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  • Writer's pictureClaire Moore

Diets for Cancer Patients: What Not to Avoid

Updated: Jun 15, 2022

Receiving a diagnosis of cancer can be devastating. You will probably go through many emotions, one of which might be the feeling of strength, and wanting to do whatever it takes to beat your cancer. This kind of positive attitude is great, but it’s important to make sure you don’t go to the extreme.

There are a lot of diets that claim to combat cancer, many of which are marketed as an alternative or complement to medical treatment. This kind of marketing can often lead to confusion, unnecessary restrictions, and stress around food.

As an oncology specialist dietitian, I am seeing more and more patients come to me with growing concerns about specific foods or food groups, and the role that they play in cancer growth and treatment. The types of myths associated with these diets can have grave repercussions on cancer patients, which is why it’s important to consult a dietitian if you have cancer and are considering changing your diet.

Here are the most common diets for cancer patients that I get asked about:

Sugar-Free Diets “I’ve cut out sugar” and “Sugar feeds cancer so I’m not eating it” are two of the most common phases I hear when I meet new patients. The idea that sugar can kick-start or fuel the growth of a cancer is a massive over-simplification to a very complex process, yet we read and hear about it everywhere.

Sugar comes in many different forms. The simplest form of sugar is an individual molecule, such as fructose or glucose. These individual molecules bind together to form pairs or chains, which form carbohydrates. This is our body’s main source of fuel – every cell needs glucose for energy.

Cancer cells grow and multiply at a very fast rate, which means they need a lot of glucose for energy. This is where the myth is born – thinking that cutting out sugar from your diet will reduce the amount of fuel available, therefore stopping cancer cells from growing. Unfortunately, it isn’t that simple. Our healthy cells need glucose too, and there is no way for our body to direct glucose to our healthy cells and block it from cancer cells.

It is important to remember that sugar isn’t just found in sweets, chocolate or cakes – it forms all carbohydrates, and is found in dairy, fruit and vegetables. The right types of sugar are part of a well-balanced diet.

The reason that we still encourage people to be mindful of their sugar consumption is because of the indirect link between sugar and cancer. A high sugar diet over time can cause weight gain, and there is robust scientific evidence to show that being overweight or obese increases the risk of developing 13 different types of cancers[1].

There is no evidence that cutting out carbohydrates or reducing your sugar intake will help to treat cancer. Avoiding a complete food group is hugely restrictive and can lead to nutrient deficiencies, increase risk of infection, and negatively affect tolerance to anti-cancer treatments.

Dairy-Free Diets

There is a belief that the hormones found in dairy products promote the growth hormone related to cancer.

The World Cancer Research Fund notes that there is some limited evidence to suggest diets which include large amounts calcium and dairy could increase the risk of developing prostate cancer. However, there is also evidence to suggest a dairy could offer a protective role in the context of breast cancer and colorectal cancer[2].

Following a dairy-free diet is not recommended for cancer prevention. It’s important to consume three portions of low-fat dairy on a daily basis in order to get the recommended calcium intake (700mg), and improve protein, vitamin and mineral consumption. Many cancer treatments can have a detrimental effect on bone health, so it is crucial to make sure you are getting plenty of calcium from the food you eat.

Ketogenic Diets

The Ketogenic Diet (KD) was first established in 1911 to manage childhood epilepsy. The KD is a low carbohydrate diet that has specific proportions of macronutrients (fat, carbohydrates and protein) depending on the type of diet[3].

Much of the research that has been done looking at the KD and cancer is through animal studies specifically looking at brain tumours. Human data comes mostly from individual case reports and small preliminary clinical studies. There is often poor compliance to the diet, limited comparison data and no standard guidance given to the participants. These human studies show no improvement in disease management or treatment outcome[4].

The KD is an extremely restrictive diet with minimal benefit. It can have a huge impact on fatigue, cause constipation (because of a low fibre intake) and dramatically affect body weight.

I strongly suggest not following a Ketogenic Diet without speaking to a dietitian first.

Alkaline Diet

The theory behind the Alkaline diet (also known as the pH Diet, The Miracle pH Diet or the Acid Alkaline Diet) is that replacing acid-forming foods with alkaline foods can improve your health.

It is said that if we consume high amounts of acid-forming foods, then our blood and other cells will become acidic, which in turn can cause an increased risk of ill health, including cancer. Foods that are suggested to be acid-forming include dairy, eggs, lentils, meat, fish and poultry. Conversely, a high intake of alkaline-forming foods such as green vegetables and fruit is said to reduce the risk of cancer and other diseases.

There is no scientific literature to support the benefits of this diet in the prevention or treatment of cancer[5]. It is known that some cancer cells do favour a more acidic environment, but others prefer more alkaline. However, our blood pH (7.4) is tightly regulated by a mechanism called acid-based homeostasis, which involves the kidneys and respiratory system excreting any excess acid in the urine. The blood’s pH cannot be altered by the food we eat. The only situation where pH can change is when someone is critically ill, through a process called metabolic-acidosis.

This diet, like many others, can be highly restrictive and lead to a very low consumption of protein (meat, fish, dairy, pulses). Cancer and treatment increase the risk of malnutrition, so imposing more restrictions on nutrition increases this risk even further.

The Bottom Line

When it comes to following diets for cancer patients, it’s best to avoid any dramatic changes. This impact of the treatment and the disease itself will have a significant effect on your body. It is crucial that you remain as fit as possible through your treatment. Muscle loss, weight loss and malnutrition are known to have an impact on treatment tolerance and overall survival. If you would like advice about your diet, at any point through your cancer treatment, a specialist oncology dietitian is your best port of call[6]. Contact us today for a consultation.


4. Sremanakova J, Sowerbutts AM, Burden S. A systematic review of the use of ketogenic diets in adult patients with cancer. J Hum Nutr Diet. 2018 Dec;31(6):793-802. doi: 10.1111/jhn.12587. Epub 2018 Jul 30. PMID: 30062812.

5. Fenton TR, Huang T. Systematic review of the association between dietary acid load, alkaline water and cancer. BMJ Open. 2016 Jun 13;6(6):e010438. doi: 10.1136/bmjopen-2015-010438. PMID: 27297008; PMCID: PMC4916623.

6. Ryan AM, Prado CM, Sullivan ES, Power DG, Daly LE. Effects of weight loss and sarcopenia on response to chemotherapy, quality of life, and survival. Nutrition. 2019 Nov-Dec;67-68:110539. doi: 10.1016/j.nut.2019.06.020. Epub 2019 Jun 28. PMID: 31522087


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