Here’s why: when your body senses that your easily accessible reserves of carbohydrate energy have fallen to 40 or 50 per cent, it starts to increase its use of fat as a source of fuel. It simply cannot let your blood sugar reserves empty completely, because your brain relies on them. The trouble is, fat can’t be turned into energy nearly as fast as blood sugar can, so your body becomes forced either to slow down, or to increase its effort dramatically to maintain the same speed. (In both cases, you’ll find yourself breathing more heavily, because fat conversion requires more oxygen.)
The simple solution is to take in fuel on any run over 80 or 90 minutes. (And also fluid on any run over an hour, in normal conditions.) Energy drinks can meet both your fuel and fluid needs, but if you have access to water on your run (eg race drinks stations, or park water fountains), gels are more practical. Essentially, they’re concentrated drinks of about 100 calories each, so nearly all require you to drink water in order to create a digestible solution in your stomach.
This test compares gels on the run, and you’ll find usage tips in ‘Refuelling rules’. Meanwhile, because most gels are pretty sweet, sticky and relatively expensive, it’s natural to wonder whether solid food could be a realistic alternative. The answer – we found – is that bite-sized chunks of sweet snacks are not inedible in training, but they are inferior in that they don’t get converted into energy as quickly, and require more water for digestion.
Dried Apricots Approx £1/100g
Per 100g: 165 kcal; 36g carb; 3.9g protein; 0.6g fat; 6.3g fibre
Cost per 100 calories: 60p
Main ingredients: Apricots only
Munchability: Tasty and soft, especially the partially rehydrated ones. Not over-sweet, which is a bonus if you have to face a lot of on-the-run fuelling.
The science: Fructose is a useful fast-release sugar, though too much of it can cause digestive problems. Also, the fibre content of fruit is not ideal for runners looking to avoid mid-run pitstops. Best combined with other snacks or eaten post-run.
High5 Energy Gel (26g) 69p
Per 100g: 272 kcal; 68g carb; 0g protein; 0g fat
Cost per 100 calories: 97p (also mix your own powder, £6.99 per 600g, 30p/100 kcal)
Main ingredients: Maltodextrin, fructose, dextrose, natural flavouring, sodium chloride, potassium chloride, preservatives
Munchability: The jammy blackcurrant and strawberry High5 flavours are the best on test. Very thin syrup, not hard to swallow and fractionally less sweet than the Leppin and Maxim gels. Comes in sachets or as powder that you mix using a special small bottle (this can be mixed to a thin isotonic solution).
The science: Designed so that one sachet of gel needs only 75ml of water to produce an isotonic mix in the stomach. Contains a blend of fast- and sustained-release carbohydrates.
Leppin Squeezy (125ml) £2.99 or 69p (25g)
Per 100g: 251 kcal; 63g carb; 0g protein; 0g fat
Cost per 100 calories: 77p/£1.06
Main ingredients: Enzymatic hydrolysate of maize (maltodextrin), water, fructose, sodium chloride, preservative, flavour, vitamin B1
Munchability: Fine. Very sweet, and a similar syrupy texture to the Maxim gels. Not the most natural tasting flavours, but a wide range to choose from. Comes in 65-calorie sachets or 387-calorie squeezy bottles; the bottles are very easy to use but the tops aren’t always screwed on properly at first.
The science: Maltodextrin is the standard way of providing a high energy content in a relatively digestible form. Fructose content helps to provide faster-release energy, though too much can cause gastrointestinal upset.
Mars Bar (65g) 35p
Per 100g: 477 kcal; 73.6g carb; 4.5g protein; 18.3g fat
Cost per 100 calories: 11p
Main ingredients: Milk chocolate, glucose syrup, sugar, hydrogenated vegetable fat
Munchability: The allure of chocolate is more attractive than the reality of swallowing it on the run, as it leaves you with a sticky mouth, and it can go down a little heavily if your stomach is feeling at all fragile.
