“What should I eat?”
If you have been diagnosed with cancer, there is a good chance you have asked someone on your medical team that question. There is an equally good chance you came away without much of an answer.
The response people most often describe to me is some version of “eat whatever you feel like — it won’t make any difference.” I hear it from my clients all the time. And my colleagues working in Australia, the United Kingdom, Sweden and the United States tell me they hear it just as often.
I want to be clear that this is not a criticism of oncologists. Oncology is among the most demanding specialties in medicine, and the people working in it are exceptionally well trained in what they do: diagnosing, staging and treating cancer. Nutrition is simply a different discipline, and it barely features anywhere in the medical training pathway. The published data are unambiguous.
- In a survey of 150 oncology professionals across the United States and United Kingdom, only 15% had received any formal nutrition education during medical school, residency or oncology training. Fewer than a quarter referred all their patients to a dietitian, and 54% named their own knowledge gap as a barrier to discussing diet — Oncologists’ Knowledge and Attitudes Towards Providing Dietary Guidance to Patients With Cancer (Kassam et al., American Journal of Lifestyle Medicine, 2026).
- In a UK national survey of 610 health professionals, 15% of doctors had received any nutrition training, and only 3.7% felt completely confident giving nutritional advice to people with cancer. Just 39% were even aware that cancer nutrition guidelines exist — The provision of nutritional advice and care for cancer patients: a UK national survey of healthcare professionals (Murphy et al., Supportive Care in Cancer, 2021).
- Among 149 people surveyed after finishing cancer treatment, 62% received no professional nutrition guidance, and only 23% had seen a dietitian. Of those who sought information themselves, 52% found what they read to be conflicting — Access to nutrition services and information after active cancer treatment (Baguley et al., Journal of Cancer Survivorship, 2024).
So the gap is structural, not personal. Nobody in the room has been given the training, time, or funding to answer the question properly, and the honest answer to “what should I eat” would take considerably longer than a fifteen-minute appointment allows. A systematic review of 15 studies covering 374 patients and 471 health professionals found exactly this: patients want specific dietary guidance, and clinicians consistently cite lack of time, funding, dietetic staffing and knowledge as the reasons they cannot provide it — Experiences of cancer patients in receiving dietary advice from healthcare professionals and of healthcare professionals in providing this advice (Keaver et al., Journal of Cancer Survivorship, 2024).
This matters because the research shows that “it won’t make any difference” is completely false. What follows is a summary of the places where nutrition has been shown, in high-quality studies, to change how people actually do — how well they recover from surgery, how well they tolerate treatment, how much muscle and strength they keep, and in some cases how long they live.
What nutrition actually changes
It is worth being clear about what nutrition does here. Its proven power in cancer care is not in attacking the tumour. It is in the condition of the person receiving the treatment. Cancer treatment — surgery, chemotherapy, radiotherapy, immunotherapy — is demanding on the body, and how well you come through it depends substantially on the physical reserves you bring to it and maintain during it. That is what good nutritional care changes, and the effects are larger than most people expect.
The scale of the problem is worth stating. Across 65 studies and more than 30,000 patients, around 41% of people with cancer are malnourished — rising to about half in pancreatic and oesophageal cancer. Being malnourished was associated with roughly double the risk of dying, and with substantially more complications after surgery — Global epidemiological characteristics of malnutrition in cancer patients (Liang et al., BMC Cancer, 2025). This is common, measurable, and responsive to treatment.
The evidence is strongest at four points in the journey. Each is a window — and each one eventually closes.
Window One: Before surgery
If surgery is part of your treatment plan, the weeks beforehand are the single most valuable and most frequently wasted period in the whole process. Most people spend them waiting. The research suggests they should be spent preparing.
Preparing the body for surgery works
“Prehabilitation” means getting physically ready for an operation in the same deliberate way you would rehabilitate afterwards. A large analysis pooling 27 randomised trials and 2,946 people having bowel cancer surgery compared different approaches. The clear winner was the combined package — exercise, nutrition and psychological support together:
- Around half the rate of complications after surgery (odds ratio 0.47)
- Just over a day less in hospital
- Meaningfully better walking capacity going into the operation, and again four weeks afterwards
- Less anxiety before surgery
Multimodal prehabilitation is an effective strategy to reduce postoperative complications and improve physical function and anxiety in patients with colorectal cancer undergoing elective surgery (Li et al., Frontiers in Medicine, 2025).
