It's only 11am and your concentration is waning. Time for another coffee? You shouldn't really - you had your first just an hour ago, along with that iced Danish for breakfast - but without it you'll never get all this paperwork done. And, since you've gone all the way to the canteen, why not grab a muffin to see you through the morning? It may even ease that pounding headache caused by those work drinks the night before.

Most of us think that our busy lifestyles require substances such as caffeine and sweet mid-morning and afternoon snacks to survive. Nowhere do we feel we need that espresso more, for concentration and alertness, than in the workplace. But it is this type of diet that makes us less efficient and can hinder our rise to the top.

Recent research from the International Labour Office in Geneva showed that simple things such as skipping meals, which triggers low blood sugar, can shorten attention span and slow the speed with which we process information. The coffee and snacks won't better equip you to do your work, or charm those clients. Instead, the caffeine will make your pulse race and your movements jittery, your stomach bloat and your brain sluggish from all those carbs.

If you look at the kings and queens of the British boardroom - Sir Richard Branson, Sir Stuart Rose and Karren Brady, for instance - one thing they have in common is a trim waistline and healthy glow. “It can be hard eating right with this kind of job,” says Dan Pierce, a solicitor in a top City firm.

“You're in early, you leave late and there's a lot of drinking involved. Sometimes you just want to grab the nearest thing. But our canteen is very health-conscious, as are lots of the people I work with. People do try to take care of themselves.”

Justin King, the chief executive of Sainsbury's, Martin Frizell, the editor of GMTV, Margot Marrone, the co-founder of The Organic Pharmacy, and Jacqueline Gold, the chief executive of Ann Summers, all bear testimony to the “eat well to get ahead” mentality. But, as the descriptions of their diets here show, all three work hard to keep their bodies - and brains - fit. “I do try to be healthy, because I think it's important. Not just to be a boss, but for life,” King says.

If you want to eat your way to the top, and stay there, says the Times nutritionist Amanda Ursell, it can be tough at first but you will see results quickly. Start by ditching rich fatty foods and alcohol, which make it hard to sleep. We all know the pitfalls of cutting breakfast, but even just a fruit juice to start the day will make you more efficient. Eat eggs, couscous and bananas for low-release energy - particularly bananas, which are high in potassium and increase mental acuity. And do eat that chocolate bar; just make sure it's the organic kind, which has an appetite suppressant.

Best of all, caffeine junkies can better their fix by sipping peppermint tea most of the time and saving that cappuccino for the afternoon, when it will hit them all the harder. Read on for the comprehensive guide on how to eat your way to the top.

BREAKFAST

People who eat breakfast score better in tests measuring reaction times and problem solving, and have improved “free recall” ability.

Slowly digested carbohydrate with a low glycaemic index, which increases blood sugar gently, is the best choice for breaking the fast. A small bowl of muesli or porridge is ideal, as is poached eggs on rye or granary toast.

If you cannot face breakfast first thing, take a slow-release carbohydrate snack with you. A medium-ripe banana and a plain yoghurt make a perfect choice; even a glass of fruit juice will help if that is all you can cope with.

What not to eat

Avoid quickly digested foods such as croissants, white toast with marmalade, muffins and Danish pastries, which will leave you tired and craving more sugary foods soon after.

LUNCH

Your body needs refuelling and requires the right kind of nutrients to get it through the afternoon ahead. Scientists believe that eating protein-rich foods helps to keep your brain focused and fight the post-lunch tendency to drowsiness.

Eggs and tofu are especially good because they are a major source of choline, a substance needed to make the key brain chemical for memory, acetylcholine. Soya beans and peanuts are also a good source. Other good protein-rich foods include lean red meats, lean chicken or turkey and fish.

What not to eat

Avoid big portions of carbohydrates such as bread, pasta, rice or potatoes, which make you sluggish. Select more salad and vegetables.

MID-AFTERNOON

Mid-afternoon is the time when our bodies dip in energy. Scientists are not sure why but, in the absence of a siesta, what you eat and drink can dramatically improve your chances of staying alert for the afternoon. Limit the number of cups of caffeine drinks you have during the day so a mid-afternoon cup of tea or coffee will have a real pick-me-up effect. Also have some fruit. Brightly coloured ones will help to keep you well-hydrated. Alternatively, have a Naturally Gorgeous Chocolate (35g) bar which is a natural appetite suppressant.

What not to eat

Biscuits and cakes. They make your blood sugar levels rise quickly and leave you craving more, while making you feel drowsy and lethargic.

