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Carbohydrates and Fiber in an Athlete's Diet

September 5, 2026 Henri Dumon 7 min read

Hiilihydraatin lähteitä ja välipalavaihtoehtoja: proteiinikeksi, puuro marjoilla, smoothie ja banaani.

We fitness folk have learned to watch our protein intake with hawk-like focus — but how much thought do you give to the role of carbohydrates and fiber in your daily energy levels and how your workouts actually go? What should you know about "carbs" and fiber in the first place when tuning your diet to an active lifestyle? Let's run through the basics, plus a few points that matter especially to exercise enthusiasts (and everyone else too).

What are carbohydrates?

What exactly are carbohydrates, what does your body need them for, what are the best carbohydrate sources, and how do they connect to your energy levels? Read the quick briefings below.

What does the body need carbohydrates for?

Carbohydrates are the human body's central energy source. Typically* we get a large share — or the majority — of the energy we use from carbohydrates, so staying up and running takes a suitable supply of carbs.

Carbohydrates — chains of monosaccharides in various shapes — come in three types: simple carbohydrates, i.e. sugars; complex carbohydrates, i.e. starch; and dietary fiber, which is a complex carbohydrate of its own kind. The simpler the carbohydrate, the faster it gets broken down and absorbed from food into the body as glucose, raising blood sugar levels. Fiber, on the other hand, slows down sugar absorption and thereby balances the amount of sugar released into the bloodstream.

Carbohydrates can also be divided into digestible and non-digestible, based on what happens to them in the human digestive system. Digestible carbohydrates do just what the name says: they're absorbed by the body, affect blood sugar levels and give the eater energy. Non-digestible carbohydrates like fiber, meanwhile, aren't really absorbed in the small intestine — instead they travel on to the colon and ferment there, which is how they can influence your gut bacteria, for example.

Here in Finland, so-called fast and slow carbs are something we learn about as kids. Who hasn't been told that white bread keeps you full for about five minutes, while a slice of rye bread powers you through hours of busywork? That's because rye bread is richer in complex carbohydrates and fiber, whereas the simpler carbohydrates in white bread absorb quickly and briefly spike your blood sugar. The body answers the sugar spike with insulin, and that sudden burst of energy vanishes as glucose levels drop. Slower absorption thus means longer-lasting energy, because the body doesn't have to react to a steep climb in blood sugar.

*In certain special diets the main energy source has been swapped to fat, but we won't cover those exceptions here.

Carbohydrate sources – here's where the carbs are!

Good carbohydrate sources include oatmeal, rye bread, whole-grain pasta, barley, quinoa, beans and legumes, root vegetables (potatoes included), lentils, fruit and berries. These foods are rich in carbohydrates — but alongside the carbs, you'll also be treating yourself to dietary fiber, vitamins and minerals.

Foods that contain little fiber and plenty of added sugar or refined white grain, on the other hand, tend to be less favorable both nutritionally and health-wise. Among others, data from Harvard researchers has shown that people who eat lots of fast-absorbing carbohydrates and little fiber are more likely than others to develop type 2 diabetes. A high carbohydrate intake in itself doesn't necessarily raise the risk — but a low-fiber diet leaning on sugar and starch is a risk factor. Getting enough fiber, and minding the quality of the carbs on your plate, are therefore central to your wellbeing.

Carbohydrates from a training perspective: energy and recovery

As noted above, carbohydrates are the human body's primary energy source, so it should be blindingly obvious that carbs have a direct effect on athletic performance: no energy, no peak performances. So how many carbohydrates do you actually need?

An active person's daily carbohydrate needs vary, of course, with the type of training, its intensity and the hours put in. For example, Terveurheilija, a Finnish sports-nutrition resource, puts the carb requirement in the range of 4–12 grams per kilogram of body weight. When the body's carbohydrate stores are full — that is, your muscles hold their maximum amount of glycogen — those stores carry enough energy for roughly 1.5–2 hours of hard exercise.

