DEAR DR. ROACH: I am an overweight 68-year-old woman who wants to exercise. I would like to know what type of exercise I can do to help me lose weight. I am allergic to the chemicals in pools, so I cannot swim. Each knee has had a total replacement, leaving me with a limited range of motion, making it impossible to bend my knee enough to ride a bike. I am not sure what I can do. I was hoping you could help me with that. I can walk half a mile without losing my breath. I live on the second floor and always use the stairs, but I only use them two or three times a day.

What type of exercise do you recommend? — M.W.F.

ANSWER: The best type of exercise is the exercise you are going to keep up. Based on what you are telling me, I would guess that walking is going to be your best bet. The more you walk, the stronger you’ll get, and the more you will be able to walk and, I hope, the better you will feel. If walking all the time takes a toll on your new knees, there are other options, including weightlifting, yoga and even chair exercises.

However, losing weight requires changes to diet as well. They don’t have to be big; often, just making one change can lead to a slow weight loss, such as giving up regular soda for diet or, better yet, water. Many people find that giving up sweets leads to a much easier time losing weight. I suggest you look honestly at your diet and find one thing you know you could do better on to start with.

DEAR DR. ROACH: What is ”early-onset dementia”? How does it contribute to death? Is there a time frame? — D.R.

ANSWER: Dementia is a progressive condition of loss of neurologic function, especially memory, judgment, movement and social interaction. Dementia usually begins slowly, and it may be hard to recognize at first, as opposed to delirium, which is a sudden, dramatic change in function usually associated with medical illness.

”Early-onset” means that the condition is recognized before the age of 65. Alzheimer’s disease is the most common cause of early-onset dementia. In fact, years ago the term ”Alzheimer’s disease” was reserved for younger individuals, and we used the term ”senile dementia of the Alzheimer’s type” to describe what is the very same disease in older individuals. That confusing and unnecessary distinction is no longer used.

Another common cause is vascular dementia, in which many small strokes lead to progressive loss of function and is associated with smoking, cholesterol problems and high blood pressure. Frontotemporal dementia, formerly called Pick’s disease, has two variants: one with prominent changes in behavior (bvFTD), and primary progressive aphasia, in which communication ability is the primary early manifestation. Other causes of early-onset dementia include alcohol-related dementia, the dementia associated with Parkinson’s disease and Lewy body disease. There are many other less-common causes.

With such a large number of possibilities, I can’t give you a timeline or prognosis. Some of these conditions progress more rapidly than the usual form of Alzheimer’s (which itself varies). Death in someone with dementia usually is related to progressive neurologic dysfunction leading to frailty, nutrition problems and susceptibility to infection.

READERS: Recurring vaginal infections are often troubling to women. The booklet on that topic explains them and their treatment. Readers can order a copy by writing: Dr. Roach Book No. 1203, 628 Virginia Dr., Orlando, FL 32803. Enclose a check or money order (no cash) for $4.75 U.S./$6. Can. with the recipient’s printed name and address. Please allow four weeks for delivery.

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Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers may email questions to ToYourGoodHealth@med.cornell.edu or request an order form of available health newsletters at 628 Virginia Dr., Orlando, FL 32803. Health newsletters may be ordered from www.rbmamall.com.