All that I've read suggests that in most cases fasting would be better for your illness. I can't find the paper atm, but even alternate day calorie restriction resulted in substantial improvements for patients with minor illnesses (and if I recall correctly these were quite elderly, frail patients). The reasons you've cited are good explanations, but also it's been shown that fasting raises the body's immune responses and provokes various (beneficial) anti-stress responses- (perhaps pre-empting a period of hardship, starvation and vulnerability to infection). Fasting (for long enough) might also be a good idea to dislodge any negative gut flora.

Some caveats would apply though. One would be that if you were suffering from any micronutrient deficiencies, you might want to make sure to get enough zinc etc, rather than limiting food too much (either indirectly as a result of IF or directly through CR). Secondly, you wouldn't want to either directly or indirectly restrict calories over a longer period of illness, since you would simply, eventually become weaker. The same applies for any-one already with too low a body mass.

I think 'make sure you eat' common wisdom is pretty wrong on this. It's intuitive that when you don't feel like eating because you're sick, it's for a reason, presumably to help the body recover. The notion that you have to eat is doubtless partly a result of our culture's general insistence on eating regularly, unnecessarily.

The old adage was "feed a cold, starve a fever' and I think the logic there was that with a cold you are usually clammy and run down, therefore calories from food would warm you up and give you energy. And when you have a fever and are running super hot, just don't throw any more logs on the fire- wait until it burns itself out. But drink water of course.

Isn't it quite common for animals/pets to stop eating for a few days when they get sick? This could be a good clue as to whats 'natural'

It makes a lot of sense if you have diarrhea, or something where eating food can inflame the problem, or not eating can make it directly feel better, or if you just don't feel like eating.

Otherwise, it may not be a good idea to intentionally dramatically reduce calories for a long period. The body responds to fewer calories by slowing the metabolism. One of the consequences of this should be reduced immune system function. The best direct evidence of this I have on hand is in GCBC on Keys's starvation study:

If the men cut themselves shaving, they would bleed less than expected and take longer to heal

The other issue is a reduced amount of micro-nutrients.

I was just learning about the immune system in bio class and one of the key role-players (either IL-1 or IL-2) suppresses appetite when the antigen is detected. It also causes lethargy and fever. This could support the claim that fasting will help fight off illness.

I do this quite often when I am ill, in particular for any gastro-related issues. I have found that fasting and sleeping as much as possible makes you better much more quickly. I don't actually get ill very often, but recently my wife and both kids got some sort of food poisoning, and I was better in 24 hours after fasting and increasing my sleep, whereas my wife and children had problems for between 2 days and a week

When I'm sick I'm not hungry anyway and sleep a lot,so I'm not consciously IFing,but I guess my body is taking care of it naturally.

i would love to read more studies on this, not sure if there is a consensus regardless. my personal experience or what i do is the following fwiw:

to a large extent i listen to my body. it's been a while since i've been sick; but if i recall correctly re: flu/colds, i would go through periods of no hunger and sleep (usually associated with a fever) and periods where i would get hungry (usually after a fever or when you "break the cold/flu"). i would eat pursuant to such signaling.

an important thing to note though, while i am not a macronutrient counting zealot, i did (do) tend to become very conscious of what i am eating (especially what i'm not/should not be eating!) during sickness to be sure that i'm providing resources to my body (and not aggrevating it with other factors) - seeking to make sure i'm getting enough vit c, d, zinc, for example. also, once i was in recovery mode, i would eat much more to feed the recovery (not sure how much of this was conscious or how much of it was actually driven by hunger stimuli).

another important note, regardless of the body signaling during a cold or flu, i would try not to go past 14-16 hours of IF (if not hungry i would have something small). my gym/crossfit/sports performance peaks before this point depending on the last meal contentand general activity levels (meaning, after 14-16 hours of IF i start to notice a difference in performance), so i use that as a general proxy for a limit in optimizing my body's general performance/strength (even though i fast for 24 hrs from time to time).

Well If you don't eat as much than your not putting all that junk in your body that causes arteries to clog ect.

I tried fasting until dinner time (liquids only) while recovering from the flu, and I felt like crap the next day--even lower in energy, needed a long nap, headachey from tiredness. Any thoughts?

That animals stop eating is at least partly to save food for their friends, since they dont need to waste food if they are dying anyways.

And it has been shown that the metabolism slows down quite slowly while fasting. Also, evolutionary it makes sense that you did not eat while sick, so the body should be able to prioritize sickness over saving calories.

However, the body release stress hormones (catecholamines) when in calorie defeceit, because it is them who tell fat cells to let out fat. And they tend to repress the immune system.

Fever is a reaction from the body to kill the bacterias, so you shouldnt try to surpress it.

But I am going to try fasting righ away!