It is all very well joining the gym, taking up jogging and generally increasing your exercise levels in a flourish of January enthusiasm, but to get the most from the extra activity, you also need to look at your diet, making sure that what you eat provides the nutrients needed to sustain your new fitness regimen. It also pays to eat the kinds of foods that bolster your immune system; nothing is more likely to throw a well-intentioned attempt to overhaul an out-of-shape body than a cold or bout of flu.

Begin by basing all meals around “good” carbohydrates. In other words, ones that are digested slowly and help to keep blood-sugar levels steady. They naturally keep your appetite under control and provide the steady supply of the fuel that your muscles need during exercise and to refuel once you have taken off your trainers.

Thankfully, a wide repertoire of foods tick these boxes, beginning with oats. Whether you fancy tucking into porridge, an oat-based sugar-free muesli or something slightly less holistic such as a couple of Oatibix, all provide loads of soluble fibre known as “beta-glucan”, which makes them the ultimate low-glycaemic start to the day.

As well as keeping glucose stores in your muscles topped up with energy, oats lower LDL cholesterol, which helps to look after the most important muscle in your body — your heart. Serve any of these options with berries or a long glass of fresh orange juice and you boost vitamin C. This can help to fight respiratory infections with its antiviral and antibacterial properties.

If oats don’t do it for you, another great breakfast carbohydrate comes in the form of toasted sourdough bread. Lactic acid in the bread slows its digestion in the stomach, making it a more filling option than standard loaves. This helps to keep you going for the morning and, again, provides a slow release of energy.

In our hearts we all know that grabbing a muffin, a Danish pastry or a croissant on the way to work is a bad idea. All cause a blood-sugar dip that leaves you tired, stressed and reaching for another sugar-packed energy hit, usually found in the biscuit tin. Avoid them like the plague.

As for other quick fixes such as caffeine, use them in an educated way. In other words, there is no need to ban coffee and tea, but make the most of their stimulating qualities at the times when you need them most. Starting the morning with a cup of coffee can be just what you need to get yourself going.

Hold off on any extra however until just before a workout. The caffeine will then optimise the use of fat as an energy source during exercise, helping to spare the limited supplies of glucose in your muscles so that, in effect, you can go on exercising harder for longer and burn more calories.

If you then need a cup of tea or coffee to get over the mid-afternoon energy slump, have one more cup, but make this your lot for the day to ensure a good night’s sleep and more energy for the following day.

At lunch and dinner time, slowly digested carbohydrates come in the form of tortilla wraps, pitta and pasta. The more wholegrain versions of each that you choose the better, since you will be upping your zinc levels. We all need this mineral to maintain a strong immune system by helping to make disease-fighting antibody “T-cells”; even a mild deficiency has been linked to more frequent colds and flu. Another great filling and healthy dinner carbohydrate option is a sweet potato, which is low GI and full of vitamin C.

Next, make sure that you get some kind of protein at each meal. Women are particularly bad at this, often making do with just toast or a cereal bar for breakfast, skipping lunch and snacking on things like crisps to get them through. But you leave protein out of your diet at your peril. Exercising muscles need it to help to repair themselves after any kind of exercise. There is no need to gulp down protein supplements, shakes or to start mixing protein powders into drinks left, right and centre (high protein intakes surplus to needs do not enhance muscle strength and even most athletes can meet protein needs from a balanced diet). But it does mean having something such as normal-sized servings of milk, soya milk or eggs for breakfast, something such as hoummos or a thick bean soup at lunch and, say, a fish dish or tofu in a stir-fry for dinner.

Not only does protein help to maintain muscle health, it is also vital for keeping the membranes of our nose, throats and lungs in good condition, providing strong physical barriers and the first line of defence against invading cold and flu viruses. Protein foods, especially lean red meat, eggs and oily fish such as mackerel and sardines, are also very good for providing the mineral iron. About 40 per cent of women in the UK appear to eat too little iron, leaving them prone to tiredness, stress and general lethargy.

Cure this problem and the prospect of a January fitness campaign will seem a lot more inviting.

FEED YOUR WORKOUT

Ensure that you are well hydrated before working out and that you drink during and after exercise. The best advice is to drink enough to quench your thirst. Just a 2 per cent loss of weight through dehydration can lead to a deterioration in physical performance of between 10 per cent and 20 per cent.

If you are working out for more than an hour, an isotonic sports drink can be useful because it contains small amounts of glucose and sodium to provide a little energy and to optimise fluid absorption.

If you are training very hard most days of the week and for more than 60-90 minutes at a time, it is worth having a small snack before and after training. This is the one time when having a quickly digested carbohydrate is a good thing. A handful of dried fruit, a banana on white bread sandwich or several biscuits will help to ensure that your muscle energy supplies are kept topped up. By Amanda Ursell, The Times