Dear Doctors: I have dandruff, and my doctor says it’s from seborrheic dermatitis. Can you explain what that is? I’ve been using the special shampoo I was given, and it helps. When I stop using it, though, the dandruff comes right back. Also, it’s making my hair really dry.

Dear Reader: Dandruff is a condition in which the skin on the scalp begins to flake and shed. It is often caused by a mild form of an inflammatory skin condition known as seborrheic dermatitis.

The word “seborrheic” refers to the sebaceous glands. These are miniscule glands embedded in the skin, almost always alongside a hair follicle. These glands secrete a waxy and oily substance known as sebum. The degree to which someone’s hair is naturally oily or dry is connected to the activity of their sebaceous glands.

When seborrheic dermatitis develops on the scalp, it is referred to as dandruff. It is not a danger to health, and it is not contagious. The most visible symptom of dandruff is the presence of the white flakes that you have described. These are dead skin cells, which become pale and translucent as they are shed. Additional symptoms can include itching on the scalp and areas of thickened, scaly or crusty skin.

Seborrheic dermatitis is often associated with an overgrowth of a genus of fungus known as Malassezia. It is a yeast that naturally occurs on the skin. This type of yeast requires fat for nourishment, which is why it is commonly found near the sebaceous glands. When something causes an overgrowth of the yeast, it can trigger an inflammatory response. Dry or cold weather can lead to flares of dandruff symptoms. So can hormonal shifts, a buildup of hairstyling products or the warm and humid microclimate created by wearing a hat or wig.

Treatment of seborrheic dermatitis on the scalp has two main targets. One is to reduce the colonization of the Malassezia yeast. The other is to manage any symptoms of inflammation that are making the person uncomfortable. When the sebaceous glands of the scalp are overactive, they produce an abundance of sebum. This, in turn, can encourage the growth of the Malassezia yeast associated with dandruff symptoms. When this occurs, modulating sebum production is also an important part of managing the condition.

The shampoo that you have been advised to use contains an antifungal agent that is designed to slow or stop the growth of the yeast. However, because it does not completely eradicate the yeast, ongoing use of the shampoo is necessary to manage the fungal infection. Because reducing sebum production is also one of the goals of treatment, these products can lead to dry hair, as you have experienced. Dandruff shampoos also contain ingredients to ease itchiness and reduce flaking. These shampoos come in a variety of formulas, with different added ingredients. With a bit of trial and error, you can find the brand of shampoo that works best for your specific situation. Some people find using a rotation of different products can be helpful.

(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.)

Dear Doctors: Ads for hydration boosters claim to make one glass of water equal to three glasses of water. These products seem to be primarily water, sugar and salt. I have water, sugar and salt in my diet every day. Does that have the same benefit of the combined solution in the so-called booster?

Dear Reader: You are asking about a vast array of products that, as you point out, promise to maximize the body’s ability to absorb water. Also known as hydration multipliers, they are offered as powders, liquids, tablets, syrups and gels. Unlike sports drinks, which come premixed, hydration multipliers are meant to be added to plain water. Last year, global spending on these products topped $1.7 billion, making the additives big business.

Hydration multipliers typically contain four main ingredients: electrolytes, carbohydrates, vitamins and amino acids. These have important roles in maintaining optimal fluid balance, providing energy and aiding in fluid absorption. The most frequently added electrolytes are sodium, potassium and chloride, each of which is lost when we sweat. Carbohydrates, to be used as fuel, typically take the form of sugars such as glucose, fructose or sucrose. Amino acids are included because they help move water from the small intestine and into systemic circulation for distribution to the tissues. When it comes to vitamins, B and C are common add-ons. The advertising on hydration multipliers claims that the products provide these components in ratios that amplify hydration.

There is no question that drinking enough water is key to optimal health. After all, the human body is about 60% water. We rely on proper -- and precise -- fluid balance in our tissues for digestion, muscle repair, joint health, blood sugar control, heart and brain health, hormone production, organ function, energy, elimination, blood pressure regulation and mental focus.

Many Americans fail to drink enough water each day. This is particularly true for younger people, who drink a lot of sugary sodas, and older adults, who gradually lose the thirst cues that remind them to drink. We do lose electrolytes in sweat, and also excrete them in urine. That said, drinking plain water is still adequate for most of us. If the taste is too boring, many people find that adding a squeeze of citrus, a slice of cucumber or some other natural flavor can help the water go down more smoothly. Unsweetened fizzy water, which also comes with natural flavorings, is another option.

You are correct that when it comes to the electrolytes, amino acids, vitamins and carbohydrates that are contained in hydration multipliers, a balanced diet will provide you with all that you need. Your body will take care of using them in the proper ratios and proportions. That means eating leafy greens, vegetables, nuts, seeds, legumes, fruit, dairy products, certain fish and seafood, healthful fats and lean meats. Unless you’re an elite athlete, you can save the hydration multipliers for those times that you are engaged in prolonged or intense exercise that is making you sweat profusely.

(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.)

Dear Doctors: My understanding is that nicotine and tar are what hurt you when you smoke cigarettes. But you recently said there’s also carbon monoxide in cigarette smoke, and that it’s even more dangerous. Can you please explain? I want my husband to quit, and this may help.

Dear Reader: Carbon monoxide is a colorless and odorless poisonous gas. It is produced when anything that contains carbon is burned. It is often referred to simply as CO, which is its chemical formula. This indicates that each molecule consists of one carbon atom and one oxygen atom. (And for our fellow science nerds, the carbon and oxygen atoms of CO are connected by a triple bond.) The most common source of CO is the incomplete combustion of fuels such as gasoline, natural gas, heating oil, propane, kerosene, wood and charcoal. And, yes, the act of burning tobacco also results in the release of carbon monoxide.

What makes CO dangerous is its enormous affinity for hemoglobin. That’s the protein in red blood cells that carries oxygen from the lungs to the tissues throughout the body. A molecule of hemoglobin has four binding sites, each of which can carry a single molecule of oxygen. Due to its chemical structure, CO binds to hemoglobin 200 to 300 times more readily than oxygen. That means that when CO is present in the air someone breathes, the CO will crowd out the oxygen. This greatly diminishes the amount of oxygen the blood can carry to the cells. Even a small amount of CO can leave the cells of the body starved for oxygen.

When someone smokes a cigarette, the burning tobacco, which contains carbon, emits CO. Because the smoke is inhaled directly into the lungs, it leads to a dramatic decrease in the amount of oxygen the tissues will receive. Every time a smoker inhales, they put their body into an increasing state of oxygen debt. Among the many ill effects are breathlessness, diminished lung capacity and permanent injury to the alveoli, which are the hundreds of millions of minute air sacs in the lungs.

Exposure to CO damages the heart muscles and impairs the ability of the circulatory system to function properly. It also is linked to poor mood and cognitive impairment -- and even to sexual dysfunction. Over time, the effects of tobacco use have been shown to do harm to virtually all of the tissues and metabolic functions in the body.

It has been several decades since the link between tobacco use and disease and death became clear. Yet smoking continues to be the cause of death for 480,000 Americans each year. It causes 80% to 90% of all lung cancer deaths and 80% of deaths from chronic obstructive pulmonary disease. The habit can double your risk of stroke, and it significantly increases the risk of heart attack. It is a grim way to end this column, but this is a difficult subject. We know that quitting is difficult. But the details of this discussion show that not quitting can lead to circumstances that are even more difficult.

(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.)