One detail from that analysis deserves emphasis, because it is the thing most often got wrong: nutrition on its own ranked well below the combined approach. Protein without a reason for the body to use it is largely wasted. Eating well and moving well are not two separate pieces of advice — they only work properly together.
Specialised pre-surgical nutrition formulas
A category of nutritional formula, usually called immunonutrition, contains arginine, omega-3 fatty acids, and nucleotides, and is given for around five to seven days before major abdominal surgery. It is one of the best-evidenced interventions in the whole of cancer nutrition.
A review pooling 16 separate research syntheses covering 41,072 patients having gastrointestinal cancer surgery found it reduced:
- Infections after surgery by around 38%
- Anastomotic leaks — where a surgical join fails — by around 32%
- Wound infections by around 36%
- Time in hospital by nearly two days
Perioperative Immunonutrition in Gastrointestinal Oncology: A Comprehensive Umbrella Review and Meta-Analysis (Marano et al., Nutrients, 2025).
For a formula taken for less than a week, those numbers are substantial. In many hospitals, this is standard practice. In others, it is not standard, or it is offered only to patients identified as malnourished. It is a reasonable question to ask your surgical team directly.
What this window looks like in practice: protein at every meal, a specific pre-surgical formula where appropriate, daily walking, some form of resistance work, and attention to iron and vitamin D status. Start as early as the diagnosis allows—three to four weeks is enough to make a measurable difference.
Window Two: During active treatment
The goal during chemotherapy, radiotherapy or immunotherapy is not weight loss and it is not detoxification. It is holding on to muscle, and finishing the treatment course at full dose.
Muscle is the thing to protect
Muscle mass is not simply a marker of general fitness during cancer treatment. Analysing 35 studies and 3,858 patients, low muscle mass was associated with a poorer response to treatment — and the association was strongest exactly where it matters most, in people being treated with the intention of cure, and in people receiving immunotherapy — Low skeletal muscle mass predicts treatment response in oncology: a meta-analysis (Surov et al., European Radiology, 2023).
Part of the explanation is straightforward. Chemotherapy doses are calculated from body size, and people with low muscle relative to that size experience more side effects, which leads to dose reductions and delays. Protecting muscle helps protect the treatment.
How much protein, and why the usual advice is too low
This is where the specifics matter. A review of studies in cancers where muscle loss is common found that protein intakes below 1.2 g per kilogram of body weight per day were associated with muscle loss during treatment — even though that figure sits within some standard guidance — while only intakes above 1.4 g/kg/day were associated with holding muscle steady — Protein intake and muscle mass maintenance in patients with cancer types with high prevalence of sarcopenia (Capitão et al., Supportive Care in Cancer, 2022).
For a 70 kg adult, that is roughly 100 g of protein a day, spread across meals rather than concentrated in the evening. European clinical nutrition guidelines recommend 25–30 kcal/kg/day and up to 1.5 g/kg/day of protein for people with cancer — ESPEN practical guideline: Clinical Nutrition in cancer (Muscaritoli et al., Clinical Nutrition, 2021).
Achieving that when appetite has collapsed and food tastes of metal is a practical problem, not a motivational one. It is most of what a consultation in this window is actually about.
Not all nutritional supplements perform equally
Where food alone will not reach the target, oral nutritional supplements help — but the composition matters. Pooling 11 randomised trials in 1,350 people receiving chemotherapy or chemoradiotherapy, standard high-calorie supplements produced no significant change in body weight, while supplements enriched with additional protein and omega-3 fatty acids produced a significant gain of nearly 2 kg and helped preserve lean tissue — Systematic review and meta-analysis of the evidence for oral nutritional intervention on nutritional and clinical outcomes during chemo(radio)therapy (de van der Schueren et al., Annals of Oncology, 2018).
The same research identified the real limiting factor: people were prescribed 2,100–2,800 calories a day and were actually managing 1,700–2,000. In one trial, only 19% took all of the supplement they had been given. Getting the prescription right is the easy half; making it achievable alongside nausea, taste changes and fatigue is the half that needs support.
Two specific findings worth knowing
For mouth ulceration during treatment. Across 16 randomised trials, glutamine reduced severe oral mucositis — the painful mouth and throat ulceration that can derail eating and delay treatment — by around half — Effectiveness of glutamine in the management of oral mucositis in cancer patients (Peng et al., Supportive Care in Cancer, 2021). This is one to discuss with your oncology team rather than to start independently.
