Homemade Isotonic Drink: Composition (Sodium, Sugar, Water), When It Helps and When It Is Unnecessary

An isotonic drink has an osmotic pressure close to that of blood plasma, that is, it contains a similar number of dissolved particles (sugars and salts) per litre. Drinks with fewer are hypotonic (for example water with a little sugar), and with more hypertonic (for example juices and sweetened fizzy drinks). The theoretical advantage of an isotonic drink is that it leaves the stomach quickly and is well absorbed, and the sugar and sodium in it help water absorption in the gut through shared glucose-sodium transport.

In practice commercial isotonic drinks contain a few percent carbohydrate and a little sodium, and their job is to supply energy, salt and water at the same time. A homemade isotonic is a simplified version: water, a sugar source (sugar, honey, fruit juice) and a pinch of salt. Its advantages are control over composition and low cost; the drawback is the lack of standardisation: the concentration depends on your spoon, not on a laboratory.

The American College of Sports Medicine (ACSM) position stand on exercise and fluid replacement stresses that the goal of drinking during exercise is to avoid excessive dehydration (loss of more than 2% of body weight from water deficit) and excessive changes in electrolyte balance, and that needs are highly individual because sweating rates and the electrolyte content of sweat vary greatly. The document notes that during exercise drinks containing electrolytes and carbohydrate can provide benefits over water alone under certain circumstances.

As for carbohydrate, Jeukendrup’s review explains that carbohydrate intake during prolonged exercise increases endurance capacity and improves performance. The old advice was 30-60 g an hour, but newer studies showed that with several types of transportable carbohydrate (glucose with fructose) oxidation can reach up to 105 g an hour, and shorter, intense exercise (about an hour at 80% VO2max) can also benefit. Recommendations therefore depend on the duration and intensity of exercise, not only on the quantity.

The practical conclusions are these: an isotonic drink with carbohydrate and sodium makes sense when exercise is long enough or intense enough that energy stores and sodium begin to matter, which in practice usually starts beyond about 60-90 minutes (the threshold is a practical rule, not a scientific constant), and in heat, when sweat losses are large. In shorter exercise of moderate intensity, plain water and a normal meal before training are usually enough.

The key practical question is whether you have to drink on a fixed schedule or can go by thirst. The meta-analysis by Goulet and Hoffman covered seven publications (82 people, cycling and running, exercise of about 96 minutes) comparing drinking to thirst (ad libitum) with planned drinking, in which on average about twice as much fluid was drunk (1073 vs 505 ml an hour). Body-mass loss was about 1.0% and 2.1% respectively. Drinking to thirst even gave a marginally better result (an improvement of 0.98%), but the difference is probably practically trivial.

In practice this means that for efforts of up to about 2 hours you do not need to drink by force, and rigidly drinking large amounts of isotonic drink brings no extra benefit. An additional argument against ‘drinking plenty in advance’ is the risk of exercise-associated hyponatraemia (too low a blood sodium). In a study of 766 participants in the Boston Marathon, 13% of runners had hyponatraemia after the race, and it was associated with weight gain during the race (OR 4.2), a finishing time over 4 hours and body-mass-index extremes. We write more about this in the article on electrolytes during training.

If you want to cut the sugar you can reduce it (to about 30 g a litre), but then the drink serves mainly to hydrate and flavour, not to provide energy. If you need more energy in efforts lasting many hours, solid sources of carbohydrate (banana, gels, dried fruit) taken with water help more than a stronger drink. Magnesium and potassium are present in a homemade isotonic only in trace amounts (for example from juice); we write about the role of these minerals in the entries on electrolytes and magnesium.

No. Oral rehydration solution (ORS) has a different composition from a sports isotonic and was developed to treat dehydration in illness. WHO and UNICEF have recommended a new reduced-osmolarity ORS formula since 2003 (of improved effectiveness compared with the previous one). The composition tested in the large CHOICE trial of 675 children was 75 mmol/l sodium, 20 mmol/l potassium, 65 mmol/l chloride, 10 mmol/l citrate and 75 mmol/l glucose, at an osmolarity of 245 mosm/l. By comparison a typical isotonic drink contains more sugar and less sodium than ORS, so on its own it does not protect against salt loss in diarrhoea.

A Cochrane review (Hahn et al.) summarised trials comparing reduced-osmolarity ORS with standard WHO ORS in children with acute diarrhoea: in a meta-analysis of eight trials there were fewer unscheduled intravenous infusions (OR 0.59; 95% CI 0.45-0.79), lower stool output and less vomiting, with no additional risk of hyponatraemia. The upshot is that the composition of ORS matters, not just a ‘sweet-salty drink’.

For a healthy person during longer exercise a homemade isotonic is usually safe, but a few groups should be careful. Sugar and juice raise blood glucose, so people with diabetes or insulin resistance should discuss use with a doctor and count the carbohydrate in their treatment plan. Sodium at a few hundred milligrams a litre is not usually a problem, but people with hypertension, heart failure or kidney disease who have sodium and fluid recommendations should take them into account. Juice is a source of potassium, which can matter in kidney disease and with certain medicines. Frequent sipping of drinks with sugar and juice promotes caries and enamel erosion.

If you train regularly, check in one session how your stomach reacts to a drink with sugar. Many people tolerate low concentrations well and higher ones worse, especially in intense exercise when digestion is limited. In competition do not test anything you have not tried before.

A homemade isotonic is a simple tool that in the right place (long exercise, heat) can be useful and in the wrong one becomes an unneeded source of sugar or, worse, a false substitute for treating dehydration. The best strategy is to recognise which situation you are in: up to an hour, water is enough; after two hours carbohydrate and sodium count; and in illness, ORS and a talk with a doctor. And in each of them, drink to thirst.