A number of studies have also shown that an imbalance in the gut microbiome can contribute to a wide range of diseases, including allergies, depression, inflammatory bowel disease, obesity, type 2 diabetes and certain cancers.
Diet, your microbiome and melanoma
What is the Gut Microbiome?
We commonly associate microbes, such as bacteria and viruses, with causing infections. However, in contrast with microbes that can invade our bodies and cause infection, a huge number and diversity of microbes live in and on our bodies. These have a significant benefit to our health. The population of these microbes is thought to be just above the number of cells in our own body .
These ‘helpful’ microbes are found in many locations including the skin, gut, oral cavities, vaginal lining and respiratory airways, and help support our normal tissue and organ functions. By far the greatest number are found in our gut, the majority of which are bacteria. Their collective name is the gut microbiota, also known as the ‘gut microbiome’.
Over the last two decades, there has been a huge increase in research to try and understand the relationship between the gut microbiome and us. Studies have shown that the microbes in the gut are essential for maintaining good health, including protection against disease, digestion, metabolism, and blood glucose regulation.
What constitutes a healthy adult gut microbiome?
The make-up of a healthy adult’s microbe population can be very different between individuals. Therefore, there is no single blueprint. However, we do know that certain bacteria categories have beneficial effects in the gut, whilst others are detrimental.
Diversity in the bacteria in the gut microbiome is beneficial and diet is a significant factor that can increase this diversity.
“There is no single blueprint for a healthy gut microbiome…. a dominance of beneficial bacterial categories, where there is lots of diversity in the different types present, is considered best”.
How do I gain a healthy gut microbiome?
Many studies have shown that a high-fibre diet is hugely beneficial (>30g fibre a day). The fibre acts as food for the good bacteria. Fibre-rich foods include: whole-grain foods such as wholemeal bread, whole-wheat cereal, brown rice, oats, plain popcorn; fibrous vegetables including peas, broccoli, baked potatoes with skin on; fruit rich in fibre for example pears, apples, raspberries and bananas; pulses like brown lentils, baked beans, black beans and chick peas; nuts such as raw brazil nuts and raw almonds.
Another suggestion is trying to eat at least thirty different types of plant products a week — ‘Thirty for Diversity,’ including as many different varieties of fruit, vegetables, pulses, nuts, spices and herbs as possible . Herbs and spices count as a ¼ point towards your quota of 30 and 1 portion of fruits, vegetables, beans, pulses, seeds and nuts count as 1 point. In essence, variety is key and one study has shown that eating 30 different plant points or more compared to those who eat 10 per week had a far more diverse microbiome.
Brightly coloured plant products often contain polyphenols that are also thought to be beneficial, for example turmeric and berries. Tea and coffee also contain a type of polyphenol known as flavonoids.
Prebiotics are foods which help feed the good bacteria. Examples include raw garlic, chicory, onion, asparagus, bananas and apples.
Probiotics are different. These introduce beneficial bacteria into the gut. Examples of these include fermented foods like kimchi, sauerkraut, miso and also live yoghurts such as kefir. Some yoghurt drinks that mention they contain a ‘good bacteria’ should be avoided as they contain a single strain and lower diversity.
High levels of saturated fats (e.g butter, animal fats) and animal proteins have been shown to have a unhelpful effect on the human gut microbiome . Alcohol also has a negative impact .
“A high-fibre diet is hugely beneficial. Another good rule is eating at least thirty different types of plant products a week — sometimes called ‘Thirty for Diversity.’ These can include as many different varieties of fruit, vegetables, pulses, nuts, spices and herbs as possible”.
Immunotherapy response in melanoma
Some patients with a diagnosis of melanoma are offered treatment with immunotherapy. In two separate studies in melanoma patients the best responses to immunotherapy (anti PD-1) were seen in patients who had a gut microbiome with a good diversity of healthy bacteria. Patients with a disrupted or unhealthy gut microbiome had a lower response [3,4,5]. This was further corroborated in a recent trial from a 2022 study by same research group [3,6].
