Steve Jobs’ letter to the Apple community about his health problems seems to have reassured investors — the stock closed up 4.22% in Monday trading.

But medically, Apple’s (AAPL) CEO raised more questions than he answered.

His eight paragraph message contains remarkably few health-related facts. They’re all contained in these three graphs:

“As many of you know, I have been losing weight throughout 2008. The reason has been a mystery to me and my doctors. A few weeks ago, I decided that getting to the root cause of this and reversing it needed to become my #1 priority.

Fortunately, after further testing, my doctors think they have found the cause — a hormone imbalance that has been ‘robbing’ me of the proteins my body needs to be healthy. Sophisticated blood tests have confirmed this diagnosis.

The remedy for this nutritional problem is relatively simple and straightforward, and I’ve already begun treatment. But, just like I didn’t lose this much weight and body mass in a week or a month, my doctors expect it will take me until late this Spring to regain it. I will continue as Apple’s CEO during my recovery.” (link)

“Cryptic,” is how Dr. William Sherman, a medical oncologist at the Pancreas Center of New York Presbyterian and Columbia University, described Jobs’ breezy summary of his medical condition. “Delightfully vague,” says Dr. Andrew Ko, a medical oncologist at UCSF. That  phrase — “hormone imbalance” — tells us neither what hormones are involved nor why they’re misbehaving.

Moreover, Jobs, who is 53, has left several relevant facts out of this account, starting with the malignant tumor that was removed from his pancreas in 2004 — along with his gallbladder, part of his stomach, the lower half of his bile duct and part of his small intestine. See the Whipple procedure diagram below. The details of this operation were first reported by Peter Elkind in Fortune. (See also Why does Steve Jobs look so thin?)

Many experts are tempted to speculate that the hormone imbalance Jobs describes is being caused by a recurrence of the original cancer — an islet cell neuroendocrine tumor, one of the few forms of pancreatic cancer that can be successfully treated. Even if it has returned, that’s not necessarily a death sentence, says Dr. Ko. With proper treatment — using injectible drugs designed to block key hormone receptors — patients with these cancers can live for years.

But Jobs has explicitly reassured his employees — and, reportedly, Apple’s board — that he is cancer free, and there is no reason not to take him at his word.

According to the New York Times, Jobs also told the board that he had a second surgery in 2008 — most likely a surgical “revision” or rearrangement of his internal plumbing to address complications stemming from the original procedure (see here.) The purpose of this second procedure, according to the Times, was to correct ongoing digestive difficulties.

Jobs’ open letter suggests that those digestive difficulties have continue to plague him and offers a new theory — confirmed, he says, by sophisticated blood tests — about what might be going on: an easily treated “hormone imbalance.”

There are several problems with this explanation. Not only is it frustratingly imprecise, but it suggests that Jobs’ hormone issues are something his doctors just discovered — prompted by his decision a few weeks ago to finally get to the root cause of his weight loss.

Hormones — a broad term for any chemical released by cells that affects cells in other parts of the body — perform a wide variety of functions, from stimulating growth and regulating mood to triggering physical changes like puberty and menopause. They interact in complex ways — through cascading reactions and elaborate feedback loops — and are known to get out of whack now and then, most familiarly in post-menopausal women.

Hormones also affect digestion and metabolism. In fact, the main function of the pancreas is to produce metabolic hormones (chiefly insulin and glucagon) and digestive enzymes. Given that Jobs lost a large portion of his pancreas in his 2004 surgery, you would expect his digestive enzymes to be affected — a condition that can be effectively treated with enzyme replacement therapy.  Hormone deficiencies are also common after a Whipple procedure, but they are usually detected and treated early. The mystery is what — besides a tumor — could cause a hormone imbalance this late in the game.

“If someone is losing weight, you do a workup of his pancreas, you do a workup for diabetes, a workup for hypothyroidism,” says Columbia’s Dr. Sherman, who is not treating Jobs. “Maybe it wasn’t abnormal enough to say so at first, and maybe now it is so a diagnosis can be made.”

What strains credibility — and sounds too good to be the whole story — is that the issue was first raised and the cause discovered only a few weeks ago. If it is something as simple and straightforward as a nutritional problem caused by a hormone imbalance, says UCSF’s Dr. Ko, “I doubt his doctors would have missed it all this time.”

What seems more likely is that Jobs, a man who knows something about controlling the message, is telling us a story as carefully crafted as any Apple product. He has said as little as possible about his medical condition — just enough to calm the waters stirred by his decision to skip this week’s Macworld. And he said it before the markets opened on the eve of Apple’s last Expo — just in time to allow the thousands of reporters, analysts, developers and fans descending on San Francisco to “relax,” as he writes, “and enjoy the show.”

What’s really going on, we still don’t know.

–With reporting by Alyssa Abkowitz.

[Diagram courtesy of the Mayo Clinic]

Dan Lyons the former Forbes editor whose brilliant Web parodies of Apple’s CEO entertained tech enthusiasts for two years, publicly apologized Friday to those who complain that he’s not funny since he stopped writing as his alter ego, the Fake Steve Jobs.

