We’re told repeatedly about the benefits of getting our five a day. However, in some circumstances certain fruits and vegetables may actually be bad for our health.
Grapefruit juice, for example, can interfere with statins, increasing the risk of side-effects.
Here, we look at the produce that may cause problems…
AVOCADO
People who are sensitive to latex can also have an allergic reaction to avocado.
Latex comes from the sap of the rubber tree, Hevea brasiliensis. A study of 137 patients with rubber latex allergy, published in the Journal of the American Dietetic Association, found that 21.1 per cent were also allergic to particular foods, including banana (18.3 per cent) and avocado (16.3 per cent).
This is because some of the proteins in latex that cause allergic reactions are also present in these fruits.
The same cross-reactivity can happen with kiwi fruit, says Professor Jean Emberlin, scientific director of Allergy UK. ‘The proteins are very similar in both the latex and the fruit, so they can trigger similar reactions.’
Symptoms include tingling in the mouth, stuffy nose, itchy eyes, wheezing and, in rare cases, life-threatening anaphylaxis.
Someone allergic to the manufacturing chemicals in latex rather than the plant won’t have allergic reactions to these foods — but you wouldn’t necessarily know which component part of latex caused your allergy.
‘So if you have a latex allergy it’s best to avoid them,’ says Professor Emberlin.
CHERRIES
‘Cherry stones can be dangerous if chewed and then swallowed, as they contain a chemical compound based on the poison cyanide,’ says Dr Prasad.
If the stone is ground down, say by chewing, then the compound amygdalin, a form of cyanide, is released.
This can cause fever, headaches, falling blood pressure and, in extreme cases, can be fatal.
Research suggests the cyanide compound in cherries could be fatal in doses as small as 1.5 mg per kilogram of bodyweight, according to the European Food Safety Authority (EFSA).
A single cherry yields roughly 170 mg of cyanide per gram of seed — which means ingesting one or two freshly crushed stones could be dangerous. Apricot kernels also contain high levels of amygdalin — eating more than three small, raw apricot kernels, or less than half of one large kernel, can be a serious health risk, says the EFSA.
The chemical is also found in apple seeds — but you would have to eat two cups of ground seeds for it to be fatal.
GRAPEFRUIT
If you’re taking statins, check with your doctor whether you can have grapefruit juice, says Dr Sanjay Prasad, a consultant cardiologist at the Royal Brompton Hospital in London. ‘Statins are broken down by an enzyme in the liver, CYP3A, which normally reduces the amount of drug that enters your bloodstream.’
But grapefruit contains compounds that affect the function of CYP3A and therefore increases the potency of the drug (because less is broken down).
Not all statins are affected to the same degree, and eating half a grapefruit is much less of an issue than drinking the juice as it takes several of the fruit to make one glass of grapefruit juice. But still check with your doctor.
BANANAS
Avoid very large quantities of bananas if you’ve had kidney problems, as they contain the mineral potassium.
‘Kidneys regulate the amount of potassium in the body and if they’re functioning normally, they can control intake,’ explains Paul Cathcart, a consultant urological surgeon at Guy’s and St Thomas’ Hospital in London.
But kidneys that don’t function normally can allow potassium to build up, potentially triggering hyperkalemia, which causes nausea, a slower pulse and an irregular heartbeat.
‘So if you have a renal impairment or have had kidney disease, you should have a lower potassium intake,’ says Mr Cathcart.
Healthy adults need about 3,500mg of potassium a day, says dietitian Dr Sarah Schenker. The average banana contains between 400 mg and 450 mg, so you’d have to eat more than seven-and-a-half bananas to reach that level.
BEETROOT
If you’ve had kidney stones, avoid beetroot, says Bhaskar Somani, a consultant urological surgeon and honorary senior lecturer in urology at University Hospital Southampton NHS Foundation Trust.
‘Beetroot contains oxalates — substances that prevent calcium being absorbed.’
Most people’s bodies can break down and eliminate these substances via the kidney or colon. But in some people, oxalates can accumulate, leading to stones.
SPROUTS
Sprouts should be avoided if you’re taking the anticoagulant drug warfarin, used to help prevent blood clots or to keep a clot from getting bigger, advises Yoon Loke, a professor of medicine and pharmacology at the University of East Anglia.
‘This is because green, leafy vegetables such as sprouts and spinach are high in vitamin K, a nutrient needed to make clotting factors in the body — the opposite of what warfarin does, which is to thin the blood.
‘So vitamin K inhibits the effects of warfarin.’
CABBAGE
Raw cabbage, along with cauliflower and kale, contains goitrogens — substances that can affect thyroid function by blocking production of the hormone thyroxine, says Dr Mark Vanderpump, a consultant endocrinologist at The Physicians’ Clinic in London.
The hormone helps to absorb iodine, which is essential for normal thyroid function.
The effects are small, so cabbage is unlikely to cause problems in healthy people and those who are taking the drug levothyroxine for underactive thyroid.
‘But these vegetables may be an issue for those who have a low thyroid function but haven’t been diagnosed. Symptoms include tiredness and weight gain,’ says Dr Vanderpump.
BROAD BEANS
Tyramine, found in foods including broad beans, chocolate and hard cheese, is an amino acid that helps regulate blood pressure.
However, if you take one of the older forms of antidepressant known as monoamine oxidase inhibitors, the drug can block the enzyme that breaks down excess tyramine.
A build-up of tyramine can lead to critically high blood pressure, says Dr Prasad.
CRANBERRIES
Cranberry juice contains salicylic acid, a key ingredient in aspirin, so reduce the amount you drink if you take aspirin regularly (it is often prescribed as a blood thinner for those who have had a heart attack), as this could thin the blood further.
‘Drinking more than three glasses of cranberry juice a day can increase the amount of salicylic acid in your body,’ says Professor Loke. ‘To be on the safe side, keep intake to less than this if you take aspirin regularly.’
Salicylic acid could also potentially affect how you metabolise warfarin.
