If the memory impairment started abruptly after the major surgery/anesthesia, it is important to rule out any reversible causes of the change. He may had a stoke or even multiple small strokes caused by cerebral hypoperfusion, loss of blood or by another reason. He may be over-medicated, deficient in vitamins.
It is important for your family member to be evaluated by a good neurologist before you decide how to help him.
That’s a terrible suggestion, humans already have a poor ability to digest plants but without a colon or becomes next to impossible. But older people need more nutritient as they get older especially when recovering, especially protein so that means more animal products not less
Can’t say enough about this issue. Anesthesia in older patients. My dad healthy 82 years… walked a few miles a day. After a knee replacement- memories totally wiped.
He got somewhat better after 6 months, but was never himself, memories, and started seeing things not there, hallucinations…odd people, little whirl winds, and such. He knew it was impossible and said so… but still saw them.
Had a 90 year old volunteer. Sharp as a tack… had Anesthesia…knee surgery… she could tell it wasn’t right memory faded week by week… in a month… knew nobody. Sad.
I was just listening to the latest Hubermann podcast and his guest is Dr christopher Palmer. He said there was clinical evidence that a substantial benefit for people with Alzheimers using a keto diet.Similarly a British doctor reported that coconut oil had slowed her husbands dementia, presumably because of the high levels of saturated fat. As Palmer pointed out, the keto diet has proven useful in epilepsy, which after all is a brain misfunction.
Thanks everyone for their ideas and suggestions. The situation is a little more complex, as @Arhu suggested, because of the loss of the large intestine. Grains and fiber cause lots of problems for people who have a colostomy bag.
But - I will do more research on the area of ketosis, and look into exogenous ketones too.
Dale Bredesen, formerly of the Buck Institute has a series of books out on preventing alzheimers - so I’m busy reading those:
The study, which comes jointly from the UCLA Mary S. Easton Center for Alzheimer’s Disease Research and the Buck Institute for Research on Aging, is the first to suggest that memory loss in patients may be reversed, and improvement sustained, using a complex, 36-point therapeutic program that involves comprehensive changes in diet, brain stimulation, exercise, optimization of sleep, specific pharmaceuticals and vitamins, and multiple additional steps that affect brain chemistry.
I work a lot with older persons and I n my experience it is common for apparent symptoms of dementia to show up after a major surgery for older persons. I’m not sure why, but after some weeks or months of healing I’ve noticed the symptoms often subside, even completely.
When the memory impairment began abruptly after surgery your relative might be dealing with Post Operative Cognitive Dysfunction. That can be different than typical dementia so might be worth exploring.
Let me tell you the story of my mom (age 83). In 2018 my dad passed away and my mom never really got over it. We saw her going backwards and about a year ago mild dementia was established. About half a year ago I read about glynac and the cognitive improvement in elderly people (among all other benefits) that was associated with it. I decided to wait no longer and put her on glynac straight away. Now 6 month later she seems so much better and the dementia seems to be stopped in it’s tracks. So much even that het caregiver noticed it without knowing about the glynac. They do know by now… Another remarkable thing is that she is now once again contacting and visiting old friends, where in the last 2 years she hardly came outside the house. 1200 mg nac and 3 grams glycine per day has done wonders
Approximately 10% of elderly surgical patients develop POD, rising to 30–65% after certain types of surgery, such as hip fracture, cardiac and emergency surgery
Post operative cognitive dysfunction
The incidence of POCD varies according to the definitions used in various studies . Higher incidences are demonstrated in populations of elderly patients undergoing major surgery. For example, following coronary artery bypass grafting, the quoted prevalence of POCD varies between 10 and 80% early after the procedure and is thought to be associated with emboli, atherosclerosis and intra-operative ischaemia 
It seems there are no proven treatments yet, but some proposed ones:
The neuroinflammatory hypothesis provides many varied targets for candidate treatments for POCD. These treatments largely fall into one of three strategies: blocking inflammation by inhibiting inflammatory mediators (anti-inflammatory), preventing the oxidative component of inflammation (anti-oxidative), or protecting neurons during and promoting neuronal health before surgery (pro-neuronal).
Anesthesiologist here. I lost both parents to dementia related causes, so am acutely sensitive to this issue. It’s a complicated problem with no magic bullet solution. We try to avoid general anesthesia in advanced age and in patients with baseline cognitive issues, but not all operations are elective(colon resection for cancer) or can be done without general anesthesia. Not all families and patients recognize and admit baseline impairment, so the memory issues seem acute, but are actually a progression worsened slightly by our drugs and techniques, so we get implicated as causing the problem.
Definitely look at post op medications, as narcotics, sleep aids, anticholinergics, and others have pronounced CNS effects which can last for many days in the frail patient.
Dementia is a tough problem for the caregivers AND the patient. It takes so many forms and is exquisitely frustrating and exhausting. I wish I could be more help.
I am curious — are you interested in rapamycin due to its possible prevention of dementia/Alzheimer’s?
If I had two parents with this issue - I would have concerns about genetics traits. Best thoughts to you.
Hi - thanks for the input. Yes - I’ve got a friend who is a pharmacist so the first thing we’ve looked at are the medications post operation to make sure they are not the issue. This was an emergency surgery, so definitely not elective surgery.
My parents had several lifestyle risk factors and comorbidities, and they developed dementia in their 80s, so not a pure AD picture. I do not carry the apoe4 gene, so Dr. Green thinks my risk is very low, but the answer is still yes to the question of rapa and brain health. Dr. Green insists on knowing apoe4 status before he consents to treat. It’s available on 23and me and as a standalone lab, I’m sure. There are now also AD blood tests available for difficult diagnosis cases, not for screening. We are lacking great treatment however. https://www.washingtonpost.com/health/2022/11/17/alzheimers-blood-test-research-treatment/