Dr. Eugene J. Fine. Dr. Feinman invited me to contribute a guest blog on our recently published cancer research study: “Targeting insulin inhibition as a metabolic therapy in advanced cancer: A pilot safety and feasibility dietary trial in 10 patients” which has now appeared in the October issue of the Elsevier journal Nutrition, with an accompanying editorial. Today’s post will focus on this dietary study, and its relation to the general problem of cancer and insulin inhibition. Part II, next week, will discuss in more detail, the hypothesis behind this study. Richard has already mentioned some of the important findings, but I will review them since the context of the study may shed additional light.

Comments to Richard’s post noted that almost 100 years ago Otto Warburg described how many cancers depend on glucose for fuel, and remarkably, that the cancers relied on anaerobic glycolysis for energy even under aerobic conditions. In contrast, normal skeletal muscle, for example, might rely on anaerobic metabolism in a sprint but would switch back to aerobic respiration (and reliance on fat) after slowing to a walk. The stubborn use of glucose and glycolysis by cancer cells has been termed “The Warburg effect.” The important point is that anaerobic metabolism relies on glucose and strictly glycolytic cells cannot use fatty acids for metabolism.

Meanwhile, the 1982 Food Pyramid encouraged Americans to eat 300-400 grams of CHO per day. Many followed their advice, now to their regret, and at least 90% of these CHOs were sugars, or starches that digested to sugars, all useable by many cancers. So at first we thought we might starve tumors by limiting dietary carbohydrate (CHO), obviously a major source of blood glucose.

It took us less than a day of literature review to recognize that CHO restriction would not starve most cancers because they are usually excellent at pirating glucose at blood glucose concentrations way below the normal range.

A few words about me: I’ve been fascinated with the Warburg effect since medical school. My specialties are Internal Medicine and Nuclear Medicine. One of the radioactive isotopes that we use in Nuclear Medicine is Fluorine-18 (written 18F) which emits positrons. (Positrons interact with other substances to emit gamma rays, high energy light waves that mostly pass through tissues and can be detected on a PET (positron emission tomography) scan. We do a lot of PET scans of patients with cancer using an injected radioactive form of glucose labeled with 18F, 2-deoxy -2-[18F]fluoro-D-glucose, or FDG for short. Cells take up FDG but do not metabolize it and we can see on a PET scan where the glucose-avid cells are. PET scans with FDG work extremely well in cancer precisely because of the Warburg effect — i.e. many cancers depend on glucose for their fuel. In Fig. 1 (left, below) the scan shows a small metastasis in the liver, just above the kidney; Fig. 1(right) shows same patient now with many new liver metastases and one new spot in the lung.

Figure 1: Metastatic Cancer from a primary tumor in the colon. The primary cancer in the colon is not visible as it was removed surgically, but the disease had already spread microscopically so that by the time of the PET image on the left there’s a metastasis in the liver, and the right image shows multiple liver mets and one small lung met. (Note that darker means more FDG uptake.)

The PET scan is done by injecting a small amount of FDG into a vein, then waiting for the tracer to distribute by blood flow to the body’s tissues. After about an hour the patient is placed on a narrow table and the PET scanner, really a very expensive camera, circles the patient while taking pictures (capturing the emitted gamma rays) showing the FDG distribution as in Fig. 1.

The heart and brain normally use a lot of glucose, so they “light up” with FDG, and, due to FDG excretion, kidneys and bladder are also seen well. There’s normally only mild background uptake detected in other tissues. Intense uptake in an unexpected location strongly suggests a primary cancer or a metastasis. For this reason, PET has become very useful in the management of many cancers, where it can show tumors clearly and can demonstrate the effects of treatment. My early interest in the Warburg effect was obviously fueled by what PET scans could show us.

A word of caution: Cancers resist overly broad generalizations. Notice that I always say “many cancers” and not “all cancers.” Otto Warburg made a very important observation, but he went a little too far by stating that persistent glycolysis was the common feature of all cancers. The observation holds up well for many aggressive malignancies but by way of counterexample, 80% of prostate cancers are not especially aggressive, nor are they avid for FDG (or glucose), i.e. the Warburg effect does not apply.

Back to our study idea: We realized that gluconeogenesis and release from glycogen stores would prevent blood glucose concentration from falling to a level low enough to starve cancer cells. However, it became clear that by reducing insulin signaling, dietary CHO reduction would cause many effects which were known to inhibit cancer growth. Some of these were systemic, such as ketosis triggered at CHO restriction to less than 50 grams/day. Ketosis had been reported to inhibit cancer growth in cell culture studies in our lab and others (1,2), animal cancer studies (3,4) and a case study of two children with brain tumors (5). Other expected effects are changes in all the intracellular signaling molecules downstream of the insulin receptor, which regulate their growth, proliferation, and resistance to apoptosis (cell death signals known in all cells), etc. In the last blogpost, Richard described an animal study which demonstrated insulin’s involvement in downstream signaling and response to diet in cancer.

