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Nutrition and daily life

Carbohydrates: the main fuel for the brain and muscles

Bread, rice, potatoes, porridge, fruit — all of these are rich in carbohydrates. They are sometimes called “bad”, but during treatment carbohydrates are the easiest source of energy. This topic explains the types of carbohydrates, what happens to them in the body and how to choose them wisely during treatment.

Last updated: ~4 min read
On a dark navy background, a wooden board with a bowl of white rice, a bowl of buckwheat, a round loaf cut open, boiled potatoes, a banana and a peeled orange.
Figure 1.Everyday sources of carbohydrate: rice, buckwheat, bread, potatoes and fruit.

01Short answer

Carbohydrates come in three types: sugars, starch and fibre. During digestion, sugars and starch are turned into glucose, which fuels the brain, muscles and blood cells. Fibre is not digested but helps the gut work. There is no need to cut carbohydrates out during treatment; it is better to choose whole grains, vegetables, fruit and legumes and limit sugary drinks and sweets. When appetite is poor or you have diarrhea or mouth sores, soft white starchy foods (rice porridge, mashed potato) may be easier for a while. Steroids can raise blood sugar — your doctor will advise you.

02Simple explanation

Carbohydrates are the sugars, starch and fibre in food. They are found mainly in plant foods, and also in milk.

Starch is a long-chain carbohydrate: rice, wheat, potatoes, buckwheat. Digestion breaks the chain into single glucose molecules.

Glucose travels in the blood to the whole body. Under normal conditions the brain runs almost entirely on glucose, and red blood cells get their energy only from it.

Fibre is not digested, but it forms the stool, feeds helpful gut bacteria and helps sugar enter the blood more slowly.

Fast carbohydrates (sugary drinks, sweets) raise blood sugar quickly and let it drop quickly. Slow ones (whole grains, legumes) release energy more evenly.

03Scientific explanation

WHO guideline, 2023: adults are advised to get carbohydrates mainly from whole grains, vegetables, fruit and legumes and to eat at least 25 g of naturally occurring fibre a day. This is general advice for healthy eating.

Free sugars: according to the 2015 WHO guideline, free sugars (added sugar, honey, sugars in juices) should provide less than 10% of total energy, and reducing them below 5% may bring additional benefit.

In cancer: according to the 2017 ESPEN guidelines, people who are losing weight and have reduced insulin sensitivity may benefit from getting part of their energy from fat instead of carbohydrate; this is decided individually. There is no evidence that “sugar feeds cancer, so carbohydrates must be cut out completely”.

Treatment and blood sugar: according to NCI, corticosteroids and some other medicines can raise blood sugar. In that case your doctor checks your sugar and adjusts treatment if needed.

04What happens in the body?

Three types of carbohydrate

Sugars (fruit, milk, honey, table sugar), starch (grains, bread, potatoes, legumes) and fibre (vegetables, fruit skins, whole grains). Each plays a different role in the body.

Digestion: from starch to glucose

Saliva starts breaking down starch in the mouth, and the process finishes in the small intestine. Glucose passes into the blood, and insulin helps it enter the cells.

On a dark navy background, an enlarged translucent grain of rice breaks into tiny golden glucose spheres that flow through a red blood vessel to the cells.
Figure 2.During digestion starch becomes glucose, which the blood carries to the cells.

Brain, muscles and blood cells

The brain uses a lot of glucose every day. Muscles store extra glucose as glycogen. When carbohydrate intake is low, the body may start making glucose from protein.

Better choices

Wholegrain bread, buckwheat, oats, brown rice, legumes, vegetables and fruit provide vitamins, minerals and fibre along with energy. Sugary drinks provide mostly “empty” calories.

On a dark navy background, oat porridge with apple slices and wholegrain bread on the left, a glass of sugary soda and a pile of sugar cubes on the right.
Figure 3.Slow carbohydrates give steady energy; sugary drinks give a quick rise and fall.

On difficult days

With diarrhea, nausea or mouth sores, soft foods are easier: white rice, mashed potato, soft pasta, porridge, banana. This is a temporary choice; when things improve, fibre-rich foods are brought back gradually.

Steroids and diabetes

Blood sugar can rise in people taking steroids and in people with diabetes. Swapping sugary drinks for water or unsweetened tea and spreading carbohydrates evenly across the day can help. Only your doctor changes medicine doses.

05Carbohydrates: four steps

Know the types, understand the fuel, choose slow ones, watch your sugar.