For people on immunotherapy. In 128 patients receiving immunotherapy for melanoma, those eating more fibre from whole foods did markedly better: each additional 5 g of fibre per day was associated with around a 30% lower risk of the cancer progressing, and response rates were 82% in those with high fibre intake and no probiotic supplement, against 59% in the rest. Notably, probiotic supplements alone showed no benefit — Dietary fiber and probiotics influence the gut microbiome and melanoma immunotherapy response (Spencer et al., Science, 2021). This is an observational finding in a single group of patients rather than a randomised trial, but the direction is consistent and the intervention — beans, whole grains, vegetables, fruit — is a safe one.
Nutritional support in hospital
If you are admitted to hospital during treatment, structured nutritional care matters a great deal. In a randomised trial including 506 patients with cancer who were at nutritional risk in hospital, individualised nutritional support reduced deaths within 30 days from 19.9% to 14.1% — Nutritional support during the hospital stay reduces mortality in patients with different types of cancers (Bargetzi et al., Annals of Oncology, 2021). Roughly one life saved for every 17 patients treated this way, from a nutrition protocol.
A related analysis of the same trial found the benefit was greatest in people whose inflammatory markers were still low — in other words, nutritional support works best when it starts early, before the body’s inflammatory response to advanced illness takes over — Comparison of the inflammatory biomarkers IL-6, TNF-α, and CRP to predict the effect of nutritional therapy on mortality (Wunderle et al., Journal of Inflammation, 2025).
If there is one message in this article, it is that one. Early is better. Considerably better.
Window Three: Recovery and rebuilding
Treatment ends and, for most people, formal support ends with it. This is the window with the least attention and some of the most striking evidence.
A landmark randomised trial followed 889 people with stage III or high-risk stage II colon cancer after they finished chemotherapy. Half were given a structured, supported exercise programme for three years; half received health education materials. The exercise group had:
- A 28% lower risk of the cancer returning or of death (five-year disease-free survival 80% against 74%)
- A 37% lower risk of death from any cause (eight-year overall survival 90% against 83%)
Structured Exercise after Adjuvant Chemotherapy for Colon Cancer (Courneya et al., New England Journal of Medicine, 2025).
This is the strongest evidence of its kind for any lifestyle intervention in cancer care. It is worth reading the design detail: this was not general encouragement to be more active. It was a structured programme with a dedicated consultant, sustained over three years. Support was the active ingredient as much as the exercise itself.
Nutrition’s role in this window is to supply what rebuilding requires. Muscle regained after treatment needs both a training stimulus and adequate protein; without the second, the first largely fails. The aim is regaining muscle and function, not simply returning to a previous number on the scales.
Exercise in this period also does the more immediate work of reducing fatigue and improving quality of life, demonstrated across 28 randomised trials — Effects of exercise interventions on cancer-related fatigue and quality of life among cancer patients (Hu et al., BMC Nursing, 2023).
Window Four: Living well afterwards
The long window, and the one where diet as a general pattern rather than a targeted intervention comes into its own.
Soy — the question almost everyone asks
Women with breast cancer are frequently told to avoid soy. The best available evidence points the other way. Pooling observational studies covering 34,567 women, higher soy isoflavone intake was associated with a 26% lower risk of recurrence — including in oestrogen receptor-positive disease and in postmenopausal women, the two groups most often warned off it — Phytonutrients and outcomes following breast cancer: a systematic review and meta-analysis of observational studies (van Die et al., JNCI Cancer Spectrum, 2024).
The same analysis found higher blood levels of enterolactone — a compound the gut microbiome makes from lignans in flaxseed, whole grains and vegetables — associated with a 31% lower risk of death from any cause.
This applies to soy as food: tofu, tempeh, edamame, soy milk. Concentrated isoflavone extracts are a different question and are best discussed individually.
Vitamin D, taken the right way
A pooled analysis of individual data from 14 randomised trials and around 105,000 participants found vitamin D3 reduced cancer mortality by 12% — but only when taken daily, at doses between 400 and 4,000 IU. Large intermittent doses showed no effect. The benefit was greatest in people over 70 and in those genuinely deficient — Efficacy of vitamin D3 supplementation on cancer mortality: individual patient data meta-analysis (Kuznia et al., Ageing Research Reviews, 2023).
In colorectal cancer specifically, pooling five randomised trials, vitamin D supplementation was associated with a 30% reduction in cancer progression or death — The effect of vitamin D supplementation on survival in patients with colorectal cancer (Vaughan-Shaw et al., British Journal of Cancer, 2020).
Given how common insufficiency is at New Zealand’s latitude, particularly through winter, this is worth testing rather than guessing.
