Carbs are needed to fuel almost every type of activity and the amount of glycogen stored in your muscles and liver has a direct effect on your exercise performance. A high muscle-glycogen concentration will allow you to train at your optimal intensity a achieve a greater training effect. A low muscle-glycogen concentration, on the other hand, will lead to early fatigue, reduced training intensity and suboptimal performance.
How many carbs should I eat per day?
Sports nutritionists and exercise physiologists consistently recommend that regular exercisers consume a diet consisting of a relatively high percentage of energy from carbs and a relatively low percentage of energy from fat. There is pletiful evidence that such a diet enhances endurance and performance for exercise lasting longer than one hour.
You can estimate your optimum carb intake in one of two ways: from your energy intake or from your body weight and activity level. From your energy intake, it is recommended that you consume a diet containing 60-70% energy from carbs. You can estimate your usual energy (calorie) intake by keeping a weighed food diary over several consecutive days and then looking up the calorie values of each food using food tables or in a calorie counter. (Alternatively, you can use the formulae based on RMR to estimate your energy needs.) However, this method can be misleading in terms of providing optimum nutrition. For those with high energy requirements (say, 4000-5000 calories daily), even a diet providing 50% of energy from carbs would contain more than enough to maintain muscle glycogen stores. Conversely, for those with low energy requirements, even a diet providing 60% energy from carbs may not contain enough to maintain muscle glycogen stores.
The other way to estimate your optimum carb intake is from your body weight and activity level. This method, based on individual’s body weight and training volumes, is more popular among sports nutritionists and sports scientists who consider it to be more accurate. It is more flexible as it takes account of different training requirements and can be calculated independent of calorie intake. The amount of carbs per kg of body weight needed per day according to your activity level is:
Activity level Optimum carb intake per day
3-5 hours / week 4-5 g / kg body weight
5-7 hours / week 5-6 g / kg body weight
1-2 hours / day 6-7 g / kg body weight
2-4 hours / day 7-8 g / kg body weight
more than 4 hours / day 8-10 g / kg body weight
Carbs are traditionally classified according to their chemical structure. The most simplistic method divides them into two categories: simple (sugars) and complex (starches and fibres). These terms simply refer to the number of sugar units in the molecule.
Simple carbs are very small molecules consisting of one or two sugar units. They comprise the monosaccharides (1-sugar units): glucose (dextrose), fructose (fruit sugar) and galactose; and the disaccharides (2-sugar units): sucrose (table sugar, which comprises a glucose and fructose molecule joined together) and lactose (milk sugar, which comprises a glucose and galactose molecule joined together).
Complex carbs are much larger molecules, consisting of between 10- and several thousand-sugar units (mostly glucose) joined together. The include the starches, amylose and amylopectin, and the non-starch polysaccharides (dietary fibre), such as cellulose, pectin and hemicellulose.
In between simple and complex carbs are glucose polymers and maltodextrin, which comprise between 3- and 10-sugar units. They are made from the partial breakdown of corn starch in food processing, and are widely used as bulking and thickening agents in processed foods, such as sauces, dairy desserts, baby food, puddings and soft drinks. They are popular ingredients in sports drinks and engineered meal-replacement products, owing to their low sweetness and high energy density relative to sucrose.
In practice, many foods contain a mixture of both simple and complex carbs, making the traditional classification of foods into ‘simple’ and ‘complex’ very confusing. For example, biscuits and cakes contain flour (complex carbs) and sugar (simple carbs), and bananas contain a mixture of sugars and starches depending on their degree of ripeness. Moreover, not all carbs are ‘equal’. It’s tempting to think that simple carbs, due to their smaller molecular size, are absorbed more quickly than complex carbs, are produce a large and rapid rise in blood sugar. Unfortunately, it’s not that straightforward. For example, apples (containing simple carbs) produce a small and prolonged rise in blood sugar, while many starchy foods such as potatoes and bread (complex carbs) are digested and absorbed very quickly and give a rapid rise in blood sugar. So the old notion about simple carbs giving fast-released energy and complex carbs giving slow-released energy is incorrect and misleading.
The Glycaemic index
To describe more accurately the effect different foods have on your blood sugar levels, scientists developed the glycaemic index (GI). While the GI concept was originally developed to help diabetics control their blood sugar levels, it can benefit regular exercisers and athletes too. It is a ranking of foods from 0 to 100 based on their immediate effect on blood sugar levels, i.e. a measure of the speed at which you digest food and convert it into glucose. The faster the rise in blood glucose the higher the rating on the index. To make a fair comparison, all foods are compared with a reference food, such as glucose, and are tested in equivalent amounts of carbs.
