"Nightshade" is a huge family of plants that covers everything from tomatoes to potatoes. And belladonna too, commonly called "deadly nightshade".

The family as a whole is not considered to be different than other fruits or vegetable.

The general reasons why anyone would try to avoid anything are "medically diagnosed allergy", "self diagnosed allergy", or "fad diet that they got suckered in to".

Solanaceae, or the Nightshades, are a family of flowering plants. It's a pretty broad category of plants, including things like tomatoes, eggplant, and potatoes, but also tobacco, goji berries, and a bunch of different decorative plants.

Some nightshades are poisonous (like the aptly named Deadly Nightshade - Atropa belladonna.) Some nightshades will make you hallucinate. Some have medicinal properties.

Nightshades contain solanine and that is thought to increase inflammation. The data for this is not consistent. Some patients do notice the difference in their arthritis pain.

Nightshade is just a family. A bunch of plants that share genetic similarities.

We consider them different for the same reason that we consider cats different from dogs. Cats belong to the Felidae family (aka felines), and dogs belong to the Canis family (aka canines). Those families are used to describe the basic characteristics of all family members.

For plants, the Solanaceae family (aka nightshades) have different characteristics from the Brassicaceae family (aka Cruciferous veggies)

One of the characteristics of the Nightshade family is poison. You can avoid the poison if you are careful and eat the correct part... but some people avoid the whole family all together.

We group similar plants into groups called families. "Nightshade" is the common name for a huge family of plants (Solanaceae). They're all flowering plants, many with edible fruits. They share a few similarities in the shapes of parts of their flower, but otherwise they are dramatically different in size (tiny shrubs to trees), shape (vines, bushes, ...), color (whole rainbow), fruits, etc.

Some common edible nightshades: tomatoes, potatoes, eggplants, bell peppers, and chili peppers.

There are also nightshades that are poisonous: belladonna, tobacco, mandrake, henbane, etc.

There's no reason to avoid the edible nightshades unless you have an allergy specific to the plant. Obviously, you shouldn't eat the poisonous ones.

As u/Red_AtNight mentioned, most nightshades are in someway poisonous. I'm not a botanist so I can't say they all are, but most are, especially the wild ones.

We've domesticated things like tomatoes and potatoes but even potatoes can be highly toxic if you eat too much of the wrong bits. I believe they are also a kind of toxin that likes to stay in your fat tissue so it can accumulate slowly over time.

I'm not sure about which diets avoid nightshades other than Macrobiotic diets which as far as I know are more based eastern philosophy rather than some sort of nutritional logic. For example, the Macrobiotic diet classifies foods according to their status in a Yin/Yang balance so certain foods can only be paired with others in a dish. Specifically the nightshades are excluded from the diet entirely, again for philosophical reasons, not for some actual nutritional or ethical logic.

There might also be a religious aspect, I believe the early European colonists believed the tomato was the supposed "Biblical Fruit" that Eve ate and so they believed it both kill you and then damn you hell if you ate it. Obviously that didn't last long. FWIW, I think 'scholars' have since agreed the fruit was a quince.

Much of the reputation of night shades in Europe for much of its history was its use as a poison (or medicine). It’s acceptance as a wide food source wasn’t until the new world (tomatoes, potatoes, peppers). In fact, In the beginning farmer were cautious of these plants because of their resemblance.

Over time we understood their value, but their stigma as more of a poison than food source gave the family a bit of a different identifier.

>What is a Nightshade.

Nightshade is a colloquial name and not a technical one and is used to refer to many different plants.

Most of the plants with the colloquial english name "nightshade" are in the family Solanacea, However there are also several important food crops in that family.

Nightshade traditionally refers to plants in the genus Atropa, such as Atropa belladonna. This is the source of the useful drugs atropine and scopolamine. Atropa is a member of the family Solanacea. However large amounts of atropine are deadly and cause cardiac and respiratory arrest. All parts of plants in the genus Atropa are considered deadly poisonous.

Nightshade also refers to many plants in the genus Solanum, also of the Solanacea family.

This genus also includes the potato (Solanum tuberosa), the tomato (Solanum x lycopersicon), eggplant (S. melongena.). There are several wild relatives of tomatoes, native to south and central america that have edible and tasty fruits, but aren't widely grown.

However other members of the family Solanum have fruits that are toxic. For example, S. dulcamera, called bittersweet nightshade.

The word nightshade can be construed to mean plants in Solanum genus which have no edible parts.

For example, all aboveground parts of potatoes become quite toxic and the toxicity is triggered by light exposure. This includes the fruits and seeds of the potato, and cooking does not destroy the toxins. Only the underground tubers can be eaten. If potatoes are exposed to light for several days they may turn green. This is typically accompanied by modest toxicity, thus green-skinned potatoes are unfit for eating.

Europeans were originally quite skeptical of potatoes and eggplants due to their similarly to other members of the genus Solanum, native to europe which were themselves, very toxic including their fruits. They only became widely adopted in the late 1700s and early 1800's. Because potatoes are a very productive and nutritious crop even in relatively poor soils and are very tolerant of adverse weather conditions, they may be considered one of the unsung heroes of the industrial revolution.

Likewise, only the fruit of the tomato plants is fit for eating and other parts are generally mildly toxic.

Other members of the family Solanacea grown as crops include chile peppers (Capsicum fructescens), Physalis fruits such as tomatillo and lantern-fruit, Lycium barbatum also called wolfberry or goji. It also includes one of the most toxic plants in the family outside of the genus Atropa, Nicotiana tabaccum. This produces the highly toxic alkaloid nicotine.