Dear Doctors: Have you ever encountered anyone who barely seems to metabolize caffeine at all? I am a 64-year-old woman, and over the last eight years, I have become increasingly unable to tolerate even small amounts of caffeine. I would like to understand why.

Dear Reader: Let’s begin with caffeine, which is a naturally occurring compound found in many leaves, plants and seeds. In the United States, it is primarily consumed in the form of a beverage, most often as coffee, tea or cola. It is also present in cacao products such as chocolate, in energy drinks and added to some medications.

Caffeine is best known for the boost it can give to energy, wakefulness and mood. The effects occur in part because caffeine blocks the receptor sites meant for a brain chemical called adenosine, which causes sleepiness. Caffeine also causes several regions of the brain to release dopamine, a neurotransmitter sometimes known as the feel-good hormone, which plays a key role in the body’s reward system.

Thanks to these pleasurable effects, caffeine is the most widely used stimulant on earth. Yet, not everyone can tolerate it. In fact, an inability to metabolize caffeine is somewhat common. For some people, like yourself, even a small amount can be disruptive. Others find they have a daily limit for the amount of caffeine they can consume, or a time of day beyond which additional caffeine begins to cause ill effects.

Side effects of caffeine sensitivity can include irritability, nervousness, anxiety, restlessness, irregular or interrupted sleep, insomnia, headache, heart arrhythmias, elevated blood pressure or gastrointestinal discomfort. This can occur for a variety of reasons, including age, metabolic health, genetics and, although rare, immune response. As a result, the range of people who are thus affected is quite diverse.

A fascinating area of research looks at the role that genetics plays in the body’s response to caffeine. Studies have found that people with a so-called “fast” caffeine gene, which helps the liver to process the molecule rapidly, can tolerate a lot of the compound without any problems. But those with the “slow” gene, which causes caffeine to linger in the body, experience the adverse effects we discussed earlier. Interestingly, the presence or absence of these genes also influenced whether people liked or disliked the flavor of coffee.

Being older can also play a role in the ability to metabolize caffeine. As we age, there is a gradual decrease in the quantities and activity of the liver enzymes. This includes an enzyme known as CYP1A2, which plays a crucial role in the breakdown of caffeine. CYP1A2 converts caffeine into its metabolite, which are further broken down and then excreted in the urine. In one study, researchers found that coffee drinkers between the ages of 65 and 70 required 33% more time to metabolize the same amount of caffeine as the younger participants. Considering your age and the timing of the onset of your caffeine intolerance, it is possible that this may be the case for you.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10960 Wilshire Blvd., Suite 1955, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)

Kansas City, MO 64106; 816-581-7500

Dear Doctors: I am 66 years old and an avid cyclist. Last winter, after we went for a long ride, I suddenly felt lightheaded and then fainted. My doctor ordered a series of heart scans that found slight mitral valve prolapse. What is that? Should I be worried?

Dear Reader: Mitral valve prolapse is a condition that interferes with the one-way flow of blood through two of the four chambers of the heart. As the name suggests, it involves a structure known as the mitral valve. This is one of the four valves in the heart that act as one-way doors into and out of the four pumping chambers. These valves ensure that blood keeps flowing in the right direction. It is the coordinated action of the valves opening and closing that creates our heartbeat.

The mitral valve separates the two left chambers of the heart. When the heart muscle relaxes, the mitral valve opens. This lets blood move from the upper chamber of the heart, or left atrium, into the lower chamber, or left ventricle. This is the chamber that sends oxygenated blood to the aorta and out to the body. When the heart contracts again, the mitral valve closes. This prevents blood from backing up and returning to the left atrium.