“the best responses to immunotherapy (anti PD-1) were seen in those who had a gut microbiome with a good diversity of healthy bacteria”
Antibiotic treatment during melanoma cancer treatment
Antibiotics cannot distinguish between the good bacteria in your gut microbiome, and the harmful bacteria that cause infection. This means that taking antibiotics for an infection can upset the balance in your gut microbiome.
There is some very early research to suggest that taking antibiotics before immunotherapy may be associated with less benefit from immunotherapy – it has been proposed that this could be because the antibiotics have disrupted the gut microbiome [3, 5, 6]. However, antibiotics can be an important part of treatment for bacterial infection and not enough research has happened yet for doctors to understand the possible impact of antibiotics on treatment for melanoma.
Until more evidence is available, it is worth bearing in mind that many infections are caused by viruses and will not not benefit from treatment with antibiotics. If you have any concern about whether antibiotics are needed in your case, you can discuss this with your medical team.
Dr Heather Shaw. Presentation. The Gut Biome explained- the inner you that’s not you. 2021. Melanoma Patient conference.
Importance of eating well during cancer treatment: St Luke’s cancer centre
British Dietetic Association page on Challenging cancer myths and diet
 Korpela K, Costea P, Coelho LP, et al. Selective maternal seeding and environment shape the human gut microbiome. Genome Res. 2018;28(4):561‐568. doi:10.1101/gr.233940.117
 Hicks, LA et al 2019. Improving outpatient antibiotic prescribing. BMJ 2019; 364 doi: https://doi.org/10.1136/bmj.l289
 Lee KA, Shaw HM, Bataille V, Nathan P, Spector TD. Role of the gut microbiome for cancer patients receiving immunotherapy: Dietary and treatment implications. Eur J Cancer. 2020 Oct;138:149-155. doi: 10.1016/j.ejca.2020.07.026. Epub 2020 Sep 2. PMID: 32889369
 Gopalakrishnan et al., Gut microbiome modulates response to anti-PD-1 immunotherapy in melanoma patients. Science. 2018 Jan 5; 359(6371):97-103
 Matson et al., The commensal microbiome is associated with anti-PD-1 efficacy in metastatic melanoma patients. Science. 2018 Jan 5;359(6371):104-108. doi: 10.1126/science.aao3290. PMID: 29302014; PMCID: PMC6707353
 Lee, K.A., Thomas, A.M., Bolte, L.A. et al. Cross-cohort gut microbiome associations with immune checkpoint inhibitor response in advanced melanoma. Nat Med (2022). https://doi.org/10.1038/s41591-022-01695-5
 Pinato DJ, Howlett S, Ottaviani D, et al. Association of Prior Antibiotic Treatment With Survival and Response to Immune Checkpoint Inhibitor Therapy in Patients With Cancer. JAMA Oncol. 2019;5(12):1774–1778. doi:10.1001/jamaoncol.2019.2785
 Purohit V, Bode JC, Bode C, et al. Alcohol, intestinal bacterial growth, intestinal permeability to endotoxin, and medical consequences: summary of a symposium. Alcohol. 2008;42(5):349-361. doi:10.1016/j.alcohol.2008.03.131
 Rowland L, Gibson G, Heinken A, et al. Gut microbiota functions: metabolism of nutrients and other food components. Eur J Nutr. 2018;57(1):1‐24. doi:10.1007/s00394-017-1445-8
 Sender R., Fuchs S., Milo R. Are we really vastly outnumbered? Revisiting the ratio of bacterial to host cells in humans. Cell. 2016;164:337–340. doi: 10.1016/j.cell.2016.01.013
 Walker RW, Clemente JC, Peter I, Loos RJF. The prenatal gut microbiome: are we colonized with bacteria in utero. Pediatr Obes. 2017;12 Suppl 1(Suppl 1):3‐17. doi:10.1111/ijpo.12217
 Yang Q, Liang Q, Balakrishnan B, Belobrajdic DP, Feng QJ, Zhang W. Role of Dietary Nutrients in the Modulation of Gut Microbiota: A Narrative Review. Nutrients. 2020;12(2):381. Published 2020 Jan 31. doi:10.3390/nu12020381