The GI of foods is very useful to know because it tells you how the body responds to them. If you need to get carbs into your bloodstream and muscle cells rapidly, you would chose high GI foods. Sports nutritionists find it useful to classify foods as high GI (71-100), medium GI (56-70) and low GI (0-55). This simply makes it easier for people to select the appropriate food. In general, refined starchy foods, including potatoes, white rice and white bread, as well as sugary foods, such as soft drinks and biscuits are high on the GI. Less refined starchy foods, like porridge, beans, lentils, muesli, as well as fruit and dairy products are lower on the GI. Factors that influence the GI of a food include the size of the food particle, the biochemical make-up of the carbs (the ratio of amylose to amylopectin), the degree of cooking (which affects starch gelatinisation) and the presence of fat, sugar, protein and fibre.
The biggest drawback of the GI is that it doesn’t take into account the portion size you are eating. For example, watermelon has a GI of 72 and is therefore classified as high GI food, which puts it off the menu on a low GI diet. However, an average slice (120 g) gives you only 6 g carbs, not enough to raise your blood sugar level significantly. You would need to eat at least 6 slices (720 g) to obtain 50 g carbs, which is the amount used in the GI test. Similarly, many vegetables appear to have a high GI, which means they may be excluded on a low GI diet. However, their carb content is low and therefore their effect on blood glucose levels would be small. Another drawback is that some high fat foods have a low GI, which gives a falsely favourable impression of the food. For example, the GI of crisps or chips is lower than than of baked potatoes. Fat reduces the rate at which food is digested but saturated and trans fats can push up heart disease risk. It is important you don’t select foods only by their GI – check the type of fat (i.e. saturated or unsaturated) and avoid those that contain large amounts of saturated or trans fats.
When and how many carbs to eat?
What, when and how much you eat affects your performance, strength and endurance in training.
Ideally, you should eat between 2 and 4 hours before training, leaving enough time for your stomach to settle so that you feel comfortable, not too full and not too hungry. Clearly, the exact timing of your pre-exercise meal will depend on your daily schedule and the time of day you plan to train. If you leave too long an interval before eating and training, you will be at risk of hypoglycaemia (low blood glucose) and this will certainly compromise your performance. You will fatigue earlier and, if you feel light-headed, risk injury too. On the other hand, training with steady blood glucose level will allow you to train longer and harder. Most studies suggest 2.5 g carbs per kg of body weight about 3 hours before exercise. You may need to experiment to find the exact quantity of food or drink and the timing that works best for you.
Whether to eat high GI or low GI foods pre-exercise has long been a controversial area. Many experts recommend a low GI meal based on the idea that such a meal would supply sustained energy during exercise. For example, most fresh fruit, vegetables, milk or yoghurt would be suitable, or a combination of carbs, protein and healthy fat. However, other researchers believe that the GI of the pre-exercise meal has little effect on performance. It’s certainly not a clear-cut case but what you have to consider is the timing of your pre-exercise meal. High GI foods can be more ‘risky’ to your performance, particularly if you are sensitive to blood sugar fluctuations. Get the timing wrong and you may be starting exercise with mild hypoglycaemia – remember, high GI meals produce a rapid rise in blood sugar and a short-lived dip afterwards. The safest strategy may be to stick with low GI pre-exercise meals and then top up with high GI carbs during exercise if you are training for more than 60 minutes.
For most activities lasting less than one hour, drinking anything other than water is unnecessary provided your pre-exercise muscle glycogen levels are high, i.e. you have consumed sufficient amount of carbs during the previous few days and eaten a carb-containing meal 2-4 hours before exercise. However, if you are exercising for more than 60 minutes at a moderate-high intensity, consuming carbs during your workout can help delay fatigue and enable you to perform at a higher intensity. It may also help you to continue exercising when your muscle glycogen stores are depleted.
During that first hour of exercise, most of your carb energy comes from muscle glycogen. After that, muscle glycogen stores deplete significantly, so the exercising muscles must use carbs from some other source. That’s where blood sugar, glucose, comes into its own. As you continue exercising hard, the muscles take up more and more glucose from the bloodstream. Eventually, after 2-3 hours, your muscles will be fuelled entirely by blood glucose and fat. Sound handy, but you cannot keep going indefinitely because blood glucose supplies will eventually dwindle. Some of this blood glucose is derived from certain amino acids and some comes from liver glycogen. When liver glycogen stores run low, your blood glucose levels will fall, and you will be unable to carry on exercising at the same intensity. That is why temporary hypoglycaemia is common after 2-3 hours of exercise without consuming carbs. In this state, you would feel very fatigued and light-headed, your muscles would feel very heavy and the exercise would feel very hard indeed. In other words, the depletion of muscle and liver glycogen together with low blood sugar levels would cause you to reduce exercise intensity or stop completely. This is sometimes called “hitting the wall” in marathon running.