Cancers are now being treated with drugs that individually target these intracellular signaling molecules that are controlled by insulin. These new drugs have shown some efficacy but are often limited by side effects due to the drug interactions with normal tissues. Normal tissues, however, are tolerant of the effects of reduced insulin signaling. This is apparent from the safety of low CHO diet investigations in overweight people and people with diabetes as well as healthy subjects of normal weight. It seemed reasonable to us that a low CHO insulin inhibiting (INSINH) diet could target the same molecules as the drugs and could plausibly inhibit or even kill cancer cells, but would be safe for normal tissues. So, if a study showed safety/feasibility as well as some evidence for efficacy, it would open the door for further investigation of this diet at least as an adjunct to drug therapies. In short, an INSINH diet, by systemic and synergistic effects on multiple signaling molecules, might eventually be shown to reduce drug doses and therefore side effects, boosting efficacy at the same time. What was most surprising to us was that nobody had done this before.

Our goal was to implement a ketogenic INSINH diet for 28 days to see if this diet was safe and feasible in cancer patients and to test for efficacy using the change in FDG uptake on a PET scan. We were striving for ketosis, i.e. the strictest type of low carb diet, with the thought that we might engage/recruit as many cancer inhibitory mechanisms as possible. Diets are hard, but almost anyone can stay on a diet for a month—and we wanted the patients to have a chance to succeed. Furthermore, PET scans are very sensitive: some cancers show changes due to treatment as early as a week, so a month might even permit us to see evidence of improvement on the scan.

What about the results? Ten patients is too small a sample to draw firm conclusions. And it wasn’t so easy after all to get ten sick patients to do this trial. Four of the patients continued to have progressive cancer by our follow-up PET scan at one month, while five patients showed stable disease and one, a partial remission. We’d agree that these findings aren’t so remarkable in themselves. But the details are much more interesting: The patients with the worst PET scan results at study’s end were principally those who had the least degree of insulin inhibition, i.e. the least amount of ketosis (only five times their baseline level); whereas those that had the best PET scan results were those that had the most insulin inhibition, or the most ketosis (17 times baseline)—see A below (where β-hydroxybutyrate is the ketone body we measured); asterisks represent a significant difference (p<0.02). Figure B confirms a general inverse relationship between the extent of ketosis and insulin concentration, expected from CHO restriction (p<0.03).

It should also be noted that all patients reduced overall calorie consumption and 9 of 10 lost weight. Calorie restriction has been postulated to have effects similar to those we’ve suggested. (We tried to over-feed the patients in order to maintain weight and calorie intake, but it didn’t work: very low CHO diets do indeed cause spontaneous calorie restriction and weight loss, even when you try to prevent that.) But in our study, neither the extent of calorie reduction nor weight loss showed any relation with the PET findings (see C and D, above). Nevertheless, we couldn’t completely exclude that the calorie reduction played some contributory role.

In conclusion the best metabolic response to the INSINH diet gave the best PET scan response, the worst metabolic response gave the worst, but calorie reduction and weight loss did not demonstrate a measurable relation to PET outcome.

Our trial might be viewed as an unremarkable pilot study with too few patients to draw many inferences. But that would miss the point. We think what’s more important is that we may have opened a door, long overdue, to systemic study of dietary compositional change in cancer therapy– an insulin inhibiting diet is now worth a further look!! We hope there will now be the opportunity to study INSINH diets in more patients; to see if standard therapies and newer drug treatments can be improved by dietary adjunctive therapy; to find biomarkers to pre-identify which patients are/aren’t likely to benefit from diet; to tease out the effects of calorie restriction from those of carbohydrate restriction.

The next post will describe in more detail the underlying hypothesis about carbohydrate restriction/insulin inhibition as a potential therapy for cancer.

References

1. Demetrakopoulos GE, Brennan MF. Tumoricidal potential of nutritional manipulations. Cancer Res. 1982;42(2 Suppl):756s-65s.

2. Magee BA, Potezny N, Rofe AM, Conyers RA. The inhibition of malignant cell growth by ketone bodies. Aust J Exp Biol Med Sci. 1979 Oct;57(5):529-39.

3. Mavropoulos JC, Isaacs WB, Pizzo SV, Freedland SJ. Is there a role for a low-carbohydrate ketogenic diet in the management of prostate cancer? Urology. 2006 Jul;68(1):15-8.

4. Moulton CJ, Valentine RJ, Layman DK, Devkota S, Singletary KW, Wallig MA, et al. A high protein moderate carbohydrate diet fed at discrete meals reduces early progression of N-methyl-N-nitrosourea-induced breast tumorigenesis in rats. Nutr Metab (Lond). 2010;7:1.

5. Nebeling LC, Miraldi F, Shurin SB, Lerner E. Effects of a ketogenic diet on tumor metabolism and nutritional status in pediatric oncology patients: two case reports. J Am Coll Nutr. 1995 Apr;14(2):202-8.