Three reasons not to eat kale: for some of us, it’s not quite the super food we thought

It does have many benefits, but kale is not as good for some of us as we’ve been told. Three reasons not to eat kale, and how to know if you should avoid it

If it’s possible for a vegetable to be trending, kale is. Even if you’re not a foodie or health conscious, it’s been hard to avoid for the past three years. With myriad recipes online, books devoted to kale (Fifty Shades of Kale? I kid you not), hundreds of Instagram images and entire Pinterest boards, the green curly stuff is now everywhere.

We’ve been cooking with it, putting it in salads, making juices and smoothies, or buying kale crisps – including raw chocolate-covered ones – at the organic shop. But it didn’t make our top ten foods to improve happiness (nor did chocolate cake!).

Reasons why kale is good for us include being high in fibre, which helps to keep our bowels clean and regular. It can also help to keep cholesterol in check, and helps us to feel full after eating.

It’s high in iron, helping to form haemoglobin, which transports oxygen to our cells for growth and renewal, and helps to purify the blood.

It contains vitamins A (which benefits our skin and vision), K (which contributes to bone health and blood clotting) and C (helping to boost the immune system), as well as sulforaphane, a chemical compound with antioxidant, anti-cancer and antimicrobial properties, that stimulates natural detoxifying enzymes, contained in cruciferous vegetables.

It’s high in calcium (per calorie, more than cow’s milk), which is needed for bone strength and to prevent osteoporosis. It’s fat free and its Omega-3 content (ten per cent of your RDA per cup of kale) makes it an anti-inflammatory food. Inflammation is the cause of many of today’s chronic illnesses.

So why am I telling you it might not be so great?

Because, like any food, kale is only healthy if you can digest it. With advice from Cassandra Barns, a clinical nutritionist at The NutriCentre, I’ll explain why:

Related: Coconut oil: ten myths debunked

1 Kale contains too much fibre for some people’s digestive systems 

It contains about 2.6 grams per cup, which is about 14 per cent of an adult’s recommended daily intake. Fibre is not digested and remains in our digestive tract, where it helps with the passage of food through our digestive system. Which is a good thing, usually.

But it’s this high-fibre content that could cause problems for your digestion, especially if you have IBS (irritable bowel syndrome) or other unexplained digestive issues.

Most of the fibre in kale is insoluble fibre – the type found in wholegrains, bran and nuts and seeds – and too much of it can be difficult for your digestive system to cope with. When it is not properly processed it gets stuck in the gut causing increased fermentation, which in turn causes problems such as bloating and gas.

2 Kale contains a sugar called raffinose, which is hard for us to break down

Raffinose belongs to a group of carbohydrates called oligosaccharides, meaning it is made up of several sugar molecules hooked together. It is found in cruciferous vegetables (the group that includes kale, cabbage and broccoli) as well as beans and legumes.

We lack the right enzymes to digest raffinose in our stomach or small intestine. So after passing through these organs it arrives in the large intestine still intact, and is then fermented by the bacteria there.

Related: Why you should be worried about inflammation

Though it can ‘feed’ our good bacteria and encourage their growth, this fermentation also produces gases such as methane and carbon dioxide, which cause the well-known problems of bloating and flatulence that some people get after eating these foods.

With groundbreaking research showing that our gut bacteria could even more key to good health and illness than previously thought, we need to look after our gut more than ever.

3 Raw kale can affect your thyroid gland function

Like other cruciferous vegetables, kale contains small amounts of substances that can have a ‘goitrogenic’ effect, meaning they can affect the function of our thyroid gland. These substances become inactive with cooking, but they are still active in raw kale.

No one should overdo raw kale. Most of us are OK if we stick to modest amounts, such as a handful in a smoothie or juice, even on a daily basis. But if you have existing thyroid problems it’s safest to stick to cooked kale.

So who shouldn’t eat kale?

If you get gas or bloating when you eat kale, cabbage or beans it’s best to avoid all aggravating foods, including kale, to give the digestive system a chance to recover. Likewise people with IBS, those with weak digestion, or if you have any sort of digestive discomfort where you can’t pinpoint the problem food.

As with any food, eat mindfully, become aware of how different foods affect you personally, rather than going on what others tell you is ‘healthy’, and if something is not digested easily (with no gas, gurgling, rumbling, cramps, constipation, acidity or bloating), act on what your body is telling you.

Related: Three three reasons we’re eating way too many avocados

How to make kale more digestible

Cooking kale improves its digestibility, as cooking starts to break down the tough fibres. Steaming it is a good method, or add it to soups or hotpots. But stick to small amounts of kale: a handful per person added to a dish is enough. Small amounts of spices, such as black pepper, cumin, fennel or fresh ginger root, also help to make if more digestible.

Don’t use it raw in salads then?

Not if you have any of the symptoms listed above, as the leaves and stalks are very tough to chew when raw.

For the rest of us, if we want to eat kale raw in salads, rubbing or massaging the leaves first can break down the fibres. It’s easiest to do this while you’re rinsing it.

Remove the tough stems first, then take a handful of leaves and rub them together for several minutes. You’ll see it wilting and breaking down, and the taste changes – it loses its bitterness. Use no more than a few leaves if you’re eating it raw.

Kale chips can be made in a dehydrator or low-temperature oven, but this doesn’t make it more digestible, so avoid these too if you have the any of the above symptoms. Particularly don’t eat them if you have dryness in the colon (signs of dryness include not having a bowel movement every days and/or your stools are sometimes hard to pass and come out in small, hard bits rather than one piece).

What about kale in juices and smoothies?

One of the most popular uses for it is in fresh vegetable juices or in smoothies. For smoothies, you need a good quality blender or smoothie maker to break down the leaves and stems properly. Use no more than a small handful of kale, and mix it with other vegetables or fruits. And if you’re in the group that should avoid raw kale, avoid juices too as the kale is still raw.

And finally, if you haven’t got it by now, remember: “A food is only healthy if I can digest it.” It’s your new health mantra, people!

Related: Six reasons eating vegan is finally cool

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