DINNER

Making time for a “proper” dinner each night will help you to avoid ordering takeaways and eating too many ready meals, which can lead to weight gain and general poor nutrition.

Increase levels of relaxing brain chemicals with carbohydrates such as sweet potatoes, buckwheat noodles and couscous, which make an ideal base for the evening meal.

What not to eat

Rich, fatty foods that will lie heavily on your stomach and make it hard to sleep. When out for dinner, check out what Britain's bosses eat (see right). They provide good examples of what to opt for. Also, avoid too much alcohol. One drink may help you to relax, but any more will stimulate your nervous system and disturb your sleep patterns.

BEDTIME

Having enough sleep is crucial to performing well the following day. A lack of sleep plays havoc with your hormones, so you wake up craving fast-release carbohydrates such as muffins, pastries and sweet cereals; this sets you on a rollercoaster of blood-sugar highs and lows for the rest of the day.

What to drink

Try chamomile, which affects parts of the brain targeted by anti-anxiety drugs. It has proven sedative effects, helping to reduce stress and improve sleep.

WHAT TO DO WHEN FACED WITH ...

Stress: A skinny cappuccino can be good to fight stress as the milk is rich in magnesium (but keep to one shot of caffeine), which boosts concentration; essential fats in fish oils help to lighten your mood. Eat oily fish two or three times a week, or invest in a fish oil supplement to give you 1g of the essential fats EPA or DHA each day.

Lack of concentration: Even mild dehydration causes poor concentration, bad moods and stress. In addition to hot drinks, consume a litre of still mineral water a day, plus a litre after exercise.

Eat potassium-rich foods, such as a medium-ripe banana or a fresh fruit salad. Iron is also essential for concentration - make sure you get 15mg a day, in tablet form if necessary.

WHAT THE BOSSES EAT

Justin King, 46. Chief executive, Sainsbury's

I was brought up to believe in eating regularly throughout the day and not to have a feast and famine approach, so this is still how I eat. I am picked up and taken to work at 6am and have breakfast in the car on the way. It always consists of a black coffee, for obvious reasons, and then a drinking yoghurt and two pieces of fruit.

Lunch is always a sandwich or wrap if I'm at work, either from the in-house restaurant or from the Sainsbury's around the corner. I'm working my way through our healthy options. I have this with a piece of fruit and a juice but I've given up the crisps. I used to have a packet, but I can't afford to at my age.

I eat out for dinner about four times a week, two to three of which are business. I always start with a tomato juice and sparkling water, have a first course of seafood, such as crab or salmon, with salad then have a grilled steak without a sauce or pork loin cooked plainly. I avoid potatoes but have vegetables; spinach is my favourite. Ice-cream and cheese are my downfall; I can rarely resist just a little bit of them.

We have stopped executive catering in business meetings, so there are no biscuits. We have a bowl of fruit on the table.

I have the odd glass of champagne or red wine and only very occasionally a lager. I do try to be healthy because I think it's important, not just to be a boss but for life generally. There is so much information about how to eat well that I'd have to make a conscious decision to ignore all of this to eat badly.

Amanda Ursell says:

Justin is wise to adapt his diet and drop the crisps. We tend to need fewer calories in our forties than in our twenties. If you can't give up the crisps, opt for baked versions and stick to 25g portions found in multipacks - they are only 120 calories a pack.

Justin is lucky that business meetings do not offer the temptation of a biscuit plate; if yours do, take your own fruit. Grapes and satsumas are the easiest to handle and make the least mess.

Martin Frizell, 49. Editor, GMTV

I think that eating well is crucial for performing well at work. My staff have to do night shifts in the newsroom. This can be particularly debilitating and you can tell the ones who take care of what they eat because they are still performing at the end of their shift when the programme is going out on air.

The camera and studio crew have bacon sandwiches at 5am. I'm in the gallery at this time and when they come round I tend to eat the bacon but not the rolls. At lunchtime, I look up on the internet what the soup of the day is in my local Eat coffee bar and have one with a wheat-free roll. Corn chowder is my favourite.

In the afternoon, I may have a few cream crackers and in the evening tend to have something light, such as a stir-fry. We have organic food boxes delivered at home with the weirdest-looking vegetables, but they taste good and no doubt do you good.

I only have lunch out once a week on business. I can't stand nouvelle cuisine, so tend to have something traditional but never pudding. When out for dinner with work it will be a smoked salmon starter, roast cod or monkfish and, again, no dessert and the odd glass of red wine.