“I really miss him,” said Lyons, speaking in New York City at the Web 2.0 Expo, a tech conference sponsored by O’Reilly Media, Inc.

Lyons stopped contributing to his popular blog, The Secret Diary of Steve Jobs, in July. He now writes a column for Newsweek and a blog called Real Dan.

According to Lyons, neither has been particularly well-received.

“I get these e-mails all the time: ‘I don’t understand how Fake Steve can be so good and you can be so bad. Your blog sucks, dude.’”

“I apologize,” Lyons told the audience. “I really am much less interesting than Fake Steve as a writer.”

Lyons says he had intended to bring The Secret Diary to Newsweek, but lost heart after Apple’s (AAPL) World Wide Developers Conference in June, when it was apparent to all who saw him that the real Steve Jobs had lost a lot of weight. (Jobs had a malignant tumor removed from his pancreas in 2004 and has been having lingering digestive difficulties as a result of the surgery, according to friends. See here.)

“I woke up one morning and I kind of lost heart,” said Lyons. “I said man, I just can’t keep doing this joke.”

But Lyons was heartened by Jobs’ appearance at Apple’s “Let’s Rock” event earlier this month. “He’s looking better now. Maybe we’ll revive it.”

Joe Nocera buried his lead.

The New York Times columnist — and former Fortune editor — waited until the end of Saturday’s 1,700-word “Talking Business” column about the health of Apple’s CEO and the secrecy that surrounds it to reveal that on Thursday afternoon, several hours after he’d gotten his final “Steve’s health is a private matter” from Apple’s public relations machine, he got a call from Steve Jobs himself.

“This is Steve Jobs,” he began. “You think I’m an arrogant [expletive] who thinks he’s above the law, and I think you’re a slime bucket who gets most of his facts wrong.”

Jobs, according to Nocera, said he would share some details about the health condition that made him to look so thin and haggard at his last public appearace — and triggered two share-punishing rounds of speculation on Wall Street — if Nocera agreed to keep the conversation off the record.

Nocera agreed, and reported only that nothing Jobs told him …

“contradicted the reporting that [Times reporter] John Markoff and I did this week. While his health problems amounted to a good deal more than ‘a common bug,’ they weren’t life-threatening and he doesn’t have a recurrence of cancer.”

The “common bug” is a reference to the explanation for Jobs’ weight loss that Apple’s PR department put out in June — an explanation that Nocera feels fell somewhat short of the truth. Markoff reported on Wednesday that Jobs had had an unnamed surgical procedure earlier this year related to his loss of weight, and Nocera adds that he had learned that Jobs was having ongoing digestive difficulties stemming from the cancer surgery he had four years ago — the details of which were first reported by Fortune (see here).

All this leads Nocera to the broader point he wants to make about Apple:

“Apple simply can’t be trusted to tell truth about its chief executive. Under Mr. Jobs, Apple has created a culture of secrecy that has served it well in many ways — the speculation over which products Apple will unveil at the annual MacWorld conference has been one of the company’s best marketing tools. But that same culture poisons its corporate governance. Apple tells analysts far less about its operations than most companies do. It turns low-level decisions into state secrets. Directors are often left out of the loop. And it dissembles with impunity.” (link)

So, yes, Nocera thinks Steve Jobs is an arrogant [expletive] who thinks he’s above the law.

But Jobs may have the last laugh. Twice in his column, Nocera refers to things that happened during Apple’s (AAPL) third quarter conference call on Tuesday afternoon.

In fact, the conference call happened on Monday.

UPDATE: The error in the printed edition of the paper has been corrected in the online version.

Steve Jobs’ health is in the news — again. Apple isn’t talking — again. And investors are freaking out — again.

So now’s probably a good time to remind ourselves what we know, and don’t know, about the health of Apple’s CEO.

In 2003 Jobs learned that he had a malignant tumor in his pancreas — a large gland behind the stomach that supplies the body with insulin and digestive enzymes. The most common type of pancreatic cancer — adenocarcinoma — carries a life expectancy of about a year. Jobs was lucky; he had an extremely rare form called an islet cell neuroendocrine tumor that can be treated surgically, without radiation or chemotherapy.

On July 31, 2004,  Steve Jobs underwent a modified Whipple procedure — or pancreatoduodenectomy — that removed large parts of his digestive system and reassembled the remaining parts in a new configuration.

Patients undergoing the procedure typically lose up to 10% of their body weight and may suffer digestive problems for the rest of their lives.

The nature of the surgery was first reported in general terms by Fortune investigative reporter Peter Elkind in a March 5 cover story, (“The trouble with Steve Jobs”) and in some detail on this site on June 13 (”Why does Steve Jobs look so thin“).

It is possible that Jobs’ cancer has returned; that fear, and Apple’s reluctance to discuss the matter during its quarterly earnings conference call this week, helped drive Apple (AAPL) share prices down more than 10% in after-hours trading Monday. Apple shares recovered most of the lost territory on Tuesday and were trading higher again Wednesday, after investors calmed down enough to take a second look at Apple’s record third-quarter earnings.

