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Nutrition and daily life

Fats: concentrated energy and essential protection

Fat is often linked with “extra weight”, but the body needs it: it builds cell membranes, helps absorb vitamins and gives a lot of energy in a small amount of food. This topic explains the types of fat, their role during treatment, and which ones to choose more often and which less often.

Last updated: ~4 min read
On a dark navy background, a glass bottle of olive oil with a falling drop, a bowl of walnuts and almonds, a cooked salmon fillet, half an avocado and a small dish of butter.
Figure 1.Everyday sources of fat: olive oil, nuts, fish, avocado and butter.

01Short answer

Fats are the most concentrated source of energy: 1 gram gives about 9 kcal, more than twice as much as carbohydrate or protein. They are part of every cell membrane and help the body absorb vitamins A, D, E and K. Unsaturated fats (olive and other vegetable oils, nuts, fish) are considered more beneficial; saturated fats (fatty meat, butter, lard) are best limited, and trans fats (some fried and industrial products) kept as low as possible. For people with a poor appetite, adding a little oil to food raises energy without making the portion bigger. If you have pancreas or gallbladder problems or diarrhea after fatty food, agree the amount of fat with your team.

02Simple explanation

Fat is the body’s energy store and the basis of every cell’s membrane. The brain, too, is largely built from fats.

Fat is needed to absorb vitamins A, D, E and K. These vitamins are absorbed well only together with fat.

Unsaturated fats are usually liquid at room temperature: olive, sunflower and flaxseed oil, as well as nuts and fish oil.

Saturated fats are usually solid: butter, lard, fatty meat, cream. Trans fats form in some industrial products and in oil that is reheated again and again.

During treatment, fat is especially useful because it gives a lot of energy in a small volume. When appetite is poor, this helps protect your weight.

03Scientific explanation

WHO guideline, 2023: in adults, saturated fat should provide less than 10% of total energy and trans fat less than 1%, replaced mainly by unsaturated fats. This is advice for general health.

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; this increases energy density and reduces the load on blood sugar. The decision is individual.

Omega-3: according to ESPEN and ESMO, there is limited evidence that omega-3 fatty acids (fish oil) may help appetite and weight in cancer-related weight loss. Supplements can interact with medicines, so they are taken only with your team’s agreement.

Practical advice: NCI’s “Eating Hints” guide suggests that people with a poor appetite raise the energy of meals by adding oil, sauces, cheese and cream, while keeping to food safety rules.

04What happens in the body?

Why do we need fat?

An energy store, cell membranes, making hormones, keeping the body warm and carrying fat-soluble vitamins. Eating without fat is not possible for long.

Digestion and transport

In the small intestine bile acids break fat into tiny droplets, pancreatic enzymes digest it, and it enters the blood through the lymph. From there it reaches the cells and becomes part of their membranes.

On a dark navy background, golden fat droplets in the gut break into glowing particles that reach the cells and become part of their shimmering double membrane.
Figure 2.Fat is digested, enters the blood and becomes part of the cell membranes.

Helpful fats

Olive oil, other vegetable oils, walnuts, almonds and hazelnuts (stored properly), oily fish (salmon, mackerel — fully cooked) provide unsaturated fats. They are kinder to the heart and blood vessels.

What to limit

Fatty meat, sausages, lard and a lot of butter and cream are rich in saturated fat. Deep-fried foods, reused frying oil and some industrial sweets can contain trans fats.

On a dark navy background, olive oil, walnuts and baked fish on the left; deep-fried snacks, pastry and fatty sausages in dimmer light on the right.
Figure 3.Unsaturated fats are the better choice; fried foods and trans fats are best limited.

Fat when you eat little

A teaspoon of olive oil or butter can be stirred into mash, porridge, soup or pasta. Pasteurized cheese, cream and yogurt also add energy. You get more calories without a bigger portion.

When to be careful

With pancreatitis, gallbladder disease, diarrhea after fatty food or raised blood fats, the amount of fat is set by your doctor and dietitian. Some treatments can raise triglycerides.

05Fats: four steps

Dense energy, carries vitamins, can be added to food, the type matters.