When travelling, I admit to a guilty pleasure: the little caravans in lay-bys that serve dreadful food and tea in plastic cups. I just have to stop for a sausage sandwich. Otherwise I think I'm pretty healthy and I think what you do most of the time really does matter to keep yourself in shape for your job.

Amanda Ursell says:

If, like Martin, you like a traditional start to the day, bacon sandwiches can surprisingly be a great choice if you use extra-lean back bacon, granary bread and ketchup. The bacon is not fatty, the bread is digested slowly to give long-lasting energy and the ketchup raises levels of heart-protecting super nutrients in your blood stream.

If you are partial to the odd full breakfast at hotels, or like diving into greasy spoons, stick with poached eggs on toast with grilled tomatoes and mushrooms. The eggs are great for choline, a substance needed for brain functioning.

When tucking into soups, avoid the cream-based ones such as Stilton and broccoli, creamy mushroom and chicken - you could have a big, filling sandwich for the same calories.

Margot Marrone, 42. Co-founder, The Organic Pharmacy

One of my vices is coffee and I have to have a cappuccino every morning. I eat regularly throughout the day because, if my blood sugar drops, I find myself getting stressed and irritable and I run the risk of making bad decisions in my business.

I have a bag of muesli at home and work so, if I don't have time to eat at home, I'll have a bowl at my desk. For lunch I try to have a wholemeal sandwich with alfalfa, nut spread and grated carrots, which makes me sound incredibly healthy. But I have to admit to another weakness; the odd organic chocolate digestive biscuit in the afternoon. When I'm being good, it's a handful of nuts.

I always cook dinner; something like lentils with vegetables and small pieces of sausage. Sadly I ususlly give in to puddings but try to stick to healthy ones such as baked apples with yoghurt and, when I'm feeling less good, cream.

I try to eat organic food so travelling, which I do a lot for my business, is tricky. I always try to eat before taking a flight. I don't drink very much when on board because I hate using the loos in planes. I know that's bad for you, but I drink lots once I land. In Japan I stick to sushi but in the US I always gain weight because of the huge serving sizes.

Amanda Ursell says:

Margot's diet is very business-friendly. She controls her blood sugar with muesli and wholemeal sandwiches. The odd biscuit will not do any harm but it's important not to kid yourself that, just because something is organic, it is good for you. Margot should definitely drink more when flying to avoid dehydration and headaches, especially if doing business directly after landing. Nuts are a nutritious snack with iron, which is needed for concentration, but stick to a handful. A 100g bag has more than 500 calories.

Jacqueline Gold, 47. Chief executive, Ann Summers

For breakfast I have porridge and a red grapefruit, a low-fat yoghurt with fresh fruit, or Weetabix with skimmed milk. All of these give me a steady release of energy and keep me going during the morning ahead.

At lunchtime it is usually chicken and vegetable grilled kebabs or roasted vegetable couscous plus a selection of fresh fruit. These foods keep me alert in the afternoon.

Whether I am at home or eating out on business, the type of food I eat is similar. For example, sweet and sour stir-fried chicken with brown rice, lobster salad or avocado and grape salad.

If I need something between meals, I'll go for fresh fruit salads or raw vegetable sticks with reduced-fat hoummos and fruit. I stick with organic produce whenever I can and always go for low-fat options, using herbs to boost the flavour. It is important to keep hydrated, so I drink freshly squeezed orange juice, cranberry juice, mineral water and lots of herbal teas.

When travelling, I don't leave my food to chance. This way I don't have to rely on service stations and aircraft food. I pack my own healthy snacks, so I know exactly what I'm eating. Mini-bars in hotels are really hard to resist, but I'm pretty good.

I tend to have meetings in my office and you won't see any biscuits to tempt me, or anyone else for that matter.

Amanda Ursell says:

Jacqueline is wise to plan ahead and take food with her when travelling. If you know that you are not going to be able to resist the treats in the mini-bar when you arrive at your hotel, ring ahead and ask the hotel to remove them and give you a big fruit bowl instead. Similarly, if you are travelling by plane, go armed, as most airport lounges are awash with complementary coffee and stacked high with Danish pastries. Take pre-packed fruit salad or a Rumblers ready-to-eat cereal.

While fruit juices are healthy, it is wise to stick to a small glass a day. This counts as one serving of your five fruits and vegetables a day. Drinking more can lead to calorie overload.