The fact is, the effects of the Whipple procedure are probably sufficient to explain Jobs’ weight loss, without assuming any recurrence of the original cancer. Wednesday’s New York Times reports that Jobs has assured friends that he remains cancer-free, and that he underwent a second operation earlier this year to address a problem that was contributing to a loss of weight. (One hedge fund hired a private detective to tail Jobs four years ago on his way to hospital appointments to determine out how sick he might be, according to a Wall Street Journal report, and some hedge fund managers are talking about hiring private eyes again.)

Apple insists that Steve Jobs’ health is a “private matter.” But it’s also a matter of public record. Jobs has publicly discussed his bout with cancer, both in a memo to his staff and — quite movingly — in a commencement speech to Stanford University’s class of 2005 (see here).

Apple and Steve Jobs may be forgiven their reluctance to delve deeper into the details of his digestive issues. But a little disclosure on that front might go a long way to calming Wall Street’s rattled nerves.

Much of the speculation about Steve Jobs’ rail-thin appearance at the unveiling of the new iPhone on June 9 has tended to be all or nothing.

Either his cancer has returned or he is recovering from a bout with a “common bug,” as his spokeswoman told the Wall Street Journal on Tuesday. “That’s all there is to it,” she said. (The talk may have unnerved investors a bit: Apple (AAPL) shares fell 4.1% on Thursday and another 2.4% by midday Friday.)

But this is not the first time Jobs’ appearance has raised concerns about his health, and the “common bug” doesn’t explain the weight loss that’s evident in a review of his keynote videos over the past few years.

There’s another possibility, one that is consistent with both Jobs’ medical history and the changes in his appearance. It stems directly from the type of cancer for which he was treated four years ago and the nature of that treatment.

In 2003 Jobs learned that he had a malignant tumor in his pancreas - a large gland behind the stomach that supplies the body with insulin and digestive enzymes. The most common type of pancreatic cancer - adenocarcinoma - carries a life expectancy of about a year. Jobs was lucky; he had an extremely rare form called an islet cell neuroendocrine tumor that can be treated surgically, without radiation or chemotherapy.

As Fortune reported in a March 5 cover story, (”The trouble with Steve Jobs“), Jobs tried various alternative therapies for nine months before the tumor was taken out on July 31, 2004, at the Stanford University Medical Clinic in Palo Alto, near his home.

“This weekend I underwent a successful surgery to remove a cancerous tumor from my pancreas,” Jobs wrote in an e-mail to Apple’s staff the next week. “I will be recuperating during the month of August, and expect to return to work in September.”

What Jobs didn’t tell the staff was that the operation he had undergone had radically rearranged his digestive organs and would permanently change the nature of his health.

The Fortune article reported - and Apple has not disputed - that his surgery was a variation on the Whipple procedure, or a pancreatoduodenectomy, the most common operation for pancreatic cancer.

Nobody who has a Whipple is ever quite the same.

The Whipple procedure, named for Allen Oldfather Whipple, the American doctor who perfected it in the 1930s, is a complex, Rube Goldberg-type operation in which surgeons remove the right-most section, or “head,” of the pancreas - as well as the gallbladder, part of the stomach, the lower half of the bile duct, and part of the small intestine - and then reassemble the whole thing in a new configuration. The severed surfaces of the stomach, bile duct, and remaining pancreas are stitched to the small intestine so that what’s left of the pancreas can continue to supply insulin and digestive enzymes.

These before-and-after diagrams, reposted with permission from the Mayo Clinic website, will give you a feel for what’s involved (e-mail subscribers click here):

A German study comparing the long-term effects of two variations of the Whipple procedure on 104 patients found an increase in diabetes and various degrees of gastric acid reflux, stomach ulcers, oily bowel movements, intolerance toward larger meals and aversion to certain foods. (Annals of Surgery, 2005)

Along with the digestive problems, patients often lose 5% to 10% of their body weight after the procedure. Weight stabilizes within the first year or two for the vast majority of patients, says Dr. Dilip Parekh, chief of tumor and endocrine surgery at the University of Southern California, who has performed more than 100 Whipple procedures. “There is a small group of people who tend to have persistent problems with weight loss and loss of energy and you often you are not able to pinpoint why,” he says. “But if they stay active and manage their nutrition well, there is no reason for them not to live a normal life.”

Jobs has never spoken publicly about what life is like after the Whipple, so we can’t be sure that he has any of the post-operative problems associated with the procedure. But they would go long way toward explaining why he looked the way did on Monday. And none of them would indicate that his cancer has returned, or that his capacity for work is diminished. Post-operative guides for patients suggest that there will be lifestyle changes but that they need not be drastic. And a survey of patients at Johns Hopkins Hospital found that the overall quality of life of long-term survivors of the surgery is nearly comparable to that of healthy people.

Apple was invited to comment on this story, but has so far declined.