Dense energy — 1 g of fat ≈ 9 kcal.1Dense energy1 g of fat ≈ 9 kcal.
Vitamin carrier — A, D, E, K are absorbed with fat.2Vitamin carrierA, D, E, K are absorbed with fat.
Add to soft foods — Mash, porridge, soup.3Add to soft foodsMash, porridge, soup.
Choose the type — Less frying, less trans fat.4Choose the typeLess frying, less trans fat.
Weight supportLots of energy in a small portion.
Care for the heartUnsaturated fats come first.
Vitamins absorbedFat-soluble vitamins are taken up.

06Types of fat (simplified)

TypeFound inAdvice
MonounsaturatedOlive oil, hazelnuts, almondsAs the main choice
Polyunsaturated, omega-6Sunflower and corn oil, walnutsDaily, in moderation
Omega-3Oily fish, flaxseed, walnutsOily fish weekly — fully cooked
SaturatedButter, lard, fatty meat, creamIn limited amounts
Trans fatReheated frying oil, some industrial productsAs little as possible
CholesterolEgg yolk, meat, dairyIndividual — with your doctor

* The table is general information. If your blood fats are high or you have pancreas problems, your doctor gives individual rules.

07Key terms

Fatty acids
The main building blocks of fats.
Unsaturated fat
Usually liquid fat from plants and fish.
Saturated fat
Usually solid fat of animal origin.
Trans fat
Fat formed by industrial processing and overheating.
Omega-3
Essential fatty acids from fish and flax.
Triglycerides
The main form of fat in the blood.
Bile acids
Substances that break fat into small droplets.
Energy density
The amount of energy in each gram of food.

08Key points

  • The body needs fat; it is not an “enemy”.
  • 1 gram of fat gives about 9 kcal.
  • Vitamins A, D, E, K are absorbed with fat.
  • Prefer olive oil, nuts and fish.
  • Cut down on fried food and trans fats.
  • If you eat little, add a little oil to your food.
  • Tell your team about pancreas or gallbladder problems.
  • Take fish oil capsules only with agreement.

09Frequently asked questions

Can I eat fatty food during treatment?

Yes, in moderation. Fat raises energy and helps protect weight. But if fatty food makes nausea worse, reduce the portion and choose lighter fats.

Butter or olive oil?

Both give energy. For everyday use, olive and other vegetable oils are the better main choice; butter can be used in moderation.

Should I take fish oil capsules?

That is an individual decision. Omega-3 supplements can affect bleeding and interact with medicines, so extra care is needed when platelets are low. Ask your team.

Is fried food forbidden?

It is not forbidden, but it can make nausea and diarrhea worse, and reheated oil forms harmful substances. Boiling, baking and stewing are better choices.

My cholesterol is high. Should I cut out fat?

There is no need to cut it out completely. Replacing saturated fats with unsaturated ones helps. Protecting your weight during treatment also matters, so find the balance with your doctor.

Can I eat nuts?

Yes, properly stored nuts without mould, packaged or roasted, are usually fine. With mouth sores, nut butter is easier.

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. Can I eat fatty food during treatment?

  2. Butter or olive oil?

  3. Should I take fish oil capsules?

  4. Is fried food forbidden?

  5. My cholesterol is high. Should I cut out fat?

  6. Can I eat nuts?

My questions

12Sources

  1. MedlinePlus (U.S. National Library of Medicine). Dietary Fats
  2. NHS. Fat: the facts
  3. World Health Organization (WHO). Saturated fatty acid and trans-fatty acid intake for adults and children: WHO guideline (2023)
  4. World Health Organization (WHO). Trans fat — fact sheet
  5. World Health Organization (WHO). Healthy diet — fact sheet
  6. Harvard T.H. Chan School of Public Health — The Nutrition Source. Types of Fat
  7. British Dietetic Association. Fat: Food Fact Sheet
  8. British Dietetic Association. Omega-3: Food Fact Sheet
  9. Clinical Nutrition (ESPEN), 2017. Arends J et al. ESPEN guidelines on nutrition in cancer patients
  10. ESMO Open, 2021. Arends J et al. Cancer cachexia in adult patients: ESMO Clinical Practice Guidelines
  11. National Cancer Institute (NCI). Eating Hints: Before, During, and After Cancer Treatment
  12. National Cancer Institute (NCI). Nutrition in Cancer Care (PDQ) — Patient Version
Medical disclaimerThis information is for educational purposes only and does not replace medical advice.
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