Dangers of very low calories

There are essential nutrients that are not in multi-vitamins (i.e., nutrients your body can't manufacture from others). As an old person, I've seen many essential or beneficial nutrients 'discovered by science' over my lifetime, after which they may be included in supplements like vitamins. They've been in foods all along, so I don't personally consider multi-vitamins a good bet except as insurance on top of good nutrition. (I've actually had personal conversation with nutritional researchers who feel pretty much the same way, that people are foolish to rely on supplements like those for core nutrition.)

Note that your calories-out can come not only from food, glycogen, or stored body fat, but also from lean tissue. Research suggests that we can metabolize a limited amount of body fat daily per pound of fat we have, though exact estimates of those limits differ. Certainly, some people lose weight rapidly enough to lose lean tissue, such as muscle.

If your calorie intake is too low, your body will slow down bodily processes it considers less vital to survival. The early ones are not so worrisome (fidgeting reduction for example), but it proceeds to some most people consider undesirable even if not life threatening (fatigue, weakness, slow hair growth (so breakage and thinning), brittle nails), to others of much greater concern. Immune system can be compromised, for one, a thing I wouldn't love to think about ever, let alone in a pandemic. Another would be rebuilding bone, which is a continuous process to maintain the body in good condition.

Certain health conditions become more likely with too-low calorie intake, such as gallbladder disease. Brain fog is common, among people who are getting too few calories.

As an extreme case, this is sobering, for example:

https://community.myfitnesspal.com/en/discussion/10761904/under-1200-for-weight-loss/p1

We don't really know what weaknesses our individual bodies may have, that will show up when we put them under unnecessarily much stress . . . and an extreme calorie deficit is a physical stress, no question.

Some of the above consequences don't show up until it's too late to do anything to prevent them, and some can have permanent impacts.

None of these bad consequences are *certain* to occur with too-rapid weight loss. It's a question of how much risk you like in your life.

Personally, I'm of the opinion that we can train our bodies, to some extent, to deal with what we require of them. I'd rather train my body to thrive as much as possible alongside a healthfully moderate weight loss (if needed), rather than training it to limp along on minimal calories. Some of the consequences of low calorie intake, over a long-ish period, can result in a lowered calorie requirement for maintenance, making it harder to stay at a healthy weight, or to lose weight next time if there's regain (as there is in the overwhelming majority of cases).

Just a few thoughts.

Above are 2 detailed explanations why it is a bad idea. My personal experience; I was on a diet, which was tracked through my health authority (at the beginning). I stopped attending meetings but continued with diet. Thinking it was safe, as NHS. (As a very sedentary, as disabled, late 40's female). Was very obese and struggling to lose weight, on my own). Didn't track calories at the time, but weighed all food and kept accurate logs. Over a few weeks, I became very unwell. Looking back, with experience gained, it was linked to inadequate nutrition, diet at that time averaged between 800 and
1,000 calories a day.

1) My heart condition was consistently worse, as my body was under stress.

2) Developed (and still have for life) an autoimmune disease. Two of my main problems, that I manage are extreme exhaustion and severe sight problems. Affecting daily life, at times I can barely see and can't go out alone.

3) Brain fog (also linked to autoimmune condition), but vastly improved and managed with balance nutrition. Especially by getting enough Fat, my cardiologist had recommended a low fat diet. But without targets, I had cut fat almost completely. At times, unable to hold conversation or drive.

4) Hair loss, remaining very thin hair, like straw, breaking away.

5) Brittle nails, that stopped growing and fell out. Bare nail beds, so sore and bled.

6) Weakened muscles, meant physically disabled condition, deteriorated.

These are the main symptoms, some are irreversible. Lived with and managed, rather than cured. Was a long road to recovery, 7 years later, still affect daily living. Was physically disabled and needed help before, but add my sight condition, I can not go out alone. No longer drive or see to cross the road.

Perhaps I would have developed the auto immune condition anyway. Who knows??? But putting my body under severe stress, certainly contributed. At the time, my BMI was still very obese. Using your theory I had fat to "support me", I didn't

Why take the risk!

A long post, but if this helps anyone else avoid my experience, then I'm glad.

(Incidentally, my daily activities, even for me plummeted. Barely able to dress, with help. Laying on sofa, barely able to sit, let alone move. I didn't exercise in the conventional way, but my neat dropped too. I stopped losing weight.

Since then, finding and using MFP, as intended. Averaging 1,600 calories a day, I'm maintaining a 66lb weight loss. So more food, improved my health and enabled me to lose in a healthy manner).

If you treat your body in this horrible way you risk also suffering negative mental repercussions in addition to the possibly permanent or fatal physical side effects. It’s been studied, look up The Minnesota Starvation Experiment. The volunteers that were able to complete the experiment developed an obsession with food that lasted long after the experiment ended. An excerpt from a book about the study:
“To everyone's relief, the subjects' moods and social behavior stabilized three months later. But when it came to eating, the men agreed they were not "back to normal." Many ate "more or less continuously" and a subgroup of the subjects continued bingeing to the point of sickness, even eight months later. At least one man was hospitalized for several days after having his stomach pumped. "Hunger differs radically from the delightful nuances of appetite," wrote the researchers in ‘Men and Hunger’. Semi-starvation had temporarily changed these men in many ways, but what seemed to linger long after was this inability to distinguish between the constant gnawing of hunger and normal appetite.”