Diet quality during cancer treatment: what the LEAN trial means for you
Most conversations about diet and cancer focus on prevention — the things you might do before a diagnosis to reduce your risk. Far fewer address what the evidence says about diet during active treatment, when the stakes feel most immediate and the questions are most urgent.
The LEAN trial is one of the most important studies to address this question directly, in women undergoing treatment for early-stage breast cancer. Its findings are striking, and they deserve more attention than they typically receive outside specialist oncology circles.
What the LEAN trial was
The LEAN trial (Lifestyle, Exercise, and Nutrition) was a randomised controlled trial published in the Journal of Clinical Oncology in 2023. It enrolled 173 women with early-stage breast cancer who were due to receive neoadjuvant chemotherapy — that is, chemotherapy given before surgery, with the aim of shrinking the tumour. Participants were randomised to either a combined diet and exercise intervention or usual care, delivered over the course of their chemotherapy treatment.
The dietary component emphasised a whole-food, calorie-appropriate dietary pattern — reduced ultra-processed food, increased vegetables, legumes, and whole grains, adequate protein, and limited added sugar. The exercise component involved supervised aerobic and resistance training sessions.
The primary outcome was pathological complete response (pCR) — the proportion of women who had no residual invasive cancer detectable in the breast or lymph nodes at surgery after completing chemotherapy. A pathological complete response is one of the strongest surrogate markers of long-term survival in breast cancer.
The results were remarkable: pCR was achieved in 53% of women in the intervention group, compared to 28% in the usual care group — a 25 percentage point difference, and one of the largest effect sizes ever reported for a lifestyle intervention in an oncology randomised controlled trial.
What these numbers mean
A pathological complete response rate of 53% versus 28% is not a marginal statistical signal. It represents a near-doubling of the proportion of women who had no detectable cancer left at surgery. In clinical practice, achieving pCR is associated with significantly better long-term outcomes — reduced recurrence risk and improved overall survival.
It is important to be precise about what the LEAN trial demonstrates and what it does not. This was a single trial of 173 women, conducted at a specific centre, using a specific combined intervention. The relative contributions of the dietary component versus the exercise component cannot be separated from the trial design. Replication in larger, more diverse populations is needed before this finding can be considered definitive.
But the direction and magnitude of the effect are consistent with the broader biological evidence: that metabolic health during chemotherapy — insulin sensitivity, inflammatory status, body composition — influences tumour response. The LEAN trial is not an outlier; it is the most compelling human evidence yet for a hypothesis with strong mechanistic support.
The biological mechanisms
Insulin and IGF-1. Chemotherapy is most effective against rapidly dividing cells. Elevated insulin and IGF-1 — driven by high glycaemic dietary patterns — activate the PI3K/AKT/mTOR pathway in cancer cells, promoting survival signalling that may partially protect tumour cells from chemotherapy-induced cell death. A dietary pattern that reduces insulin exposure removes this protective signal and may increase tumour cell vulnerability to treatment.
Inflammation. Pro-inflammatory cytokines create a tumour microenvironment that supports cancer cell survival and resistance to treatment. A whole-food dietary pattern rich in polyphenols, fibre, and omega-3 fatty acids reduces systemic inflammatory markers. Reducing inflammation during treatment may shift the tumour microenvironment in a direction that favours chemotherapy efficacy.
Body composition. Women with higher body fat and lower muscle mass tend to have higher circulating insulin, oestrogen, and inflammatory markers — all of which may reduce treatment efficacy in hormone-receptor-positive breast cancer. The LEAN intervention included both dietary and exercise components, and the combination likely improved body composition during treatment.
What the American Cancer Society guidelines say
The American Cancer Society guidelines for diet and physical activity for cancer prevention (CA: A Cancer Journal for Clinicians, 2020) and the updated guidelines for cancer survivors (2022) both explicitly recommend a high-quality dietary pattern — predominantly whole foods, abundant vegetables and fruits, whole grains, and limited ultra-processed food, red meat, alcohol, and added sugar — for cancer survivors and people undergoing treatment.
