How do you “force” an adult to eat and drink more?

You can’t force her. At her age I would let her eat whatever she wants. As folks get older it appears they eat much less but are ok with it. My mother ate like a bird unless it was a favorIte which for her was a McDonalds fish filet, fries and a coke. She also loved potato chips with a coke. She would eat every last chip and finish every fish filet sandwich. Mom also had a sweet tooth. Good thing she wasn’t diabetic because she enjoyed her desserts too.

At 99 she’s got the right to eat and drink as she pleases, and it sure sounds like she’s doing well. You can encourage, you can’t force. Doctor doesn’t sound like the best...

This appetite is a normal part of extreme age. If your Mother's doctor doesn't know this, then you are with the wrong doctor. There is very little drive to eat as activities decrease. I have seen people force elders to drink and send them into congestive heart failure as a result. Your Mother is 90. It is time to consider that things will go downhill now, and this is all a part of it. With your good care she has maintained longer than most, and apparently still is. Time to relax a bit.

You don't force a 99 year old to eat and drink more. You may be able to make eating and drinking more enticing. My mother drinks water well, but not as well as she drinks lemonade or a strawberry milkshake so when she needs more fluids, I replace or add a flavored drink. I also offer watermelon or cantaloupe to increase fluid intake. My mother has always like sweets so I usually add a sweet to her meals. Often she starts the meal with the sweet, but she also usually finishes the "real" food in the meal too. There are relatively good sweets choices: fruit flavored yogurt cups, rice crispy treats, ice cream and sherbets, pound cake, peanut butter on nogut crackers or vanilla wafers and so forth. Just stuff with a sweet flavor but not over the top on the sugar. My mother loves fruit too so there's a bowl on her side table and included with each meal. My mom also likes the Healthy Choice cafe steamers frozen meals, which makes it easy to provide some quick and easy variety in the offerings.

My mother is 88 and although I try to keep her overall meals reasonably healthy, I also believe in giving what she enjoys which in her case meals a hamburger with all the fixings, onion rings, and a milkshake at least once a week.