioxidant and, therefore, has powerful anti-inflammatory properties. It also has protective effects on cognitive function, fertility, and the immune system as well. However, selenium can be toxic in higher amounts, and consuming more than about 300 μg/d of selenium is not recommended. Since 4-5 Brazil nuts is pushing this upper limit, you must be careful not to eat more, because you may develop selenium toxicity. Signs of selenium toxicity include hair loss, nail loss, digestive problems, and memory deficits.

So beware when you are dipping into that mixed nut bowl at your next picnic, barbeque, or get-together. Stick to the peanuts, almonds, and cashews—and eat only one or two Brazil nuts.

Since the 1st century BC, the Roman statesman, orator, lawyer, and philosopher Cicero preached moderation: "Never go to excess, but let moderation be your guide."

It turns out that this centuries-old advice is still good. Practice moderation in your diet—even with foods that are healthy, and you will not go wrong.

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