What should I eat if I have cancer?
The field of nutrition is incredibly complex to study - recording exactly what patients eat over a long period of time is extremely difficult. Patients forget the details of their eating habits and over/underestimate the quantity of food consumed. Furthermore, individuals digest food in different ways and have different gut flora (microbiota) that can further complicate how the nutrients are processed.
Still, this area has been one of intense research over the past several decades. Perhaps the most general theme is adequate nutrition - that is, adequate calorie intake. Malnourishment in cancer patients is common. Having cancer typically translates to burning more calories daily, and patients can have poor appetites due to the medications they are receiving. It's very important to ensure patients have appropriate nutritional stores and avoid unintentional weight loss/muscle loss, a state known as "cachexia."
In this vein, there are many supplements patients can use to help increase calories, including Boost, Ensure, as well as appetite stimulants (cannabinoids, mirtazapine, megestrol). Patients respond to these appetite stimulants differently. So if you are considering taking one of the stimulants, talk to your local oncologist first.
A number of diets exist that patients will try upon cancer diagnosis, including vegetarian/vegan, ketogenic, and macrobiotic. No study has indicated these studies provide any benefit, but all are thought to be safe in cancer patients.
Specific supplements are also taken by patients including vitamin C, probiotics. melatonin, and various amino acids. Some of these nutrients have been studied in small studies, mostly examining their effect on malabsorption/diarrhea and reducing chemotherapy side effects. All are considered safe.
The bottom line is that nutrition is important not only upon being diagnosed with cancer but in general. Eating a well-balanced diet and maintaining adequate intake of minerals and vitamins is paramount to a healthy life. Testament to its importance is that there are currently over 30 clinical trials ongoing to study various perturbations of nutrition and its impact on cancer outcomes. Hopefully over the next decade, we will be able to provide information on an individual-level based on genetics, microbiome, and cancer type.