Clearly, then, consuming additional carbs would maintain your blood sugar levels and allow you to exercise longer. An intake of between 30-60 g carbs per hour is recommended by leading researchers. This matches the maximum amount of carbs that can be taken up by the muscles from your bloodstream during aerobic exercise. Consuming more carbs will not improve your energy output nor reduce fatigue. It is important to begin consuming carbs before fatigue sets in. It takes at least 30 minutes for the carbs to be absorbed into the bloodstream. The best strategy is to begin consuming carbs soon after the start of your workout, certainly within the first 30 minutes. While consuming carbs during exercise can delay fatigue, perhaps by up to 45 minutes, it will not allow you to keep exercising indefinitely. Eventually, factors other than carb supply will cause fatigue.
It makes sense that the carbs you consume during exercise should be easily digested and absorbed. You need it to raise your blood sugar level and reach your exercising muscles rapidly. Thus, high or moderate GI carbs are generally the best choices. Whether you choose solid or liquid carbs makes little difference to your performance, provided you drink water with solid carbs. Most athletes find liquid carbs (i.e. sports drinks) more convenient. Carb-containing drinks have a dual benefit because they provide fluid as well as fuel, which reduces dehydration and fatigue. Obviously, you do not have to consume a commercial drink; you can make your own from fruit juice, or sugar, or squash, and water. If you prefer to consume food as well as drinks during exercise, energy or ‘sports nutrition’ bars, sport gels, ripe bananas, raisins or fruit bars are all suitable. Have a drink of water at the same time and aim to consume at least 1 litre of fluid per hour. Recent studies have suggested that consuming a drink containing protein as well as carbs (for example, skimmed milk) during exercise may minimize protein breakdown following exercise and improve recovery.
The length of time that it takes to refuel depends on four main factors: how depleted your glycogen stores are after exercise; the extent of muscle damage; the amount and the timing of carbs you eat; your training experience and fitness level.
The more depleted your glycogen stores, the longer it will take you to refuel. This, in turn, depends on the intensity and duration of your workout. The higher the intensity, the more glycogen you use. For example, if you concentrate on fast, explosive activities or high-intensity aerobic activities, you will deplete your glycogen stores far more than for low-intensity activites of equal duration. The minimum time it would take to refill muscle glycogen stores is 20 hours. After prolonged and exhaustive exercise (e.g. marathon), it may take up to 7 days. The duration of your workout also has a bearing on the amount of glycogen you use. For example, if you run for one hour, you will use up more glycogen than if you run at the same speed for half an hour. Therefore, you need to allow more time to refuel after high-intensity or long workouts.
Certain activities which involve eccentric exercise (e.g. heavy weight training, plyometric training or hard running) can cause muscle fibre damage. Eccentric exercise is defined as the forced lengthening of active muscle. Muscle damage, in turn, delays glycogen storage and complete glycogen replenishment could take as long as 7-10 days. The higher your carb intake, the faster you can refuel your glycogen stores. This is particularly important if you train on a daily basis, so that over successive days of training your glycogen stores do not become progressively lower. Therefore, if you wish to train daily or every other day, make sure that you consume enough carbs. If not, you will be unable to train as hard or as long, you will suffer fatigue sooner and achieve smaller training gains. Efficiency in refuelling improves automatically with training experience and raised fitness levels. Thus, it takes a beginner longer to replace his glycogen stores than an experienced athlete eating the same amount of carbs. That is why elite sportspeople are able to train every day while beginners cannot and should not! Another adaptation to training is an increase in your glycogen storing capacity, perhaps by as much as 20%. This is an obvious advantage for training and competition.
The best time to start refuelling is as soon as possible after exercise, as glycogen storage is faster during this post-exercise ‘window’ than at any other time. Research has shown that glycogen storage following exercise takes place in three distinct stages. During the first 2 hours, replenishment is most rapid at approximately 150% the normal rate. During the subsequent 4 hours the rate slows but remains higher than normal. After this period glycogen manufacture returns to the normal rate. Therefore, eating carbs during this time speeds glycogen recovery. This is most important for those athletes who train twice a day. There are two reasons why glycogen replenishment is faster during the post-exercise period. Firstly, eating carbs stimulates insulin release, which, in turn, increases the amount of glucose taken up by your muscle cells from the bloodstream, and stimulates the action of the glycogen-manufacturing enzymes. Secondly, post-exercise, the muscle cell membranes are more permeable to glucose so they can take up more glucose than normal.
