We all know that freshly prepared, nutritionally balanced meals and snacks are better for us, and that processed foods should ideally be restricted to the occasional treat. The term 'junk food' has been around since the 1970s, and now experts are increasingly warning us to avoid consuming ultra-processed foods or UPFs. But what are they, exactly – and what's so bad about them? The term was introduced in 2009 by a group of Brazilian researchers, who defined UPFs the most industrially compromised foodstuffs. Their long ingredients lists can be hard to decipher, with elements such as palm sterate, partially reconstituted whey powder, and maltodextrin.

Ultimately, these ingredients have been manufactured for low cost and convenience, and deployed in complex formulation to make 'hyperpalatable' snacks and meals. "This processing completely changes the structure of food," says registered nutritionist Dr Lucy Williams. "We need food to actually work for us; it drives our biology so it needs to fuel us, rather than just satisfy our appetite." UPFs displace healthier foods from our diet, filling us up (and encouraging us to overeat) without delivering our essential nutrients. Now, a growing body of studies suggest diets high in UPFs are causing a 'tidal wave of harm' to public health. 

Unless you're willing to radically redesign your life, though, it's incredibly hard to cut out UPFs entirely. Industrially manufactured foods are engineered around our busy lives and tight budgets, so it's not surprising we've become so reliant on them. What we can do is shop more mindfully. "My rule of thumb is look at the label," says Lucy. "If you don't recognise an ingredient as one you'd put in your kitchen cupboard, do you really want to be eating it?"

Click or scroll through our gallery to discover 23 ultra-processed foods we should try to avoid, according to experts:

It's everywhere in our diets, but the vast majority of bread on supermarket shelves, and in pre-packed sandwiches, qualifies as UPF. "Industrially produced bread has lot of refined carbs, additives for shelf life and added yeasts," says Lucy, who suggests these strong commercial yeasts carry on fermenting in the stomach causing some people discomfort. Check labels, or make your own. True sourdough is made from just flour, salt and water – so any other ingredients could mean you're actually buying a UPF imitation, or 'sourfaux'.

Whole grains lock carbohydrate up with fibre and micronutrients, causing energy to be released slowly as we digest. The reverse is also true. "When carbohydrate is processed, the sugars within are already broken down for us so we absorb them really quickly," says Lucy. These damaging sugar spikes may be 'clean-washed' in UPF cereal by added nutrients proudly advertised on the label. "The truth is we don't know how well we can absorb them in this isolated form," adds Lucy.

This kids' favourite, often advertised a being made exclusively from 100% breast meat, isn't always what it seems. Known technically as a 'comminuted meat product', which means it's reformed from tiny fragments, the soft bites commonly hover around 50% actual meat content. In fact, a US study found levels of actual lean muscle are often much lower. The fried, refined wheat coating is high in refined vegetable oil, another ultra-processed ingredient.

Chocolate itself is not a UPF, but the type contained in, or topping, your average biscuit almost certainly is, and will contain refined fats, sugar and emulsifiers. Add the ingredients of the bake itself and you have a problem. "Once in our gut, the additives and high sugar content make a really poor environment for our digestive microbes," says Lucy. This precious microbial population impacts everything from our levels of disease-triggering inflammation and immune system to our mood.

Beside a high quantity of sugar and other refined ingredients, this toast topper almost always contains palm oil, an ingredient that's gained a bad reputation for the deforestation caused by palm plantations. There's another reason to avoid it, though. "When freshly pressed, it’s an almost luminous crimson, highly aromatic, spicy and flavourful, and full of antioxidants like palm tocotrienol," says infectious disease doctor and documentary maker Dr Chris Van Tulleken, adding that the palm oil used in UPFs are 'refined, bleached and deodorized'.

OK, so no one turns to sweets for a healthy hit of nutrition, but all confectionery is not created equal. The original classification of ultra-processed foods highlighted those with five or more ingredients, and a quick glance at a few filled chocolate bars shows the leading products tend to have triple this number. The presence of glucose-fructose syrup, typically made from starch, is problematic, too, described by researchers as delivering a 'one-two punch to health'.

Deep-fried slices of fresh potato seasoned with a little salt are a processed indulgence, but it's the reconstituted starch or puffed maize snacks with tongue-shocking flavours that many of us really crave. So, once we've popped, why is it so hard to stop? Research suggests it's the flavour enhancers (monosodium glutamate, for example) regularly deployed in these ultra-processed crisps that help transform a nutritionally bankrupt puff of starch into something we can't resist.

As you'll be gathering, it's often when a natural ingredient is being synthesised that foods turn into an 'industrially produced edible substance', to echo Fernanda Rauber, one of the original Brazilian researchers looking into UPFs. The resulting products can be aspirational and expensive, as in the case of faux meats. Often lower in fibre and higher in saturated fat, salt and sugar than simpler options like tempeh and tofu, the labels of plant-based meat substitutes need careful checking.

The sugar tax has done a great job of encouraging manufacturers to reformulate drinks, but do check what's now bringing the sweetness to your favourite fizzy pop. "Artificial sweeteners are a big no for me," says Lucy, who points to a slew of emerging research reporting negative impacts on the all-important gut microbiome, and ramifications for how our bodies subsequently metabolise sugars. If aspartame, sucralose, acesulfame K, saccharin or other common sweeteners crop up on the label, consider an alternative.

