How should the human digestive system change to be able to go without food for many months?

Question

A healthy adult without special training can endure complete deprivation of food for 25-30 days, provided that water is available and physical effort is saved. But, a 1973 study by doctors at the University of Dundee School of Medicine, published in the Postgraduate Medical Journal, described a 27-year-old "severely obese" Scotland starving for 382 days in an effort to lose weight, which is ten times higher than normal.

Amazingly, the man managed to reduce his weight from 207 to 82 kilograms and remain in surprisingly good health. 5 years after extreme starvation, his weight remained stable at 88 kilograms.

Throughout the hunger strike, the man did not eat any solid food, although he was given potassium and sodium supplements, vitamins, and a small amount of yeast (which contains protein). And if you are wondering about bowel movements, doctors note that he defecated once every 37–48 days.

The man regularly visited the hospital for blood and urine tests to check blood glucose levels and plasma electrolyte concentrations. Although these rates declined over time, he managed to maintain remarkably good health and a cheerful mood.

“Despite hypoglycemia [low blood sugar], the patient remained asymptomatic, felt good and walked normally,” the report said.

And here we come to the heart of my question: how should the human body be changed in order to be able to fully do without food (additives) for a long time, like this person?

So when we eat regularly throughout the day, our body gets most of its energy from glucose in the blood and liver. Cells use this glucose to synthesize ATP, a molecule that carries energy into cells.

If there is no glucose in the blood or in the liver, we can also make it from glycogen, a polysaccharide formed by residual glucose that accumulates in the liver and muscles until needed. In addition, the body can break down body fat into glycerin, which can eventually be converted to glucose, and free fatty acids, which can be converted to ketones.

Since the man had an extremely plentiful supply of glycerin in his body fat, he was able to survive this latter process for a surprisingly long time with a little help from supplementation.

The latter is precisely the problem, because if it were not for potassium, sodium and yeast supplements, which are necessary for the biological functions of the body. For example, potassium is absolutely essential for nerve impulses and muscle contractions, along with many other processes in the body (this just needs to be corrected somehow).

Note: a person should lead a relatively normal life, except, of course, large expenditures of energy, such as marophons and other sports activities.

Answer 1

Step one is large fat reserves. That is obvious.

Humans should be designed to put on much more weight so they have more calories to burn. This is clearly needed, as you need energy to live. But what happens after? We can look to bears hibernating to find out how they handle it.

https://www.sciencedirect.com/science/article/pii/S0085253815557360

Step two is hibernation.

Subsequently, the bearsretreat into their winter dens, where they remain for 5–7months. During this time, they are physically inactive and sleep with slightly depressed body temperatures (30–351C). Bears maintain this body temperature by undergoing periodic muscular shivering, and do not appear to use brown fat–mediated nonshivering thermogenesis. As part of a reduction of their metabolic rate, they become bradycardic, with a heart rate of 8–10 b.p.m. compared with a rate of 40 b.p.m while being active. Their basal metabolic rate decreases by 40%, and their oxygen consumption 50% of normal. Importantly, throughout the hibernation period they do not move, eat, drink, urinate, or defecate. Survival during this period is provided almost entirely from their fat stores, which are metabolized to water to keep the animal hydrated.

Burn less calories, and you have less need to feed. They also don't urinate or defecate, reducing the amount of minerals and vitamins they would lose.

Next you need to handle urea normally lost in sweat and urination.

Humans can stop sweating as much, as it's very costly, and learn to recycle urine more. Bears don't dump massive amounts of water and electrolytes out their skin.

Bears are hypothesized to use a symbiotic bacteria which can recycle urea, allowing them to reclaim it as protein. This reduces their need to urinate, and reduces waste.

You need to handle lost muscle mass.

Extended weight loss tends to reduce muscle mass, making you much less effective. To reduce this, the above urea can be recycled into protein, and bears tend to not use burnt fat to warm themselves, they shiver. They also secrete more testosterone and secrete a powerful proteolytic inhibitor which reduces muscle wasting. They also secrete special proteins to help maintain the cell structure so that less damage is done. They also have a very good ability to synthesize proteins from reycled urea to replace damaged muscle tissue, allowing them to mitigate the damage.

You need better wound healing.

It's a common issue for people with kidney failure, which causes similar loss of metabolites that their wounds heal less well and so they tend to fester. Bears secrete delta-opioid receptor antagonist and the major bile acid ursudeoxycholic acid which mitigate these issues, letting them heal wounds while they are hibernating.

You need to mitigate clots.

Prolonged low activity, necessary to reduce your calory use, tends to cause clots which can damage you. Your human who is surviving on nothing at all but their fat needs to have less of this. They have a number of adaptions, like platelet inhibitors and a2-macro-globuli which inhibits clotting, reducing the degree to which their blod clots.

You need to maintain bones.

Bones are a vital reserve of calcium and are needed to function. Prolonged bed rest tends to reduce them. Bears have enhanced calcium recycling, minimal excretion, and enhanced bone growth hormones. They also have an enhanced to retain vitamin D, which helps maintain your bones.

You need to have massive vitamin C reserves.

Vitamin C tends to reduce inflammation, which is common during fat metabolism. Bears tend to consume massive amounts of berries to build up their reserves and can recycle them better.

With all of this, your human could better survive not eating at all. Hibernation doesn't need to be as extended as a bear's, but more hibernation is better of course.

Summary. Sweat less, recycle more, repair the common damage of not eating and lying around a lot, and be a bear.

Edit. Extra requirements. A human contains around 120000 calories and people start to face severe consequences from starvation at around 30 days, and die at around 60. so the main limiting factor to survival isn't necessarily micronutrients, but lack of calories, since 120000 calories/60 is 60 days, two months, same as when people die. While the above will work to mitigate the consequences of starvation a bit longer, if you do live a normal life then when your calories run out you will die.