Energy Gels vs Isotonic Drinks vs Chews: A UK Guide to Fuelling Long Sessions

Anything past about ninety minutes — a long ride, a half marathon, a hilly hike, a full football match plus extra time — turns into a logistics problem as much as a fitness one. You have to get carbohydrate and fluid in while moving, and the products designed for that job come in three formats that behave differently. Picking between them is mostly about your stomach and your access to water, not about brand.

The word that decides everything: tonicity

Isotonic, hypotonic and hypertonic are not marketing adjectives. They describe concentration relative to blood, and they determine what happens when the product hits your gut.

• Hypotonic — more dilute than blood. Leaves the stomach fastest. Prioritises fluid over fuel.
• Isotonic — roughly the same concentration as blood, around 280–300 mOsm/kg. The compromise position: reasonable fluid delivery and reasonable carbohydrate at the same time.
• Hypertonic — more concentrated than blood. Most standard energy gels are here. They deliver a lot of carbohydrate in a small packet, and they pull water into the gut to dilute themselves — which is exactly why a thick gel taken without water can feel unpleasant.

That last point is the single most useful thing in this article. A hypertonic gel needs water with it. An isotonic gel does not. Most of the "gels upset my stomach" complaints come down to taking a concentrated gel dry, at pace, with nothing to wash it down.

If you run without carrying a bottle, isotonic gels such as SIS Isotonic Gels or SIS Go Isotonic Energy Gel are the sensible default. If you are on a bike with two bottles, a smaller concentrated gel is more efficient to carry.

How much carbohydrate, and how often

The number that matters is grams of carbohydrate per hour, not number of gels.

The range used across endurance sport is broadly 30–60 g per hour for sessions of one to two and a half hours, rising towards 60–90 g per hour for longer efforts. The upper end is only reachable with products that combine glucose with fructose, because the two are absorbed by different intestinal transporters — which is why you will see "2:1 glucose:fructose" on some labels and why a single-source gel taken twice as often does not simply double the delivery.

Practically: a typical gel carries 20–30 g of carbohydrate. So one every 30–45 minutes gets most people into the useful range, and the amount your stomach tolerates is highly individual and improves with practice. Rehearse it in training. Race day is the wrong place to discover you cannot face a fourth gel.

What the authorised claim actually requires

Sports drinks are one of the very few supplement categories in Great Britain with a genuinely authorised health claim, and the conditions attached are specific enough to be useful as a buying filter.

To carry the carbohydrate-electrolyte solution claims — relating to the maintenance of endurance performance during prolonged endurance exercise, and to the absorption of water during physical exercise — a drink must, as made up for use:

• provide 80–350 kcal per litre from carbohydrate;
• derive at least 75% of that energy from high-glycaemic carbohydrates such as glucose, glucose polymers or sucrose;
• contain 20–50 mmol per litre of sodium (roughly 460–1,150 mg/L);
• have an osmolality between 200 and 330 mOsm/kg of water.

That last line is the legal definition of the isotonic window. If a drink meets it, the brand is allowed to say so; if it does not, a compliant label will stay quiet. It also explains why zero-sugar electrolyte tablets cannot carry the endurance claim — no carbohydrate, no claim. They are hydration products, not fuelling products, and both have a place.

Format by format

Gels

Highest carbohydrate density per gram carried, easiest to time precisely, and the format most likely to cause gut trouble if you get the water wrong. HIGH5 Energy Gel, Applied Nutrition Endurance Velocity Energy Gel and Vitargo Gel cover the range from small concentrated sachets to larger boxes.

Drinks and powders

Fuel and fluid in one, and generally the gentlest on the stomach because they arrive already dilute. The catch is that you can only carry so much liquid. HIGH5 Isotonic Hydration Drink, Vitargo Electrolyte Sachets and ready-mixed options such as OSHEE Isotonic sit here.

Chews and bars

Slower to eat, easier on people who cannot face the texture of gel, and better suited to steady efforts than to a sprint finish. They need water for the same reason a gel does.

Caffeine versions: use them deliberately

Most gel ranges include a caffeinated version — HIGH5 Energy Gel Caffeine is a typical example, usually carrying 30–100 mg per sachet. Two practical points. First, there is no authorised claim in Great Britain for what caffeine does, so labels state the milligrams and leave it there; treat any listing that promises more as a red flag about the seller. Second, if you take four caffeinated gels across a long event you may be well past a normal daily coffee intake, so plan which ones are caffeinated rather than buying a whole box and finding out afterwards.

Comparing cost sensibly

Gels are sold singly, in boxes of 20, and in mixed variety packs, which makes shelf prices hard to line up. Convert everything to cost per 30 g of carbohydrate: read carbohydrate per sachet off the panel, divide the pack price by the number of sachets, then scale. A large gel with 40 g of carbs at a higher unit price can easily be better value than a small one at 22 g.

Variety packs such as the Endurance Energy Gel Variety Pack are worth the premium once — flavour fatigue is real over three hours, and finding out which one you can still stomach at the end is worth more than saving a few pence.

Have a look

Gels, tablets, powders and ready-to-drink options are in Hydration & Electrolytes, with fuelling powders in Carbohydrates and pre-mixed cans in Energy & Drinks. For the training side rather than the product side, see our guides to supplements for endurance runners and when you actually need electrolytes.

A quick note

This article is general information, not medical or nutritional advice. Food supplements are not a substitute for a varied, balanced diet and a healthy lifestyle, and should not be used to prevent or treat any illness. Always read the product label, do your own research, and speak to a qualified doctor, pharmacist or registered nutritionist before starting a new supplement — especially if you are pregnant, breastfeeding, under 18, taking medication or managing a health condition.