Most researchers recommend consuming 1 g per kg body weight during the 2-hour post-exercise period. Even if you finish training late in the evening, you still need to start the refuelling process, so do not go to bed on an empty stomach! For efficient glycogen refuelling, you should continue to eat at least 50 g carbs every 2 hours until your next main meal. Therefore, plan your meals and snacks at regular intervals. If you leave long gaps without eating, glycogen storage and recovery will be slower. Are high GI or low GI carbs best for recovery? Since high GI foods cause a rapid increase in blood glucose levels, it seems logical that foods with a high GI would increase glycogen replenishment during the initial post-exercise period. Indeed, a number of studies have shown that you get faster glycogen replenishment during the first 6 hours after exercise (and, in particular, the first 2 hours) with moderate and high GI carbs compared with low GI. It makes no difference to the glycogen storage rate whether you consume liquid or solid forms of carbs. However, the difference between high GI and low GI is probably less important for you if you do not train every day. After 24 hours, muscle glycogen storage is about the same on a high GI as on a low GI diet. In other words, high GI foods post-exercise get your glycogen recovery off to a quick start but low GI foods will result in the same level of recovery 24 hours after exercise. But there are other performance benefits of a low GI recovery diet – it may improve your endurance the next day. Furthermore, it may encourage the body to use more fat to fuel the muscles during exercise. So, a low GI diet encourages greater fat burning, which not only benefits your performance but may also help you achieve faster weight (body fat) loss.
Moreover, combining protein with carbs has been shown to be more effective in promoting glycogen recovery than carbs alone. Consuming a protein-carb drink also appears to enhance recovery following resistance training, as it promotes more efficient muscle tissue growth as well as faster glycogen refuelling. The combination of protein and carbs promotes the release of insulin, which stimulates muscle glycogen replenishment as well as the transport of amino acids into muscle cells, thereby promoting protein synthesis, and blunts the rise in cortisol that would otherwise follow exercise. Cortisol suppresses the rate of protein synthesis and stimulates protein catabolism. The optimal post-workout meal or drink, it seems, should include 20-40 g protein and 60-120 g carbs, whether from solid food or commercial sports drinks or bars. Carbs should be the foundation of your post-workout meal, with protein and some healthy fat supporting your recovery. This will lead to optimal glycogen recovery and muscle rebuilding or growth – depending on your training mode – between training sessions.
After you have taken advantage of the 6-hour post-exercise window, when and what carbs you eat for the rest of the day are still important for glycogen recovery. To optimise glycogen replenishment, you should ensure a relatively steady supply of carbs into the bloodstream. In practise, this means eating carbs in small meals throughout the day. Slowly digested carbs – that is, meals with low GI – cause much smaller rises and falls in blood sugar and insulin and create the ideal environment for the replenishment of steady glycogen stores. Avoid consuming large, infrequent meals or lots of high GI meals as they will produce large fluctuations in blood sugar and insulin. This means there will be periods of time when blood sugar levels are low, so glycogen storage will be minimal. Surges of blood sugar and insulin are more likely to result in fat gain.
Are there any other benefits of a low GI daily diet? While it is important for regular exercisers for promoting glycogen recovery, it also has numerous health benefits and is widely promoted to the general population for weight loss. Reducing the GI of the diet increases satiety (feelings of satisfaction after eating), improves appetite control and makes it easier to achieve a healthy body weight. Studies have shown that the lower the GI of a meal the more satisfied and less hungry you are likely to be during the following 3 hours. A low GI diet has also been shown to increase the resting metabolic rate, which increases daily energy expenditure and increases the rate of weight loss. What’s more, low GI diets can help reduce the risk of cardiovascular disease by lowering total and LDL (‘bad’) cholesterol levels. This is due to the lower insulin levels associated with low GI eating. High insulin levels stimulate cholesterol manufacture in the liver and total cholesterol may drop by as much as 15% on a low GI diet. A low GI diet is also promoted for the management of type 2 diabetes. It can improve blood glucose control as well as manage high cholesterol levels, typically associated with type 2 diabetes. A low GI diet can help prevent and manage the metabolic syndrome – the concurrent existence of raised blood glucose, high blood pressure, obesity and insulin resistance – and polycystic ovary syndrome.