What are the possible effects of eating 900 calories a day for a year?

Those things probably aren't related to his weight loss. A vitamin D deficiency would occur from not being outside enough. Though, he does seem to have some physiological issues, as you have noted. It's a good thing he's getting help.

To answer your other question, if a normal (non-obese) person consumed only 900 calories a day for a year, his/her body would go into "starvation" mode and cling onto every calorie as fat. Things like hair loss and random fainting would happen (look up side effects of anorexia). It's possible they could even die. However, your friend is in an okay position.

Honestly this is not enough info to truly predict your average weight loss per week there are to many variables. But 3500 calories is equal to 1 pound. Considering if you did not work out prior to this you will lose weight much quicker at first and as you gain muscle your weight loss will slow a bit. If you only intake 900 calories (which is a bit low if your exercising a lot) then calculate how many calories your burning and do the math. Also weigh yourself first thing every morning for your most accurate weight. Oh and just a health tip I would increase your calorie intake to atleast 1500 calories a day (500 per meal) and never skip breakfast.

Short answer: Yes, the things you've mentioned can be caused by excessive weight loss due to an extremely low calorie diet.

Long answer: Bariatric patients typically consume between 500 and 700 calories per day and have to follow some rules to stay heathy. They include:

1. Protein should be main source of nutrition as it keeps you from losing muscle mass and animal protein is the only complete protein, which is the only thing that builds body cells. (aka, keeps you healthy)

2. You absolutely MUST take a bariatric multi-vitamin, calcium supplement, vitamin D supplement (unless you spend a lot of time in the sun) and sometimes a B vitamin complex (a mixture of B vitamins) otherwise you become malnourished.

3. You must begin or continue to be active as inactivity can cause serious bowel and digestive issues.

Malnourishment can cause breakdown of cells, improper digestion, lack of energy, depression, light-headedness (or feeling an altered sense of physical reality, like floating), low pulse and low blood pressure, among others.

Inactivity can cause indigestion, heartburn, acid reflux, IBS, constipation and, in severe cases, fecal impaction, high blood pressure and respiratory problems, among others.

Also, for someone who is obese, food is not just nourishment, it's a form of emotional support. Losing that emotional support due to dieting means the person needs to find some other way to deal with emotions and stress. If someone doesn't, they can become depressed, disconnected, argumentative, irritable, disillusioned, lonely and even suicidal. Which is why bariatric patients are strongly encouraged to seek counseling from a professional before beginning an extremely low-calorie diet (surgery or not) to learn how to replace food as a source of comfort. Obesity is a form of food addiction and, often, is replaced by another addiction (like smoking, alcohol, excercise or sex) when the person can no longer eat the way they used to. In addition, extreme low-calorie diets can alter a persons body chemistry and hormone levels (cortisol, testosterone, seratonin) which can also lead to depression or feelings of hopelessness or anger and aggression in the case of increased testosterone.

Poor blood circulation is the main cause of coldness in the extremities and should be taken VERY seriously, especially when accompanied by tingling/numbness as that can be a sign of diabetes. Even if it's not diabetes, poor circulation can lead to serious issues in the extremities including gangrene and amputation in severe cases.

If your friend's attitude is a NEW and drastic change in behavior that was only noticed AFTER excessive weight loss due to a low-calorie diet, it's probably caused by a mixture of the things I've mentioned. He should have his hormone and vitamin levels tested as well as having his circulation checked. He should also see a psychiatrist or psychologist and be as honest as possible with them.