It is not very healthy but it won’t harm you to a significant level.
You will experience an increased amount of sodium, water retention and probably a gain of 10 pounds which is mainly water. It will speed up up your metabolism for a few days but you will be back to normal in maximum 5 days.
Well the 10k challenge won’t do your body much good. Yes you’ll be in a surplus of calories that will help you build muscle however this is really up and beyond what the body requires. It will cause severe bloating, water weight and weight gain!
A lot of it depends on your genetics and level of activity. If you are sitting around all day without any physical activity, obviously you will gain a lot weight/fat which will lead to many different types of diseases (obesity, heart disease etc…). If you are active throughout the day and working out a lot, you will gain muscle, but you will also gain a lot of fat. Athletes can get away with these types diets because they are active for hours, and majority of them are born with a good metabolism. For the average person, 10k is way too much to burn in just one day and it will most likely lead to gaining more fat than muscle.
If you do this on a one off, then most of your physiology will return to baseline the day(s) after.
You could expect to see glycemic spikes if you are ingesting a lot of simple sugars or white flour products. The glycemic load (amount) would also produce this same response. Whilst this does induce fat storage, even with 10K calories in one sitting you probably wouldn’t see a difference.
You won’t expect to see any muscle gain from simply eating 10k Kcal – you need to tear your muscle fibers first. Then the building blocks which repair them become a factor through good nutrition.
The foods which comprise those 10 Kcal are a big factor in determining whether it is ‘healthy’ or not. The excessive amount of calories, unless used as energy is just a waste for the body to process as this requires energy to do so.
Even eating 10 Kcal of plant food wouldn’t be that healthy, considering the sheer amount you would have to ingest to reach that number of Kcal (most plants are low in calories). You would simply impair the fluidity of your digestive function – important for extracting precious nutrients for cellular energy, growth and repair.
In theory, all excess should be converted and stored. However, I think there are a few slight caveats. One would be the body’s ready absorption of all the food presented. The reason for this is that the body does not seem to readily have available enough enzymes to break down everything at once, especially in such large quantities. Another thing to keep in mind is that, for all intends and purposes, de novo lipogenesis seems to be very minor in humans. This is the pathway used to convert carbohydrates into fatty acids, to speak frankly. So, you would also be limited by that.
Aside from all this, there is anecdotal evidence of people doing this in the form of a “refeed” day and gaining very little weight, if any, other than water weight that eventually came off a few days later when they resumed their normal diet.
As for health, I do not think there is much to worry if you do this once in a few months and you don’t force it to the point that you throw up. Sure, it is not inherently healthy or good or some such thing, but I don’t see this as being exceedingly destructive in moderation.
As for muscle gain, this should not do much. Muscle mass responds to regular stress and regular supply of nutrients.