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It's not often you need to examine someone else's tongue during a business event. But that's precisely what Gillian McKeith asked us to do when she addressed the London Business Forum at Waterloo's iMax theatre. The tongue is "a window to the organs," she said, "so if you haven't got anybody sitting next to you then find someone and introduce yourself... we'll wait for you." Those of us without neighbours duly clambered across the steeply raked seats, reasoning that, if we showed an internal organ to a complete stranger, it would be less embarrassing than an inspection by McKeith herself, who had the most youthful skin and highest energy levels of anybody in the room.

We were told to look for "a line down the middle of the tongue," and "teeth-marks around the sides, a bit like scalloped edging." The former suggests digestion is weak, McKeith explained, while the latter suggests "the spleen is having a bit of trouble". In either case, your body isn't getting the nutrients it needs and, even when you do eat something healthy, "you're not converting enough of that nutrition into energy." The causes? Poor diet, too much sugar, too much alcohol and not enough exercise. The consequences? Bloating, wind, debilitating energy slumps and endless other symptoms. "A lot of people don't know how much energy they could have, because they've never really been there," she said wistfully.

By this stage I had an attractive young woman sitting beside me. "You've got both symptoms," she said grimly. Her tongue, of course, was totally unspoilt - pink and glowing as the day it first took over from her umbilical cord and, evidently, weaned on flaxseed, blueberries and wheatgrass ever since. By comparison, I was an abject wreck. But I wasn't as unhealthy as many other people in the room. Their tongues had small cuts along the flanks - a common sign, according to McKeith, of a B-vitamin deficiency. "This is an epidemic in this country," she said, "it means you're not getting as much energy as you'd like... and you're going to find it hard to lose weight, because it affects the metabolism." Some people even had a horrible white discolouration, suggesting a bacterial imbalance in the gut.

Everyone in the room had at least one symptom.

Our only reassurance, at this point in the event, was that we were typical office workers. City types have been an inspiration to McKeith for most of entire career, she said, explaining that she first became interested in the links between food and productivity when an investment banker became her client many years ago. "He was very successful," she recalled, "but he had suddenly started having problems: tiredness, making mistakes, lack of mental clarity, lack of focus, lack of concentration. Not really enjoying his job any more."

Like all of McKeith's clients, he was asked to monitor all the food and drink he consumed in one week, but the best he could manage was to bring in a box containing a week's worth of food packaging, and empty it onto her desk. He was living on chocolate bars and Diet Coke, McKeith said, "with the occasional croissant, pie or cake, because he said he needed the sugar to give him the energy; that's the only way he could get through the day."

The man was trapped in a spiral of poor eating, a dependency on short-term stimulants - sugar and caffeine - and declining performance. "You can't live like that and get away with it forever," McKeith stressed. "He managed to get away with it through his twenties, but it didn't last into his thirties. You do start to make mistakes; you do start to find that the energy will go. And not only that, it will affect other aspects of your health as well."

So if you're in the same kind of slump, how should you start to change your diet for the better? Firstly, McKeith suggested, you need to open your mind to new kinds of foods. Aduki beans, for example. Since her Channel 4 show You Are What You Eat first aired, Tesco has started selling them in tins. "They're amazing for strengthening the spleen," she claimed, "an amazing food for digestion, and for this mucusy, nose-blowing, hankie habit that a lot of people in this country seem to have [in other words, aduki beans help "dry up" excess catarrh]."

Next, you need to keep an accurate diary: "You've got to write down everything you eat and drink for the next seven days, with all the beverages you put into your body. And be as precise as you possibly can, because you need to see what you're doing. Deep inside you will know that to live on kebabs every day is not the answer." Alongside the times of your meals, you should keep a "mood diary", recording how you feel so that you can see how certain foods affect your emotional state.

Before you can begin your new diet, you've also got to learn to eat properly, McKeith said. "If you keep your blood-sugar levels constant... then you will feel great all day long. So for those of you who've decided to miss breakfast, you've got to train yourself to eat it again. What I would like you to do is eat breakfast like a queen, lunch like a king (because lunch should be your biggest meal of the day), and dinner like a pauper." Indeed, you should actually be eating healthy snacks in between, so that your total number of meals per day is six.

You should also aim to rotate your meals so that you don't eat the same thing twice over any four-day period. Otherwise, you risk becoming "sensitive" to particular foods and suffering adverse reactions - allergies, tiredness, even arthritis - as a result.

The question of what we should be eating was not such a difficult one to answer. We all knew what healthy food looked like. We just didn't have the discipline to eat it regularly enough. Indeed, this is why McKeith forces each of her clients to sign an affidavit - one that incurs penalties, such as charitable donations, if they stray from her advice.

For the slow release of energy during the day, she recommended pulses such as lentils and chickpeas, and fruits such as apples, pears, apricots, strawberries and blackberries. For essential fatty acids (such as Omega-3, which is vital to good brain function but typically consumed too little in the West), she recommended oily fish, nuts, seeds and avocados. She also ordered the audience to eat eight portions of fruit and veg per day, to make sure we ate at least five.

To make your tongue look better, she said, you should "start to eat brown rice, not that white rubbish that you're always eating at lunch time." Other good sources of Vitamin-B include yams, sweet potatoes, and nettle tea (especially when mixed with a herb called astragalus). "And, to be honest," McKeith said, "if you have those cuts, I suggest you get yourself a Vitamin-B complex supplement and go on it for six months." Within a week, she claimed, "You will see a difference in your energy levels, and in how you digest your food." Anyone suffering from the horrible white coating was advised to get probiotics into their diet using a supplement called acidopholous (in its powdered form, for extra potency!).

McKeith also wanted to make sure we were clear about which kinds of food and drink to avoid, and near the top of her list of forbidden pleasures was caffeine. "It's got to go," she said. "You can give yourself a treat on the weekend if you want, but get rid of it during the week... Why? Because it's just going to 'wham-bam your adrenal glands'." She explained that caffeine not only dehydrates you but also starves your body of B-vitamins. "It will create more stress in your body, and stress has the same effect every time," she said. "Cortisol pouring into the system, and all these other stress hormones... it's like giving your brain an acid bath: you can't think."

You don't have to go cold-turkey, she added, comfortingly. Just wean yourself off using something called dandelion coffee granules. "Make that coffee weaker and weaker and eventually the dandelion coffee will take over and then you can move into nettle tea, dandelion tea, roibos and all the amazing mineral-rich teas that hydrate your body."

The key thing to remember with any new diet is it is designed to smooth out the peaks and troughs of energy that you previously experienced during the day. If you persist with habits such as breakfasting on sugary, processed snacks, McKeith said, then the chances are you will "end up hungry, craving, irritable, apathetic, not able to concentrate, having a headache, poor memory, no energy. And what use is that to anybody? Do you want your employees behaving like that? They might as well go home, because they're not going to do the best job."