For the reasons already mentioned (fiber, vitamins, minerals, blood sugar levels), even an exerciser should fuel most of their carbs from slow-absorbing carbohydrate sources. Around demanding workouts, though, fast carbs can earn their keep too. They're readily available to the body, which helps both during the performance itself and in kick-starting recovery after a session or a race.

An essential part of recovery is refilling the muscles' glycogen — i.e. carbohydrate — stores, and this process is at its most efficient right after training. Because the refill rate settles back to normal within a few hours of exercise, it pays to get carbohydrates in soon after you finish.

Based on our own observations, many keen exercisers also miss one thing: the proteins and amino acids you load up for recovery get broken down into glucose if there aren't enough carbohydrates available. This process is called gluconeogenesis. That's why the best post-workout recovery drink or snack contains both carbs and protein. That way the carbohydrates end up in your muscles' energy stores and the amino acids go to tissue repair.

Why is it important to get enough fiber?

Dietary fiber covers a wide range of non-digestible carbohydrates, which are further divided into soluble and insoluble fiber. Insoluble fiber is found in many vegetables and in the outer layers of grains. From a health perspective, insoluble fiber's biggest benefit is its effect on gut function: it binds fluid in the intestine, bulks up the gut contents and helps prevent constipation along the way. The fermentation of non-digestible fiber also supports digestion by shaping the gut's bacterial population.

What are good sources of fiber?

The gel-forming — i.e. soluble — fiber mentioned above is what we eat in berries, fruit and legumes, as well as in oats and barley. This type of fiber forms a jelly-like structure with liquid. Soluble fiber puts the brakes on rising blood sugar and curbs cholesterol absorption too. Fiber is exactly why eating a piece of fruit, natural sugars and all, doesn't cause the same blood sugar spike as the fast-absorbing carbohydrates in white bread or refined sugar. Eat fiber-rich and you'll keep your blood sugar swings in check.

Fiber and protein

The fiber in plant-based protein sources also affects how protein is absorbed in the body. Plant protein absorbs slightly slower than whey protein, for example, but according to research this makes no notable difference to an athlete in terms of protein synthesis or muscle growth. We've written about this before, when we compared plant protein and whey protein and went through the most common misconceptions about plant protein.

Frequently asked questions about carbohydrates and fiber

What should I eat before a workout?

The best pre-workout snack contains easily digestible carbohydrates and a moderate amount of protein. A protein cookie and a banana, for example, make a winning combination: the carbohydrates give you energy for the session and the protein supports your muscles. Have a bigger snack 1–2 hours before training, or a small one 30 minutes before.

Why is a protein cookie a good snack?

The Power Cookie protein cookie is rich in fiber, carries a sensible amount of carbohydrates and packs 20 grams of protein. The carbs keep you going, the protein looks after your muscles, and the excellent taste lifts your spirits.

Where do I find easy snacks for work?

Planet Amino's protein cookie is an easy, filling snack for the office — or anywhere else. The tasty Power Cookie snack cookie contains a snack-sized amount of carbohydrates, dietary fiber that steadies sugar absorption, and 20 grams of protein. Try our lowest-sugar flavor, Crunchy Salt & Pepper, while you're at it. As another option, your favorite fruit plus a drink mixed from our plant protein powder together make a quick, delicious and nutritious snack.

What would make a healthy snack?

A healthy snack follows the same logic as a healthy diet overall: it should be nutritious and filling, with enough energy, fiber and protein. A snack loaded with added sugar and light on fiber only gives you energy for a moment — the sudden blood sugar rise and insulin spike it triggers leave you feeling tired. Planet Amino's protein cookie contains a snack-sized amount of carbohydrates, dietary fiber that steadies sugar absorption (try our lowest-sugar flavor, Crunchy Salt & Pepper) and 20 grams of protein. A piece of fruit and a drink mixed from plant protein powder also team up into a first-rate snack.