"I've tried shakes before," she says, "but this one is far and away more effective than any other. It does actually properly fill me up and it means that if I do the shake once a day I don't really have to watch what I eat the rest of the time. It's the best thing I've done. It's done so much for my body and my confidence."
Kelly's efforts haven't gone unnoticed. Protein World runs a weekly competition called Slender Sundays in which it encourages devotees to share their weight-loss success stories. Kelly's Twitter feed, along with many other Slender Blend girls, is littered with photos of her tiny, near-naked frame embracing a hefty Protein World tub, gushing about how she is beating the bloat. Recently Kelly was victorious and received a month's supply for her efforts.
Not so long ago, protein shakes such as these were the preserve of brawny bodybuilders, not slimmers such as Kelly. The powdered ingredients were sold in large plastic tubs from backstreet gyms, promising to help bulk up muscles after a workout. Usually they were made from whey protein, a by-product of cheese-making, but sometimes other slightly suspect ingredients found their way into those tubs – steroids such as nadrolone or testosterone, for example.
The products of the Surrey-based Protein World, on the other hand, comes in sophisticated neutral packaging which is, cleverly, entirely transparent. Its online shop spells out exactly what's in it and what those things will do to your body. It has acquired 300,000 customers since its 2013 launch, 84 per cent of whom are women. Apart from an objectionable poster, what on earth has changed?
"Our level of nutritional understanding has increased rapidly in the past 10 years," explains Richard Staveley, the head of global marketing at Protein World. "There is a much greater appreciation of protein within diet – how the complex structure takes more work for the body to digest and makes you feel fuller longer. As a result we now appeal to the masses rather than niche markets such as bodybuilding."
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The turning point for the industry started around a decade ago when MaxiMuscle, a leading protein supplier, decided to break out of its bodybuilding shackles and go a little more mainstream. It started advertising in lifestyle magazines using more human-looking athletes rather than freakish Schwarzenegger physiques. A subtle shift in our perceptions began to take place. Not long after we saw the rise of the "protein princesses", celebrities such as Kim Kardashian, Millie Mackintosh and Miranda Kerr who took to sharing intimate pictures of their gym workouts on social media, trusty plastic protein shaker always somewhere in shot. Demand rocketed. Between 2007 and 2012 world sales in the protein industry doubled to reach £260m. It's now estimated that it will be worth around £8bn by 2017.
In fact it's gone so mainstream that in April this year Coca-Cola decided it wanted a piece of the action and bought a line of Chinese drinks made from plant-based proteins such as green beans and walnuts for around $400m. And for high-street health food store Holland & Barrett, protein products now account for more than 10 per cent of total sales. "We offer around 700 different protein-based products," says Nick Janda, the product and marketing manager for sports at the chain. "It's an area that has grown exponentially. When we started, you only really found products like these in specialist stores or in gyms. Now even Waitrose is doing it."
Part of the reason for the dramatic growth of supplemental protein is that it's backed up by scientific findings. It's well established that people who are working out have a higher protein requirement. And it's also well established that protein is the most satiating nutrient we've got, making us feel fuller for longer, so useful as a slimming aid. "Protein is one of the building blocks of muscle," says Janda. "Anyone taking part in exercise needs it to support the muscle rebuilding process. However, it is the weight-loss claims which have enabled protein to move into the mainstream and become such an important part of the market."
And where a protein shake would once have come in either chocolate, strawberry or banana flavour, now it's all gone a bit Willy Wonka. In 2013 an Irish company called V12 Shots launched a product which is a 60ml shot of apple-flavoured liquid that delivers 24g of protein straight into your body (the recommended daily intake is about 50g). It's now stocked by Ocado and sales this year have risen tenfold.
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In December 2012, Ross Edgley, a former international water polo player and sports scientist at Loughborough University, helped launch the online nutrition brand The Protein Works. It claims to have created the world's first protein popcorn as well as protein porridge and protein nutties, which Edgley swears taste exactly like Ferrero Rocher. Its protein bars come in flavours such as cherry bakewell, lemon shortcake and apple and cinnamon. And last year, after months of experimenting with ingredients and technologies, it opened a purpose-built 20,000ft protein bakery in Cheshire, using a fusion technology, the details of which are closely guarded.
"Before we arrived, the protein industry had lots of holes in it, people weren't doing things well and lots of companies were just buying product in and slapping their own label on it," says Edgley. "Our founders have decades of experience in sports nutrition. We wanted to do things differently. Traditionally, if you were going to make a protein bar, you'd make the flapjack and the protein would get thrown in as an after-thought. We don't think that's right. With the bakery, no longer is protein an additional product, it is the base ingredient."
It's a formula that's clearly working. Two years after launching, The Protein Works was crowned New Business of the Year at the National Business Awards and is now one of the UK's largest online nutrition brands. It is also one of the first protein companies to work directly with a Premier League football club, having been employed by Everton to create bespoke protein formulas for each of its players. Now it's investing heavily in online tools that allow customers to do the same. The idea is that anyone can log on and create similarly individual protein mixes to suit specific needs.
"We are bringing elite sports performance technology to the masses. Customers can now buy one-off blends which they can create themselves right down to the gram of amino acids or extract of green tea or guarana," says Edgley.
Such is their popularity, protein shakes have their own section in many supermarkets (Alamy)
Meanwhile, at GNC (General Nutrition Centre), Holland & Barrett's specialist nutrition stores where it launches its more cutting-edge products, there has been a surge of interest in different forms of protein. "People are realising that the protein industry has legs, so we are seeing brands investing in things other than shakes and bars," says Janda. "Protein-infused food is a big growth area. Now you can get things such as pasta and bread and other everyday staples fortified with protein so people who don't want a shake can find different ways of taking adequate amounts."
