Management gurus have long presented us with reams of slick advice on how to get ahead at work: from delivering PowerPoint slides with suitable gravitas to perfecting the firmness of our handshake to reflecting the body language of an unreasonable client. And very useful they are too.

But the more time I spend in boardrooms as a health coach (rather than the boss I used to be), the more I see that there’s something that’s stealthily sabotaging the effectiveness of even the most competent, intelligent and experienced professionals. And that is rampant, brain-quashing blood sugar crashes.

Again and again, I talk to people in positions of considerable authority who tell me that, at regular points throughout every working day, they’re hit by complete brain freeze. Yes, they may be physically present and correct, sitting in front of their screens and ostensibly at work. But the lights aren’t on. And, if they are managing to slowly squeeze out some work, it’s more than likely to be below par (let’s not forget that today’s mistakes are tomorrow’s workload).

No matter what the sector, how big the company is or what position people hold, the same story plays out day after day. Productivity is being eaten away by dietary choices.

So, what’s happening here? Well, let’s start with professionals opting to start a day’s work on a bowl of Coco Pops; or a more “adult” version of a breakfast cereal; or a bagel; or a sugary drink. The resultant leap in blood sugar is followed, as surely as night follows day, by an almighty crash. This then shrinks their mental faculties and has them craving another sugar hit. And that won’t generally be too hard to find, given that workplaces are now awash with cakes, doughnuts etc. to be wheeled out because Sarah from accounts has a birthday at the weekend… or for no other reason than it’s a Tuesday.

Well, you might say, where’s the harm in a bit of sweetness at the office, given that we’re working longer hours than ever in increasingly pressurised environments? The problem is that most of us are combining vast amounts of sugar and refined carbs with a low-nutrient diet that’s low in protein and good fats: in other words, a diet perfectly designed to render you completely rubbish at your job.

Working in an office may not be physically taxing but it’s certainly hungry work. If you’ve ever been trapped in a three-hour meeting with no decent food, you’ll know just how difficult it is to maintain your professionalism and composure while battling a rumbling tummy. For my entire career in PR, I was regularly starving at work, even though I was eating industrial amounts of toast. And, believe me, when my blood sugar’s low, the one and only coherent thought in my head is: “I need food. Now.”

I remember many years back sitting in a meeting with 10 lawyers and munching on a custard cream. I was reaching for a second one when I heard my name and looked up to see the 10 lawyers looking at me expectantly, waiting for an insightful, consultant-like response to a question I patently hadn’t heard a word of because, in the fog of a blood sugar crash, I’d briefly zoned out.

Let’s take a quick look now at what’s actually going on inside our head. Although the brain makes up around 2% of our body weight, it’s pretty insatiable as organs go, gobbling up a whopping 20-30% of our energy intake. Our prefrontal cortex, the “executive” bit of the brain, needs stable blood sugar for us to plan and function effectively – and to behave reasonably. It doesn’t matter what your job description is, you rely on your brain to make well thought-out decisions and, just as importantly, to get on well with your colleagues (a blood sugar crash can transform even the most mild-mannered employee into an irritable energy-vampire).

Rather belatedly in my career, I’ve finally learned to keep my blood sugar stable. With just a little bit of planning and tweaking of your schedule, you can do the same. Here are four easy ways to ensure you’re at your sharpest throughout the day:

• However rushed you are in the morning, make time to have a proper breakfast that includes a high quality protein and some good fats: these are what will keep you feeling full and your blood sugar stable. For me, switching to eggs for breakfast was the single most important factor in being able to concentrate all morning for the first time in my life.
• Don’t fall into the trap of just having a plain veggie salad for lunch with no protein – top it with last night’s chicken or salmon etc. and throw in some avocado and pumpkin seeds for extra good fat punch.
• Keep a bag of nuts, such as walnuts, in your bag to head off that 4pm danger zone. They’re a great source of protein and those brain-friendly good fats. (Our brain is 60% fat and needs them to function properly.)
• Stay hydrated. It’s all too easy to get through five cups of tea/coffee a day when there’s constantly someone offering to put the kettle on. Although this might seem to be giving you an energy boost, it’s really just sapping you of nutrients and energy. Keep a glass of water on your desk and make sure you drain it several times a day. Research has shown that even the mildest dehydration of only 1-2% of body water can affect your ability to concentrate, your alertness levels and your short-term memory function.