Does fasting help cancer? It depends.
Two forms of fasting- one, comprised of eschewing food but taking unrestricted water intake, and the other, called Fasting Mimicking Diet (FMD) have been shown to be useful.
How does it work?
Glucose, amino acids and certain hormones or hormone like substances are thought to encourage cancer growth, including insulin, IGF-1, and leptin. Think of these as anabolic pathways, which provide the nutrients and the drivers for cancer growth and survival. Turn them off, and the cancer cells are disdavantaged.
Note that I haven't mentioned fatty acids, non-intuitive though it may seem. Cancer cells are highly dependent on anaerobic metabolism, epitomised by the Warburg effect, which involves hijacking the glycolytic pathway (normally anaerobic) even in the presence of oxygen, and producing ATP therefrom. This is seen in fully 70-80% of cancer cells.
Conversely fatty acids are metabolised inside mitochondria by beta-oxidation and therefore is a highly aerobic process, not normally utilised by cancer cells.
The corollary is that cancer fighting diets must contain very little carb or protein and any calories, limited albeit in amount, must come from fat. This is exactly what happens with FMD, which consists of 300-1100 calories perday, derived from broths, soups, juices and nutty bars, with some herbal teas thrown in for taste.
It is thought that by abrogating the anabolic hormones mentioned above, you shut down two key canonical intracellular pathways- the PI3K-AKT-MTOR pathhway and the cAMP-Protein Kinase A pathway. Both are proliferatogenic and indispensable for cancer cells.
However, the claim that FMD alone will cure cancer is inaccurate. This diet works well in tandem with chemotherapy and radiotherapy, and must be administered during the peri-chemotherapy period, usually 48 hours before and 24 hours after. It stops the cancer cells from finding "escape" pathways to circumvent chemotherapeutic agents. But the piece-de-resistance of such diets is that it protects normal, healthy cells from being destroyed by chemo, while the cancer cells perish.
That is to say, the normal cells display Differential Stress Resistance (DSR) to cancer cells. At the onset of fasting, normal, non-cancerous cells go into a sort of hibernant, low metabolic state, which reduces their vulnerability to chemotherapeutic agents. As they are no longer actively taking up nutients, metabolising or dividing, they become relatively immune to chemo and radiotherapy.
You might well ask- does this work for all cancers?
Unfortunately not. It's particularly effective for breast cancer, which is ER/PR positive, but not for ER/PR negative, HER-2 positive cases. Thus, it potentiates the action of both Tamoxifen, without causing endomtrial hypertrophy, and Fulvestrant.
Similar benefits are seen for prostate and colon cancer.
If a relatively fat rich diet works, what about the ketogenic diet (4:1 fat:carb+protein in terms of weight)? After all, the blood ketone levels can rise by >0.5 mmol/L with FMDs.
Neurosurgeons here will know that ketogenic diets have considerable benefits for some intractable childhood epilepsies and has been advocated for certain gliomas/glioblastomas. However, in general, ketogenic diet does not work for cancers, and can worsen prognosis in melanomas. Hence, best avoided.
A few caveats. Most of the data for these findings were from animal, mainly murine studies and from yeasts. Human data is limited.
Secondly, some cancer patients are cachexic at treatment. They would be at risk from FMD like diets.
Thirdly, trying to achieve these metabolic benefits pharmacologically does not seem to work. The MTOR inhibitor Rapamycin (Sirolimus) is not generally useful for cancers.
Fourth, chronic calory restricted diets don't seem to work for cancer. FMD must coincide with the chemotherapy cycles. This is therefore intermittent fasting.