The science: As with the Joosters and the gels, the sugar and glucose in a Mars Bar are fine, but the 18 per cent fat makes it hard to swallow and hard to digest. (And although its energy count is very high, over 40 per cent of the energy comes from fat, which is very slow to be processed.)
Maxim Energy Gel (100g) £1.39
Per 100g: 310 kcal; 77g carb; 0.1g protein; 0g fat
Cost per 100 calories: 45p
Main ingredients: Glucose syrup, water, acidifier, flavourings, vitamin B1
Munchability: Very acceptable, with a genuinely convenient re-sealable top. Eye-wateringly sweet, as you’d expect from glucose syrup. Nonetheless, not hard to swallow. Flavours quite strong and sharp.
The science: Designed to provide energy as fast as possible, by using only glucose, which does not need to be broken down by the body. However, it must be taken with plenty of water to be moved from the gut quickly and comfortably. (Simple sugars such as glucose are associated with a sugar high followed by a sugar low, but this effect is suppressed during exercise.)
PowerBar (65g) £1.50
Per 100g: 342 kcal; 65g carb; 15g protein; 3.1g fat
Cost per 100 calories: 68p
Main ingredients: High fructose corn syrup, oat bran, maltodextrin, milk protein, brown rice, vitamins and minerals
Munchability: Surprisingly not bad, for a bar with a reputation as a chewy monster. Carry it in small chunks, and as you chew, the bar dissolves in your mouth, is fairly easy to swallow, and leaves you with a cleaner mouth than the Mars Bar or Joosters. Bite off too much, though, and you half-suffocate.
The science: A good mix of fast- and sustained-release energy, plus useful vitamins and minerals. Deliberately designed to dissolve into a thick fluid that sits easily in the stomach, though the 15 per cent protein content means it’s better after high-intensity exercise than during it.
PowerBar PowerGel (41g) £1.25
Per 100g: 270 kcal; 67g carb; 0.3g protein; 0g fat
Cost per 100 calories: £1.13
Main ingredients: Maltodextrin, water, fructose, dextrose, sodium, potassium, vitamins C and E, amino acids
Munchability: More of a paste than a gel – too thick to swallow easily without water. Still, the consistency reduces messiness, and most of the flavours are rich and tasty. Like any gel, it can be diluted in small bottles to make it easier to swallow.
The science: Contains a mix of fast- and sustained-release carbohydrates (80:20). The added antioxidant vitamins and amino acids are useful for the diet in general, but you don’t particularly need top-ups during normal exercise.
Science in Sport Go Gel (70ml/87g) £1
Per 100g: 116 kcal; 29g carb; 0g protein; 0g fat
Cost per 100 calories: ££ Main ingredients: Water, maltodextrin, natural flavouring, gelling agents, sweetener, preservative
Munchability: A big hit, even though the packs are relatively large and heavy for their calorie value, and the mild flavours have a faint aftertaste. By far the easiest to swallow; the thin, clean gel goes down more easily than water. Also, because it’s the only isotonic ready-mixed gel on the market, it’s easy to digest, even without water.
The science: An isotonic gel is easy to swallow and easy to digest. It makes the pack size big, but at the end of the day, the easier a gel is to eat, the more likely you’ll use it. The use of maltodextrins and gelling agents mean this is a sustained-release energy gel.
Starburst Joosters (45g) 36p
Per 100g: 355 kcal; 88.6g carb; 0g protein; 0.1g fat
Cost per 100 calories: 23p
Main ingredients: Sugar, glucose syrup, starch, fruit juices (five per cent), citric acid
Munchability: Pretty good, as lively debate on the runnersworld.co.uk forums testifies. Mouth-watering and very manageable one at a time – though they turn into a gloopy, hard to swallow mass if you give in to the temptation of eating a handful at once.
The science: Simple sugars make for good quick-release energy, and they’ll keep your levels topped up if you eat little and often. Joosters are fat- and protein-free, so their digestibility is high, provided you sip water regularly with the aim of keeping the mixture in your stomach isotonic.