It's three in the afternoon and already you're beginning to slide down your office chair. Your eyes close and you think how you would do anything, anything, for a lie down. The afternoon slump is a feeling that is familiar to many of us. But did you know that one major cause of fatigue is the food we eat? It's not just about lack of sleep or boredom; we need all the vitamins and minerals from a healthy, balanced diet to give us enough energy to make it through the day.  Registered dietitian and nutritionist Katie Peck (pecknutrition.com), says: "The food you eat is essential in controlling your energy levels and preventing fatigue. Food produces energy. Glucose, for example, is the final breakdown product of carbohydrate or starchy foods, such as potatoes, and glucose is used by our cells to generate energy. Therefore, we need to listen to our appetite and ensure we eat when we feel hungry, allowing the body to stay energized throughout the day.  "Poorly organised or irregular eating habits cause fatigue. The brain uses the most glucose of any organ in our body, so if we don't eat regularly then we are likely to flag or tired." Magnesium-rich foods  It may sound like an element you shouldn't really be eating, but magnesium actually plays a vital role in regulating our energy levels. Peck says: "Magnesium is a mineral like calcium, and it helps to build enzymes that are involved in energy production in our body's cells. Magnesium is plentiful in foods like milk, cheese, yoghurt, nuts, seeds, legumes, wholegrain cereals and dark green leafy vegetables." Drink lots of water  Is there anything water can't do? It keeps us looking young, helps to detoxify the body and improves cognitive function. It is also one of the best ways to boost energy levels and keep tiredness at bay. Peck says: "Moderate dehydration causes us to lose strength and stamina. The body has no provision to store water and therefore the amount of water lost over 24 hours needs to be replaced. We get water from all the fluids we drink and also foods such as fruit and vegetables, which have a high water content. We also make water as a by-product of metabolised food. So, if you don't drink enough, you can always eat more. The average adult needs approximately 2.5 litres of total fluid per day. This includes all fluids obtained from food and drinks other than just water. Be an iron woman  Not getting enough iron in your diet? You're not alone - lack of iron is one of the most common nutrient deficiencies around, especially among females, who need almost double the daily amount of iron as men (14.8mg compared to 8.7mg). The most recent National Diet and Nutrition Survey, capturing data from across the UK between 2008-10, found the average intake for women aged 18-64 was just 9.8mg per day. Iron deficiency can lead to anaemia, the main symptom of which is lethargy. Thankfully, you should be able to get enough iron by eating a balanced, healthy diet. Nutritionist Cara Lewis says: "Women lose iron each month through menstruation, hence the need to ensure adequate replacement with a healthy, balanced diet. Dietary iron is found in two forms: haem iron in meat, fish, eggs and poultry, which is absorbed more readily than non-haem iron, which is found in fruits, vegetables, dried beans, nuts and cereals." Skip the chocolate bar  Around 99.9% of women have, at some point in their lives, reached for a chocolate bar when they feel their energy levels dipping. However, despite the initial kick you get from these sweet treats, the overall effect they have is to send your power supply into freefall. Lewis says: "Highly refined sources of sugar, such as chocolate bars, generally contain few nutrients. These foods also often come hand in hand with low fibre content and lead to rapid rises in blood glucose levels, contributing to a burst of energy, but do not provide slow release energy to keep you going. Choosing a snack to provide more slow release energy will help you avoid the peaks/troughs and keep you feeling fuller for longer. Better choices include fresh fruit, yoghurt (preferably low fat or plain), a fruit scone or currant bun." Choose the right carbohydrates  Don't be fooled into thinking that carbohydrates are the main dietary cause of fatigue. Though refined white carbs such as pasta, rice and bread should be limited, other carb sources will actually help you to stay full of beans throughout the day. Lewis says: "Ensuring a healthy diet, in line with the NHS's eatwell plate with regularly spaced meals and snacks will help promote steady levels of energy throughout the day. Especially ensuring regular intake of carbohydrates, preferably wholegrains, spaced throughout the day will help avoid peaks and troughs in your blood glucose levels. Go for cereal grains, vegetables, pulses and milk and aim to get 50% of your energy intake from carbohydrates." Eat regularly  This applies even (especially) if you are trying to lose weight. Skipping meals and going hungry will trick your body into storing fat and burning valuable muscle instead, and you'll be so fatigued you will find it almost impossible to exercise. Failing to eat three meals a day, with sensible snacks in between (such as fruit, nuts and seeds), will cause your blood sugar to fall, leading to a crash in energy levels - evidence that eating is never cheating. Limit caffeine intake  Another case of 'what goes up, must come down', caffeine has a similar effect on the body as sugar-laden chocolate bars. If you drink too much coffee and rely on a hit to get you going in the morning, you are exposing yourself to symptoms of caffeine withdrawal such as headaches and, you guessed it, fatigue. And, according to netdoctor.co.uk, caffeine can interfere with iron absorption, so it should be avoided at mealtimes, especially by those at risk of caffeine deficiency. Try switching to herbal teas instead to reduce your caffeine intake and benefit from all the extra antioxidants. Eat lots of fibre  Dietary fibre doesn't just keep you regular, it also fights fatigue. Fibre - found in abundance in foods like beans, lentils, fruit, vegetables, and wholegrain cereals and bread - is known to slow down digestion after eating, meaning your body gets a steady stream of energy throughout the day, rather than a quick jolt followed by a sharp crash.