The overall pattern
The American Cancer Society’s survivorship guideline sets out what the broader evidence supports: a predominantly plant-based dietary pattern built on vegetables, whole grains and legumes; avoiding alcohol; 150–300 minutes of moderate activity each week with resistance work at least twice weekly; and maintaining muscle and a healthy weight — American Cancer Society Guideline for Diet and Physical Activity for Cancer Survivors (Rock et al., CA: A Cancer Journal for Clinicians, 2022).
What ties the four windows together
Three things run through all of it.
Timing beats intensity. The same intervention delivered early does considerably more than the same intervention delivered late. This is true before surgery, true during treatment, and true in recovery.
Combination beats components. Protein works when there is a training stimulus. Prehabilitation works as a package. Nutrition on its own consistently underperforms nutrition as part of a coordinated approach.
Specificity matters. The right protein target rather than “eat well”. The right supplement composition rather than whichever tub is on the shelf. Vitamin D taken daily rather than monthly. Fibre from food rather than a probiotic capsule. The details are where the effect sizes live.
None of this replaces oncological treatment, and none of it should be undertaken without your medical team knowing about it — particularly supplements, some of which can interact with chemotherapy. What it does is make sure that the treatment you are given has the best possible chance of working, and that you come out of it in the best condition you can.
A question worth sitting with
Before the practical part, I would like to invite you to pause on something.
Read back over what is in this article. Around half the rate of complications after surgery. Roughly a third fewer anastomotic leaks. Nearly two fewer days in hospital per patient. Better treatment tolerance, fewer dose reductions, fewer delays. A 28% lower risk of recurrence or death in the exercise trial.
Then add this. In a matched analysis of 23,696 hospitalised patients with cancer and malnutrition, those who received nutritional support had roughly a quarter lower odds of dying in hospital, and were discharged into post-acute care facilities less often — Association of Nutritional Support With Clinical Outcomes in Malnourished Cancer Patients: A Population-Based Matched Cohort Study (Kaegi-Braun et al., Frontiers in Nutrition, 2021).
And this. Across the published economic evaluations, nutritional counselling combined with oral supplements has been found to be cost-effective or outright cost-saving — Are Nutritional Interventions Worthwhile in Cancer Patients? A Systematic Review on Economic Evaluation (Nguyen et al., ClinicoEconomics and Outcomes Research, 2025).
Now hold all of that against the figures at the beginning of this article. Fifteen per cent of oncology professionals with any nutrition training. Under four per cent of doctors completely confident advising on it. Sixty-two per cent of people receiving no nutritional guidance at all.
So here are the questions I would invite you to sit with.
- What would it mean for you, or for someone you love, if nutritional care were simply part of standard cancer treatment — offered at diagnosis, rather than sought out in a crisis or stumbled upon on the internet?
- What would it be worth to a health system already under enormous strain, if a meaningful share of surgical complications, extended admissions and post-acute care placements could be prevented at a small fraction of what they cost to treat?
- And what would actually need to change for that to happen? In most cases, not very much. A referral, made early, to someone whose specific job is to know this material and apply it.
I do not think the barrier here is evidence. The evidence has been accumulating for two decades and every study behind this article is linked above for you to read. The barrier is that in a busy oncology service, it is almost nobody’s job to keep up with it and act on it.
That is the role I am asking to fill — working alongside your medical team, never instead of it.
If this article makes sense to you, the single most useful thing you can do is pass it on. To someone facing treatment. To your GP, your specialist nurse, your oncologist. Change of this kind rarely starts at the top. It usually starts with a patient asking a good question.
Working with me
I am a registered naturopath, clinical nutritionist and medical herbalist based in Christchurch, with 25 years of clinical experience.
My role is not to replace anything your oncology team does. It is to fill the gap described at the start of this article — to be the person whose actual job is to keep up with the nutritional science, and to apply it to your situation specifically. Ideally that means working as part of your oncology team. Realistically, and just as usefully, it means being part of the support network around you: the person who has read the research, knows what applies to your treatment and your stage, and has the time in the appointment to work through it with you.
In practice that means assessing where you actually are, setting protein and energy targets you can genuinely reach, working out how to hit them when appetite and taste have changed, coordinating the timing around surgery and treatment cycles, keeping your oncology team informed of anything you are taking, and keeping the plan moving through recovery.
If you or someone you care about is facing cancer treatment, the most useful thing you can do is start early. Appointments can be booked online at stephenroigard.com.
This article is general information, not individual medical advice. Always discuss dietary changes and any supplement use with your oncology team before starting.