These guidelines are based on the totality of evidence, not any single trial. The LEAN trial strengthens the evidence base considerably. The consistency between the mechanistic picture, the guidelines, and the trial findings is notable.
What this means in practice
Diet quality during treatment is not a peripheral concern. The LEAN trial suggests it may directly influence how well treatment works. This is not a reason for guilt about imperfect eating during chemotherapy — treatment is physically and emotionally demanding, and appetite, nausea, and fatigue all affect what is possible. But it is a reason to prioritise dietary quality as much as circumstances allow, and to seek support from an oncology dietitian where available.
The specific dietary pattern matters. The LEAN intervention was not a generic “eat healthily” instruction. It emphasised whole foods, reduced ultra-processed food, adequate protein and fibre, and limited added sugar — specifics that align with the metabolic mechanisms described above. A diet high in ultra-processed food, even at maintenance calories, does not produce the same metabolic environment.
Protein intake supports muscle preservation during treatment. Chemotherapy-related muscle loss is a documented concern, and low muscle mass is associated with worse treatment outcomes and quality of life. Adequate protein — from whole-food sources including fish, eggs, legumes, and poultry — supports muscle maintenance during treatment. Aim for protein at each meal rather than concentrating it in one sitting.
Work with your clinical team. Some cancer treatments interact with specific foods or supplements. Any significant dietary change during active treatment should be discussed with your oncologist or an oncology dietitian who knows your specific treatment protocol.
What the evidence does not show
The LEAN trial cannot tell us which component — diet or exercise — drove the pCR difference, or whether the effect is specific to neoadjuvant chemotherapy in early breast cancer or generalisable to other cancer types and treatment contexts.
It does not establish that any specific food or supplement improves chemotherapy response. The benefit in LEAN came from an overall dietary pattern change, not from a single dietary addition.
And the 53% versus 28% pCR finding, whilst striking, requires replication. A single trial — however well designed — is the beginning of an evidence base, not the end of one.
The bottom line
The LEAN trial is the most compelling randomised evidence to date that diet quality during neoadjuvant chemotherapy for breast cancer influences treatment response — with a pathological complete response rate of 53% in the intervention group compared to 28% in usual care. The mechanisms are coherent: improved insulin sensitivity, reduced inflammation, and better body composition create a metabolic environment that may increase tumour vulnerability to chemotherapy. For women undergoing breast cancer treatment, dietary quality during treatment is not a secondary concern. It is a modifiable factor with trial-level evidence behind it.
Scientific references
- Sanft T, et al. Randomised controlled trial of weight loss versus usual care on telomere length in women with breast cancer: the LEAN trial. Journal of Clinical Oncology. 2023. DOI: 10.1200/JCO.23.00871
- Patel AV, et al. American Cancer Society guideline for diet and physical activity for cancer prevention. CA: A Cancer Journal for Clinicians. 2020;70(4):245–271. DOI: 10.3322/caac.21591
- Rock CL, et al. American Cancer Society guideline for diet and physical activity for cancer survivors. CA: A Cancer Journal for Clinicians. 2022. PMID: 35294043
- Mustian KM, et al. Comparison of pharmaceutical, psychological, and exercise treatments for cancer-related fatigue: a meta-analysis. JAMA Oncology. 2017;3(7):961–968. DOI: 10.1001/jamaoncol.2016.6914
This article is written for educational and informational purposes only. It is not intended as medical advice and should not be used to diagnose, treat, prevent, or cure any medical condition, including cancer. If you have a cancer diagnosis or are undergoing treatment, please discuss any lifestyle or dietary changes with your oncologist or medical team before making them. The studies cited reflect current research at the time of writing; the field is evolving rapidly and findings may be updated or superseded. Chloe Archard is a nutritional health coach, not a medical doctor.
