When you’re trying to eat more healthily, it's easy to be tripped up by foods that masquerade as good for you, but really aren’t. From “fat-free” snacks to surprise sugar traps, nutritionists say you should think twice before tucking into these 20 foods.

These must be healthy as they're just slices of dried banana, right? Wrong. Crispy banana chips are usually fried in coconut oil and often have some added sugar too. A tiny 30g serving (about a handful) typically supplies 150 calories, over a teaspoon of sugar and a whopping 8g of saturated fat ­– that’s 40% of your daily limit of this potential cholesterol-raiser.

The average trendy energy/protein ball has around 160 calories and almost 2tsps of sugar. For the same calories and protein, with less than half the added sugar, you could have a 250ml glass of skimmed milk and a digestive biscuit. The milk and cookie duo are also a lot cheaper and packed with calcium too.

Beetroot, carrot and parsnip crisps are still deep fried. They also still have added salt and a 25g portion has around 127 calories, which is just five less than standard potato crisps. Crisps are crisps – i.e. an occasional treat food – whatever they're made of, so just choose your favourite ones.

Clear, colourless and made with spring water, flavoured waters seem innocent enough, but they’re really not any different to a can of diet lemonade or zero sugar cola. Just like a diet soft drink, these waters contains citric acid, flavourings and artificial sweeteners – they sometimes contain preservatives too. To hydrate, you’re much better off with plain tap water: add a squeeze of lime juice if you need to give it a lift.

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Cranberries are so tart when fresh that they're infused with added sugar when they're turned into dried fruit. Typically, nearly 18g of the 20g of sugar in a 2tbsp portion of dried cranberries is added. That’s over 4tsps of sugar – more than the amount in five custard creams.

Fish is low in saturated fat and high in protein, minerals and vitamins: in short, something we should all aim to eat more of. But swordfish (and also marlin and shark) comes with a special health warning – it contains more toxic mercury than other types of fish and should not be eaten at all by pregnant women or women trying to get pregnant. Everyone else should eat no more than one serving per week.

Low-fat dressing may often be higher in sugar than its full-fat counterpart – but there’s another reason to use a little oil rather than a fat-free dressing. You need some fat to properly absorb healthy fat-soluble antioxidants (like beta carotene and lycopene) from ingredients such as leafy greens, tomatoes and peppers.

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Coconut oil has far more saturated fat than butter. And most of the hype around it, including that it can help weight loss and Alzheimer’s disease, is based on very little evidence with hardly any human studies. Some of the specific saturated fats in coconut oil, known as medium chain triglycerides or MCTs, may have benefits, but for now it’s better to mainly use a monounsaturated oil, such as olive oil, which has much stronger evidence to support its health benefits. 

The idea of a smoothie is great – what could be wrong with a pile of fruit served up in a drink? But juicing, pureeing or crushing packs all the sugars from the fruit into a much smaller volume, while also freeing sugars from plant cell walls, so they’re able to harm your teeth and spike your blood glucose levels. A 250ml glass of an average fruit smoothie has 27g of free sugars – almost all the maximum recommended amount of 30g a day.

Green juices are rarely much healthier (at least the ones that are palatable enough to drink, anyway). Most need to contain plenty of fruit to provide sweetness – you can expect a bought green smoothie to have about 18g, or over 4tsps of sugar, per 250ml glass. Green vegetables work best and taste nicest on your dinner plate or in a salad bowl.

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Diet fruit yoghurts may be low in calories, but they come with a raft of flavours, sweeteners, colours and starch, tasting as artificial as they sound. A creamy 100g portion of full-fat natural Greek yoghurt has just 95 calories and is much higher in protein than diet yoghurt, helping to fill you up. You can add some subtle sweetness, vitamin C and antioxidants with fresh or frozen berries.

Baked crisps have around 23 fewer calories per 25g bag compared with standard deep-fried crisps. But they are a much more processed product, generally made with potato flakes and starch rather than whole cut potatoes. They also have more than a third of a teaspoon of sugar per bag. For healthier blood sugar and insulin levels, and if you are trying to avoid refined carbs, standard crisps are typically a better choice.

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It sounds like a healthier option to spread on your toast, but don’t be fooled. The sweetness in fruit spread may come from fruit juice concentrate, but this is just added sugar by another name. In fact, fruit spreads typically contain 55% sugar and jam usually around 60%, so they are almost identical.

This syrup from the Mexican blue agave plant (also used to make tequila) has been promoted as a healthy alternative to sugar, and has a low glycaemic index (GI), so won’t spike your blood sugar. But the downside is that it is almost entirely made up of fructose, the type of sugar that is generally agreed to have the most harm as it can trigger a fatty liver.

Protein bars are a popular post-workout snack, but their 220–250 calories per bar is equivalent to a Mars bar. You also need to watch for polyol sweeteners in some bars, which can cause stomach issues. Exercising at high intensity for more than 30 minutes at least three times a week means you can benefit from a little more protein than average, but it’s obtained easily from real food. Most protein bars have 10–20g protein, which is the amount in 75g prawns or a couple of small hard-boiled eggs.

These may sound healthier than chocolate raisins, but they're not. In fact, anything “yoghurt-coated” is as sugary and as high in calories and saturated fat as the equivalent milk chocolate-coated product, if not more so. Unpack the ingredients of the yoghurt-flavoured coating and you will see why: it’s usually mostly sugar and palm oil, with just the teensiest bit of yoghurt powder.

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Organic doesn’t necessarily mean healthier – and in the case of this popular breakfast cereal, it’s actually a nutritional disadvantage. That’s because standard Weetabix is fortified with a third of the recommended daily intake of B vitamins and iron, but the organic version isn’t allowed to have these nutrients added due to organic food-production rules.

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Crunchy granola averages around 200 calories, 8g of sugar and 3g fibre in a 45g serving. You would be better off with a bowl of Cheerios, which have 173 calories, 8g sugar and 4g fibre per 45g serving, plus a wealth of extra vitamins and minerals, given they are fortified (but granola isn't).

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There’s no specific nutritional downside if you opt for reduced-fat cheese, but it’s likely you’ll just end up eating more, trying (and failing) to get a satisfyingly cheesy taste and texture. A small portion of full-fat Cheddar is often a better option, or go for a cheese that is naturally lower in fat, such as goats’ cheese, camembert, feta or brie.

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An açai bowl typically contains frozen, sweetened, açai berry puree mixed with other fruits or juices, yoghurt or non-dairy milk and a topping of granola. Honey or agave is often drizzled over and nut butters sometimes feature, but whichever way you look at it, that’s a whole lot of carbs and sugars layered into a breakfast bowl. Açai bowls start at around 375 calories and go up to 600 plus. And, açai aren’t any more “super” than other berries such as blueberries and strawberries.