The most likely explanation for Dr. Atkins' weight gain, imho, is one which any nurse who has ever worked in critical care, either ER or ICU, has sadly seen too many times.
Dr. Atkins "mysterious" (not!) and very large weight gain was most likely due to a combination of massive third spacing of fluid into the area between the fascia and skin and probable abdominal compartment syndrome as well.
Abdominal compartment syndrome is what I'd call a more localized version of widespread, total body third spacing of fluid. Patients in critical care settings, particularly those who have suffered trauma, as he did in his fall, or who have had certain kinds of surgeries, as well as many other critical medical conditions are at risk for these complications of treatment.
Patients who are treated in ERs for trauma and must be infused many liters of blood or fluids are at very, very high risk for third spacing of fluids. Likewise, those who have serious neuro trauma, as Atkins did, (think of the brain as the regulator that it is, which includes regulation of fluid balance) and those who have serious cardiac issues, such as cardiomyopathy, are also at obvious higher risk. Cardiomyopathy, in and of itself can lead to hypoperfusion of major organs...and hypoperfusion of major organs can lead to third spacing of fluids and electrolyte imbalances. Those with neurosurgeries are also at higher risk.And third spacing will also contribute to already existing hypoperfusion in this scenario.
The idea that this level of dysregulation, and ultimately, multiple organ system failure can be simply treated with "diuretics" is a very facile, uninformed idea. Part of the big problem with massive third spacing is that you have a person who is infact HYPOvolemic in the intracellular spaces and thus the person has a high heart rate, a low BP, and is often not making enough urine and is not breathing well, particularly if they have abdominal compartment syndrome. This all becomes a vicious cycle as the graphic in the provided link demonstrates. The odds of NOT surviving this, as a critically ill person, are very great.
Additionally, it is also not unusual for critical patients with substantial neuro insults and or surgeries, to suffer very rapid declines in nutritional status. They can become very malnourished in a matter of days, rather than weeks. This only adds to the issue as blood levels like albumin/protein drop which further aggravate the third spacing of fluids and fluid and electrolyte imbalances are part and parcel of the picture.
Some of the the most heartbreaking situations I have seen are family members who live far away seeing a person who dies in ICU after developing massive third spacing. Their shock and grief are made nearly unbearable by the surreal human before them, who was formerly "Jack Sprat" all his life, and now looks like a monster edition of the Pillsbury Dough Boy inflated over his entire body, to the point of near bursting.
I have seen trauma patients resusitated in the ER gain 20+ lbs in a few hours. And yes, it is fluid, not fat, of course. I have measured the abdominal girths, as well as arms, thighs, etc on many, many patients, as well as weighed them daily, who were severely third spacing fluids and fighting for their lives.And I have also stopped measuring them when multiple organs failed, they'd been on a ventilator for weeks, and loved ones made the decision to provide supportive care only, in the face of looming death. I have certainly weighed persons who in a week or two were blown up beyond recognition and were 40-50 lbs heavier in the end, due to third spaced fluids.
Neurogenic shock and cardiogenic shock, as well as hypoperfusion are no little things.
What an awful, awful way for Dr. Atkins to die.
And what an awful thing for his wife to have to go through - seeing her husband of many year turn into a discolored, massively bloated, unrecognizable distortion before her eyes, and then having to deal with all the drama spinners. How very, very sad.