When someone has mitral valve prolapse, it means there is a malfunction in a pair of structures in the valve known as leaflets. These are flaps of strong tissue that keep blood pumped into the left ventricle from returning to the left atrium. Rather than forming a tight seal, one or both of the leaflets collapse back into the left atrium, which can allow a small amount of blood to return to the upper chamber. When this happens, not enough blood exits the left ventricle and flows into the aorta. It can leave the tissues of the body short of the oxygen and nutrients they need at that moment, and cause an array of symptoms, including the lightheadedness you experienced. In some people, the return of blood to the atrium makes a sound, known as a heart murmur.

When you are diagnosed with mitral valve prolapse, it is recommended that you see a cardiologist. They will order tests to assess the condition and plan any needed treatment. These include an echocardiogram, which maps the heart with high-frequency sound waves, and a cardiac MRI, which creates a detailed image of the heart. Each helps to evaluate the degree of mitral valve prolapse and whether blood leakage is occurring.

The good news is that mitral valve prolapse does not always require treatment. However, when the prolapse is severe, the resulting leakage can lead to the enlargement of the upper left chamber of the heart. This, in turn, can trigger heart arrhythmias, including atrial fibrillation, and increase the risk of having a heart attack of a stroke. This all makes it important for you to get a thorough cardiac exam and a complete diagnosis. Depending on the results, treatment may consist of monitoring the condition, medications and, in a small number of cases, surgery.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10960 Wilshire Blvd., Suite 1955, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)

Kansas City, MO 64106; 816-581-7500

Dear Doctors: Is there such a thing as pre-prediabetes? My doctor says it’s happening to me, and he wants me to use one of those blood sugar monitors that you stick to your arm. I thought those were for when you actually have diabetes. I am a 44-year-old man in good health.

Dear Reader: There is no diagnosis known as pre-prediabetes. However, it sounds as though your doctor may be warning you that your blood glucose readings are trending close to the threshold of a condition known as prediabetes. This is both a real diagnosis and a serious warning sign. Prediabetes means that someone’s blood sugar levels are consistently higher than normal. While not yet high enough to qualify as Type 2 diabetes, the elevated blood sugar levels of prediabetes increase the risk of escalating to Type 2 diabetes. This is when the body stops responding appropriately to insulin, or no longer produces an adequate supply.

Over time, the chronic high blood sugar levels of Type 2 diabetes cause damage to tissues throughout the body. Complications include heart disease and vascular disease, kidney disease and nerve damage. These can lead to vision problems, vision loss, pain, numbness or tingling in the limbs or extremities, slow wound healing and cognitive impairment.

Type 2 diabetes has reached epidemic proportions in the U.S. This is reflected in the number of questions we get on the topic. The good news is that with wise lifestyle choices, including eating a healthful diet and getting enough exercise, you can maintain safe blood glucose levels. That brings us to the second part of your question.

Your doctor has asked you to consider using a continuous glucose monitor, or CGM. It is a small, wearable medical device that provides real-time blood glucose readings. CGMs are equipped with a tiny sensor that pricks the skin and measures blood sugar levels in the interstitial fluid. This is a clear, watery substance that bathes the cells throughout the body. Among the substances dissolved in interstitial fluid is glucose from the bloodstream, which has been forced from the capillaries by hydrostatic pressure. The data collected by CGMs is sent to receivers, such as a smartphone or smartwatch app, a dedicated reader or an insulin pump.

It is true that continuous glucose monitors are most often used by people living with diabetes. However, doctors have begun to recommend them for healthy nondiabetic patients. When you wear a CGM, the multiple data points give you a detailed picture of how your diet affects your blood sugar levels. What’s the difference between a serving of broccoli and a handful of potato chips? A CGM will let you know. The devices can also offer information about the effects of fasting, various forms of exercise and even not getting enough sleep.

When we recommend CGMs to nondiabetic patients, it is as a short-term educational exercise. Typically the sensor in a device is good for 14 days. Although insurance does not cover their use for nondiabetics, some brands offer manufacturer coupons. If you decide to move forward with this, your doctor can help guide you.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10960 Wilshire Blvd., Suite 1955, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)