DEAR DR. ROACH: I recently had an episode of food poisoning with vomiting and diarrhea. It lasted four hours. I live alone. I’m 84, and in good health except for chronic atrial fibrillation and well-controlled high blood pressure.

My question is, When does it become serious enough to call a doctor, and how do I determine if it is one of the serious strains that results in hospitalization? I don’t want to go to the emergency room unnecessarily.

— B.R.

ANSWER: Foodborne illness — that is, disease passed by contaminated food or water — is something that most people will suffer one or more times in their life. Bacteria, viruses and parasites all may be the source of foodborne illness. These germs may cause symptoms due to a toxin they make or they may invade the system. In general, toxin-mediated illness comes on faster and tends not to have fever. Invasive organisms usually take at least 24-48 hours to start causing symptoms, may cause fever or bleeding and tend to last longer. However, both kinds can be serious.

Although most cases of “food poisoning” will go away by themselves, there are some worrisome symptoms and signs that should raise concern and should make it more important for you to seek urgent care.

For most people, the biggest risk is losing too much fluid. This is called volume depletion or dehydration. Diarrhea can cause an enormous fluid loss that a person might not recognize. When combined with vomiting, the risk becomes serious, especially since the person cannot replace those lost fluids. Fever also causes increased fluid loss, and a temperature over 38 C/100.4 F should prompt concern. Finally, blood in the diarrhea or vomit should prompt a visit to your doctor, urgent care center or emergency room.

Being older is a significant risk — as is being very young; infants and toddlers are at increased risk — so keep that in mind when deciding whether to seek medical attention. Atrial fibrillation causes at least modest but sometimes significant reduction in your body’s ability to withstand volume depletion. Because of your age and atrial fibrillation (an abnormal heart rhythm), you should adhere to a much lower threshold for calling your doctor than a younger person with no chronic conditions.

Hospitalization is not commonly necessary with foodborne illness. Intravenous fluids work wonders at making people feel better until they are able to keep fluids down orally. These often are given in a doctor’s office, acute care centers or ERs.

DEAR DR. ROACH: I read that every pound you lose translates to 4 pounds less stress on the knee. Can that be possible?

— S.N.

ANSWER: That is true. A 2005 study combined advanced gait analysis with a mathematical model to estimate the amount of stress on the knee in normal, everyday activity. They found that every pound of body weight dropped meant 4 pounds less stress at the knee. The authors noted, “Accumulated over thousands of steps per day, a reduction of this magnitude would appear to be clinically meaningful.”

The effect of weight loss is even more dramatic when looking at the effect of stairs on the knee. Most people with arthritis know that going up and down stairs is more uncomfortable than regular walking. That’s because the stresses are at least two to three times more with stairs than walking on level ground. Each pound of weight loss will then have even greater effect on knee stress with stairs.

Losing weight is not easy, but it remains one effective treatment for reducing symptoms of arthritis.

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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 send mail to 628 Virginia Dr., Orlando, FL 32803.