Cheaper biscuits, cakes and pastries can be hiding places for partially hydrogenated fats and oils too. According to the British Dietetic Association, spotting partially hydrogenated fats or oils on a label means that food 'will almost certainly contain trans fats too'. The higher up the ingredient list they appear, the more trans fats the product will likely contain. They may also be declared as 'mono and diglycerides of fatty acids', says Diabetes UK.

"I'd avoid 99% of fast foods," says Lucy. "Many have trans fats: fats or oil that have been synthetically altered." Banned in some parts of the world, but only restricted by voluntary agreement in others, including the UK, trans fats are linked to higher risk of stroke and heart disease. They occur when fats or oils are 'partially hydrogenated', sometimes as a side effect of being raised to high temperatures, for example in deep fat fryers.

How is it a frozen pizza can deliver 60% of your daily saturated fat allowance – but absolutely none of your fibre? Because it's often formed from highly processed cheeses, meats and dough, plus stabilisers and preservatives. Even premium, high protein and free-from options are often UPF hotpots, with toppings such as 'non-dairy sunflower oil-based food preparation', hydrolysed vegetable protein, or chicken breast that in turn contains 'dextrose, salt, maltodextrin, maize starch, rice flour and concentrated lemon juice'.

It may surprise you, but these healthy-sounding dessert alternatives aren't always the wholesome choice they appear. Expect to find modified starches, thickening agents, artificial sweeteners, gelling agents and emulsifiers, especially in the low-fat options where manufacturers have worked hard to replicate the mouthfeel of full-fat dairy. "Buy plain yogurt and add your own fruit," advises Lucy.

What could more wholesome than a good old British roast dinner? Well, UPFs are sneaking in on the action, with flavour enhancers, modified starches (for example maltodextrin) and emulsifiers such as soya lecithin often lurking in gravy granules. "If you take fat out you have to put an emulsifier in to give back that lovely mouthfeel," says Lucy, "but there's really good research to how those gloopy ingredients impact on our gut microbiome." An unmodified starch, such as cornflour, will thicken meat juices without compromising health.

When University of Michigan researchers set out to calculate the dietary changes that would have most benefit for health and the environment, their scrutiny across a complex set of metrics left processed meats nowhere to hide. While hams and bacon are processed, reconstituting that meat into a hot dog hikes the health risk. Incredibly, the study concluded that eating a single hot dog could knock 36 minutes off a health lifespan, while swapping it for a serving of nuts could add 26 minutes.

The expense of transporting a delicate blend of pricey natural ingredients (traditionally cream and egg yolks) from the manufacturing plant to the wholesalers to the retailer, all at around -18°C, means some ice cream producers often take serious shortcuts. Re-freezable 'blocks of foam stable at room temperature' is the aim of some manufacturers, according to Dr Chris Van Tullekan's book Ultra Processed People. In contrast, small local producers typically use as few as three simple culinary ingredients.

They may be super convenient, but your pot snacks could be packing a lot more than you bargained for. While instant noodles themselves are typically low in fibre, full of additives and fried during production to speed up cooking times, the accompanying sauce sachet is often high in sodium. In fact, research conducted in the US found significant consumption of instant noodles increases the risk of cardiometabolic syndrome, which can lead to heart disease, diabetes and stroke.

You know you're in trouble with a jar or tube of mayonnaise when 'starch' appears higher up the ingredient list than 'egg'. But then, creating a spoonably thick emulsion without the classic constituents of oil and yolk is a conundrum. Food technicians have cracked it by using gums and starches to magically make water and a few flavourings feel oily in your mouth. A homemade mayo typically uses four ingredients. A leading low-fat brand? A staggering 17.

A strong candidate for the first UPF (early trials took place in 1869), margarine is the quintessential example. These semi-solid blocks are another modern miracle of science. They're predominantly made from vegetable oils that were previously liquid blended with emulsifiers, preservatives and water. Common in manufactured products of all kinds, margarine also pops up in our home cooking. Pricier plant-based and nutrient-fortified options often have the most complex formulations.

It's no wonder earlier generations fell in love with packet soups; an instant meal reconstituted straight from a powder is a kind of magic. Unfortunately, it's the kind that requires disodium 5'-ribonucleotides (a kind of salt made from two E numbers) and glucose syrup. A 'vegetable' soup can contain as little as 1g of fibre in a 250g serving.

Protein makes a great basis for a snack, but the challenge to cram it into a tasty and convenient bar sees some manufacturers employing a cocktail of components spliced apart from their whole food origins. Turning soy or pea protein (and a host of additives that may include lecithin, starches and even bovine collagen hydrolysate) into an appealing snack isn't easy; cereal bars are often high in sugar and fat, and relatively low in fibre.

An estimated 91% UK shoppers buy chilled ready meals and, while standards have improved, you'll still find gelling agents, preservatives and meals that are higher in sugar than essential dietary fibre. "And fibre is a key one," says Lucy. "Gut microbes feed on it, and when you think that 80% of our immune system sits in the gut wall regulated by our microbes, you realise what we're missing out on."

Many UPF researchers warn us to suspect foods that carry a health claim, a marketing move that generally enhances a product's profitability. Check the ingredients in 'healthier' syrups and besides the many artificial sweeteners you'll find some seriously complex thickeners. Sodium carboxy methyl cellulose, anyone? More familiar may be xanthan gum, a ubiquitous thickener produced as a kind of slime by bacteria, which a researcher at University of Michigan found to impact our gut microbiome.

Now discover our simple tips to help you cook and eat more healthily