I am in the middle of my annual 7-10 day fast as a cancer prevention measure. I plan to fast until the election is over, which will make it 8 days total. The logic is in my posts linked below in the section on “Anti-Cancer Eating,” especially my post “How Fasting Can Starve Cancer Cells, While Leaving Normal Cells Unharmed.”

I first heard of this recommendation of an annual fast—in which I drink water, but don’t eat—from Jason Fung’s book The Obesity Code, which I feature in “Obesity Is Always and Everywhere an Insulin Phenomenon” and “Five Books That Have Changed My Life.” Before any of you try an extended fast yourself, let me give several cautions:

1. Do not try to do a fast of more than 24-48 hours without first reading The Obesity Code, and possibly Jason Fung and Jimmy Moore’s The Complete Guide to Fasting

2. Do not try an extended fast unless you are of prime age and not pregnant: that is, don’t try an extended fast if you are a child, a teenager, elderly or pregnant.

3. If you are on any medication, you must consult with your doctor before trying an extended fast. Fasting can make the dosage of your medication wildly inappropriate. You should worry about this even with over-the-counter medication; for over-the-counter medication, lean very low on the dosage while you are fasting.

4. You should take in some minerals/electrolytes. If not, you might get some muscle cramps. These are not that dangerous but are very unpleasant. What I do is simply take one SaltStick capsule each day.

What I can report is that this extended fast has not been that hard. I do find food a little more interesting than usual, and try not to dwell in the kitchen, but there is no physical discomfort from the fasting. I was less hungry after 3 days than after 2 days—both levels of hunger were mild. Then the level of hunger has been the same after 4, 5, and 6 days as after 3 days. The secret to keeping the level of hunger mild in the first day or two is to have been eating low on the insulin index before starting the fast. (See “Forget Calorie Counting; It's the Insulin Index, Stupid.”) Indeed, I try to have a meal especially low on the insulin index right before my fast. Theoretically, the further one gets into an extended fast, the less it should matter what you were eating beforehand. Eventually one should reach the same steady state either way (steady state except for the gradual decline in body fat).

I did have one person whom I mentioned my fast to ask “Couldn’t that harm your body?” The first part of the answer is that, in general, human beings who are in good health and of prime age (not children, teenagers or elderly) are well-designed for extended fasts. With the uncertainty of food sources at many points in human history, the contemporaries of our ancestors who weren’t adapted to extended fasts died, and didn’t become our ancestors. We are descended from those who managed to survive extended fasts.

The second part of the answer is that even if one concedes that fasting for 7-10 days might cause some negative physical stress, any such physical stress has to be compared to the harms of chemotherapy if one does get cancer. I know this only second-hand, but I am confident in saying that chemotherapy is a horrible experience and quite damaging to the body.

Of course, for me prevention of cancer by an annual 7-10 day fast is being done with a probability close to 1, and every year, while I would only need to do chemotherapy if I actual got cancer, which is a probability significantly below 1. And chemotherapy wouldn’t be done every year. But I have no problem saying that the ratio between the harm of chemotherapy and any plausible harm of an annual 7-10 day fast is likely to be quite large. In terms of relative effectiveness, as far as I know, no long-term studies have been done. But the theory for starving cancer cells is clear enough that that study definitely should be done. And studies to update our views on the safety of fasting can be done much more quickly and cheaply.

Although the primary motivation for this annual 7-10 day fast is cancer prevention, I have to admit that the fast-forward for weight loss is a great ancillary benefit. On that score, let me give a little perspective by saying that theoretically, fat loss should only be about 3/5 of a pound per day of fasting. But during your fast you will see a lot more than that. I am finding there are huge mass-in/mass-out effects during my extended fast. (See “Mass In/Mass Out: A Satire of Calories In/Calories Out.”) I don’t quite understand why the numbers on the scale are as low as they are right now, but I expect anything beyond 3/5 of a pound per day to be fairly quickly reversed once I end my fast.

I dread cancer. Cancer is bad enough that, for me, a non-painful annual fast of 7-10 days seems like a reasonable sacrifice in order to reduce my chances of cancer. Check out the logic in the posts linked below, read up on fasting, and see what you think.

For organized links to other posts on diet and health, see:

• Miles Kimball on Diet and Health: A Reader's Guide