Another trend is in alternative sources. For a while now soy protein has been popular, as the soybean is what's known as a "complete protein", meaning that it contains all the essential amino acids for human nutrition. There is currently much debate about whey vs soy, with the old-school and bodybuilding fraternity dismissing soy as protein for cissies. And there's also the environmental impact of soy protein to consider, given that Europe imports 39 million tonnes of the stuff a year and its production process is synonymous with deforestation.
"As well as soy we are also starting to see powders made from things such as pea, hemp and rice," says Janda. "All these are good sources of protein so they are being used as alternatives to whey, which is beneficial for individuals with food intolerances." Which is all well and good for the manufacturers – the world's whey makers in particular are enjoying a windfall, as between 2005 and 2008 the market value shot up by almost a quarter. But what about our bodies? Are many of us stuffing our bodies full of protein that we don't actually need? And what happens if we have too much? One Slender Blend shake, for example, has 31g of protein in it, which is more than half the recommended daily intake.
Dr James Betts, a senior lecturer in nutrition and metabolism at the University of Bath, thinks that protein fans have got a little carried away: "The question is: do we need more protein? We have quite a lot of protein in our diets in the West anyway. Even individuals who tend to have lower amounts of protein such as females, vegetarians and non-active people are still usually getting more than enough. Scientists will tell you protein supplements can be beneficial to athletes, but whether that supports selling tubs and tubs of the stuff to everybody is a very different question."
Dr Betts says that studies also prove protein is only really effective when used in specific amounts, of a specific type, at a specific given time. "Most of the research suggests we don't need more protein overall," he says. "The total amount doesn't seem to be the important thing. We know that protein acts as an important signal so ingesting certain metabolically active forms and ingesting them at key times, such as immediately after exercise: those are the real focuses of research nowadays. Actually, it's not about eating more, it's about eating smart."
Another concern for Dr Betts is possible contaminants. In 2004 there was a groundbreaking study conducted by Dr Hans Geyer, from the Institute of Biochemistry at the German Sport University in Cologne. He got hold of every supplement he could get his hands on – bought over the counter, online and from the high street – which came to 634 products from 13 different countries, and tested them all.
"It was discovered that about 15 per cent of those supplements were contaminated," says Dr Betts. "And when you get into the minutiae of that paper, it's interesting. Your instinct might be to think this would be from less tightly regulated countries, but actually it was places such as the UK and USA. It wasn't necessarily contamination with things that are going to fail you a drug test, but with other, not very hygienic things that you wouldn't want in your food. That paper may be 10 years old now, but it was shocking and I still hold it up as an example. It's a question of whether you think that the situation will have got better or worse. I think it's a worry when younger and younger people and even child athletes are taking these products now."
Shaking all over: protein shakes have become a huge and profitable industry (Alamy)
Another issue with the protein industry is its obsession with food on the go. It's an idea that's been imported from the US, where convenience is king. Many of the products seem to be aimed at people who haven't got time for a meal so it's a chance to grab a protein bar or a shake instead. A number of new brands are capitalising on this idea, including the Australian company Up & Go, which makes vanilla, chocolate and strawberry liquid "breakfasts" which claim to have as much fibre, calcium and protein as a bowl of cereal. There's also Feeling Upbeat, a protein drink dubbed "a bit of extra willpower in a bottle". It's another beef of Dr Betts who found, in a three-year study published last year, that people who skip a proper breakfast are actually likely to expend less energy – by as many as around 442 calories – afterwards.
But there is some good news in all this. It was once thought that too much protein in our diet could be harmful, in particular to our kidneys, but recent research seems to suggest otherwise. "There have been all kinds of things in the literature about the negatives of a high-protein diet in terms of being bad for your bones and putting strain on your organs," says Dr Betts. "But there have been a few papers recently that have put some of that concern to bed. Our bodies are actually quite efficient at handling nutrients and successfully secreting the excess."
That's fortunate for the steady stream of men queuing for a top-up shake at the Muscleworks Gym in east London (which suggests a little excess may be going on round here). Muscleworks, founded by former bodybuilder Savvas Kyriacou, has three branches in London and regularly turns out bodybuilding champions. The area behind the counter is so stacked with protein products that it's beginning to resemble a supermarket. Alongside traditional tubs of powder, there's a freezer filled with a range of protein-enriched microwave meals called Pot O'Gold, which come in flavours such as chilli black bean and chicken tikka and deliver around 30g of protein. Many of the men training here will have one straight after a workout.
There's also pasta from a brand called Dr Zak's made from pea protein which gives around 50g of protein per serving. Dr Zak, real name Zak Pallikaros, a former champion bodybuilder, owns a gym down the road. His range of "functional foods", which also includes high-protein bread and bagels, is now sold in gyms and sports nutrition stores all over the UK. Who knows if we'll all be eating this stuff in years to come?
Ahmad has been working out here for four years. "A lot of people are going for a much leaner look these days," he says, although he still looks pretty big to me. "I have to top up with protein shakes. High-protein food is all very well but I need something I can get into my system quickly." When he's training he'll take 200-250g of protein supplements a day and if he's building up for a competition this could go up to around 300g. At £50 for a 2.25kg tub, it must be costing him a fortune.
Ahmad is just one of many. A 2012 YouGov report discovered that one in 10 men was now using protein supplements at least once a week. While over-consumption may not be actively doing us much harm, clearly we are in the middle of a massive triumph of marketing: there's a lot of money being made in our passion for protein. Dr Betts, however, has one final word of warning. "This is a relatively recent phenomenon," he concludes, "so we may find that in 20 years or so youngsters who have been taking it for their entire adult lives might throw up some problems we just didn't see coming."