Three types — Sugar, starch, fibre.1Three typesSugar, starch, fibre.
Glucose is fuel — For the brain and muscles.2Glucose is fuelFor the brain and muscles.
Choose slow ones — Whole grains, legumes, vegetables.3Choose slow onesWhole grains, legumes, vegetables.
Watch your sugar — On steroids or with diabetes.4Watch your sugarOn steroids or with diabetes.
Steady energyLess weakness through the day.
Clear thinkingThe brain does not run out of fuel.
Weight supportEnergy balance is kept.

06Sources of carbohydrate (simplified)

FoodWhat it givesNote for treatment
Whole grains, buckwheat, oatsEnergy, fibre, B vitaminsCan be reduced for a while with diarrhea
White rice, pasta, potatoesEasily digested energyA comfortable choice on hard days
LegumesEnergy, protein, fibreCook until very soft; may cause wind
FruitSugars, fibre, vitamin CWash thoroughly
Milk, yogurtLactose, protein, calciumPasteurized only
Sugary drinks, sweetsFast energy, few nutrientsLimit, especially on steroids

* The table is general information. If you have diabetes or take steroids, your doctor gives individual rules.

07Key terms

Carbohydrates
A food group made up of sugars, starch and fibre.
Glucose
The main sugar in the blood, fuel for cells.
Starch
A long-chain carbohydrate found in grains and potatoes.
Glycogen
Stored glucose in the liver and muscles.
Insulin
The hormone that helps glucose enter cells.
Free sugars
Added sugar, honey and sugars in juice.
Whole grain
Grain that keeps its bran and germ.
Hyperglycaemia
Blood sugar above the normal range.

08Key points

  • Carbohydrates are the main fuel for the brain and muscles.
  • There is no need to cut them out.
  • Choose whole grains, legumes, vegetables and fruit.
  • Limit sugary drinks.
  • Soft starchy foods help on hard days.
  • Steroids can raise blood sugar.
  • Do not change diabetes medicines yourself.
  • “Sugar feeds cancer” is not a reason for a strict diet.

09Frequently asked questions

Does sugar feed leukemia cells? Should I cut sugar out completely?

All cells, including healthy ones, use glucose. There is no evidence that cutting out carbohydrates helps treatment, but it can cause weight and muscle loss. Reducing added sugar is good for general health.

Is white bread bad for me?

It is not harmful; it just gives less fibre and fewer vitamins than wholegrain bread. With diarrhea or nausea, white bread may be easier to tolerate.

I am on steroids and my sugar is up. What should I eat?

Cut down on sugary drinks and sweets, spread carbohydrates across the day and eat them with protein and vegetables. Follow your doctor’s advice on testing and medicines.

Does a ketogenic diet help leukemia?

There are no reliable studies showing benefit from a ketogenic diet in leukemia, and it can increase weight loss. Agree any special diet with your team.

Does juice replace fruit?

Not fully. Juice has little fibre and its sugar is absorbed quickly. During treatment choose only pasteurized juice and limit the amount.

Is honey healthier than sugar?

Honey also counts as a free sugar. During neutropenia some centres advise avoiding raw (unprocessed) honey. Ask your team.

10Questions for your doctor

Take these questions to your appointment. The ones you save are collected on the “My questions” page and stay on this device only.

  1. Does sugar feed leukemia cells? Should I cut sugar out completely?

  2. Is white bread bad for me?

  3. I am on steroids and my sugar is up. What should I eat?

  4. Does a ketogenic diet help leukemia?

  5. Does juice replace fruit?

  6. Is honey healthier than sugar?

My questions

12Sources

  1. MedlinePlus (U.S. National Library of Medicine). Carbohydrates
  2. NHS. Starchy foods and carbohydrates
  3. World Health Organization (WHO). Carbohydrate intake for adults and children: WHO guideline (2023)
  4. World Health Organization (WHO). Guideline: Sugars intake for adults and children (2015)
  5. World Health Organization (WHO). Healthy diet — fact sheet
  6. Harvard T.H. Chan School of Public Health — The Nutrition Source. Whole Grains
  7. Harvard T.H. Chan School of Public Health — The Nutrition Source. Added Sugar
  8. British Dietetic Association. Wholegrains: Food Fact Sheet
  9. National Cancer Institute (NCI). Nutrition in Cancer Care (PDQ) — Patient Version
  10. National Cancer Institute (NCI). Eating Hints: Before, During, and After Cancer Treatment
  11. Clinical Nutrition (ESPEN), 2017. Arends J et al. ESPEN guidelines on nutrition in cancer patients
  12. U.S. Food and Drug Administration (FDA). Added Sugars on the Nutrition Facts Label
Medical disclaimerThis information is for educational purposes only and does not replace medical advice.
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