You probably eat these foods every day, so why did the ancient Romans think they were disgusting poison?

So why did some of the most influential physicians in history warn against what we now consider basic nutrition?

According to Dr Claire Bubb, a historian of ancient science and literature speaking on the HistoryExtra podcast, these views existed within the context of a broader dietary philosophy.

“They don’t think in terms of vitamins,” she explains. Instead, she says, “they’re trying to pay attention to what food seems to do to the body.” The conclusions might sound strange now, but they were based on close empirical observation of food, digestion and health – at least, the version of health that was understood by ancient science.

Why the ancients hated fruit

“Fruit is pretty universally viewed with scepticism,” says Bubb.

She explains that Galen of Pergamon, one of the most celebrated physicians of the Roman empire, eyed fruit (and especially peaches) with a wary eye, because they “decompose like crazy”.

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From the modern perspective that might sound obvious (especially if you’ve ever left fruit in a lunchbox too long) and mostly an inconvenience rather than a cause for avoidance. But for ancient doctors, that spoilage could be much more than an unpleasant but inconsequential disappointment. It was seen as actively harmful.

“[Galen] says, ‘Look, it’s also decomposing in your stomach and it’s causing everything else in there to decompose,’” Bubb explains. “So, you need to be thoughtful about eating it.”

But as well as stemming from observation about how quickly fruit could spoil – and drawing conclusions about what impact this could have on the inside of the body – Galen’s take was also based on personal experience.

During Galen’s youth, his consumption of fruit was strictly regulated. “His father dictated his diet and said, well, you can only have [fruit] one time a year,” says Bubb. But like many teenagers, Galen rebelled.

“He and his friends just went crazy. He had these bad influence, fruit-loving friends… and then he develops some sort of chronic infection, which he links to the fruit eating, that takes forever to heal from.” Years later, Galen would swear off fruit entirely – and insisted that his father had been right.

Therefore, due to its quick decomposition, fruit was viewed by ancient dieticians as a risky indulgence. Especially in a world without refrigeration or sanitised water, where foodborne illness was common and hygiene standards varied greatly, the softer and wetter varieties of fruit were seen as a potentially serious threat to health.

The cursed herb: why basil was best avoided

If fruit was a source of scepticism, then basil – a staple herb in many modern cuisines – was downright alarming. While today it’s celebrated for its flavour and aroma, the ancients treated it with serious suspicion.

“Basil is poisonous,” says Bubb, summarising ancient medical opinions. “If you leave it out in the sun, you let it rot, [and] either worms or scorpions, depending on who you talk to, will spontaneously generate from the basil.”

This idea – known as spontaneous generation – was widespread in antiquity. The belief was that decaying matter could spontaneously produce insects or vermin.

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Why did such an extreme theory take hold? “Rotten basil is foul,” Bubb adds. “It goes from delicious to really gross pretty quickly.”

To the ancient observer, the smell of decomposing basil, the sudden appearance of larvae or insects, and the speed with which it spoiled all pointed to something deeply unnatural. “They see and smell the decomposition of it and think, ‘Is it decomposing in my body?’” Bubb explains. “Is that what’s happening when I eat it? We should avoid this at all costs.”

The belief wasn’t short-lived, either. It persisted in various medical and herbal texts for centuries, influencing both diet and pharmacology well into the medieval period.

Lentils and flatulence

Unlike basil, lentils were considered acceptable to Greco-Roman medical thinkers – but only in moderation. Too many, and the body would suffer serious imbalances.

In ancient Greek medicine, particularly after the writings of Hippocrates (c460–c370 BCE) and his followers, food was understood through the lens of humoral theory – the idea that health depended on the balance of four bodily fluids: blood, phlegm, black bile and yellow bile. Foods were classified as hot or cold, dry or wet, and their properties could tip that balance in dangerous ways.

Taking lentils as an example, the ancients believed that “lentils [could be] bad for you,” Bubb says. “They’re fine in moderation, but if you go all in on the lentil diet, like [some] philosophers are telling you to, you’re going to get all these things building up, and you’re not going get the right quality of nutrition and flesh in your body.”

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The digestive responses to lentils – and other pulses – were also considered valuable diagnostic tools.

“If most foods don't give you gas, but then you eat a bunch of beans and suddenly the flatulence is out of control, that's going to suggest to you that beans are doing something in your digestive tract that other foods aren't. And maybe there's a problem,” says Bubb.

So, the next time you enjoy a light lentil soup, or a basil pesto pasta, bear in mind that the Greco-Romans might have considered it a medical disaster waiting to happen.

Claire Bubb was speaking to James Osborne on the HistoryExtra podcast. Listen to the full conversation.