Dear Doctor: A lot of my friends are talking about pickles these days because they're a way to get probiotics into your diet. But I read that pickled foods are associated with higher rates of cancer. Are pickles good or bad for you? I'm confused.

Dear Reader: While it's true that researchers have identified the consumption of pickled vegetables as a potential risk factor in certain gastric cancers, it doesn't necessarily mean you have to ditch those delicious dill spears that came with your sandwich. In the big picture, the data suggest a link between pickles and some cancers, but when you drill down into the details, it turns out that additional factors appear to be at play.

Let's start with pickles themselves. Virtually every culture in the world has a tradition of pickling. Methods vary, but most use some combination of salt, liquid and sometimes oil to induce and support fermentation, which preserves food for use beyond its growing season.

In the United States, cucumbers and cabbage account for the majority of pickled vegetables. At just 4 pounds per person per year, they're treated as a condiment. However, in many other parts of the world, including China, Japan, Korea and Turkey, a wide range of fermented vegetables (as well as fruit, soy products and sometimes meat) are a dietary mainstay, eaten daily for nine to 12 months of the year.

The idea of a pickle-cancer link arose when scientists began to look at cancer registries established in China in the 1960s. As researchers crunched the numbers, they found a high rate of gastric cancer in regions where pickled foods were eaten daily throughout the year. Since then, though, other groups of researchers have analyzed a wide range of these studies. They found that differing methodologies, as well additional lifestyle factors, made the direct link between pickles and cancer more tenuous.

In some cases, the populations being studied also ate high-fat diets and a lot of barbecued meat, both of which have a link to increased cancer risk. In other study groups, smoking was widespread and thus also implicated in potential disease. And, finally, there were the pickles themselves.

For example, a traditional way of pickling vegetables in parts of China involves packing them in salted water, sometimes seawater, for a period of weeks or months. During this time, fermentation occurs. However, the process also produces yeasts and fungi, some of which have the potential to produce compounds that are carcinogenic. A combination of the pickling method and the volume of vegetables consumed could be a factor in the higher gastric cancer rates.

In the U.S., by contrast, most commercially produced pickles are made without fermentation, in a process that uses vinegar and pasteurization. Although this eliminates the risk posed by those carcinogenic compounds, it also means these pickles won't be a source of probiotics. In addition, pickles are high in sodium, with a single dill pickle packing a hefty 50 percent of a day's supply of sodium.

Bottom line, with what we know now: Whether it's fermented or commercial pickles that you're eating, moderation is the key.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: My husband and I have been reading about a possible connection between aluminum and Alzheimer's disease. Some reports even discourage the use of aluminum foil for cooking. Is there a connection between aluminum and Alzheimer's?

Dear Reader: Your worries center around the most-abundant metal and the third-most abundant element, one that by mass makes up 8 percent of the Earth's crust. Plants and animals have evolved in an environment in which aluminum is common. In fact, aluminum is even present in the human brain. This most fundamental of organs contains, on average, 1.02 micrograms of aluminum per gram of brain tissue. Further, brain levels of aluminum increase with age, causing researchers to take a look at how that happens -- and what might be the result.

The human body absorbs aluminum from compounds in water, food, aluminum-based utensils, deodorants and drugs. But at toxic levels, aluminum can damage neurons in the brain as well as DNA within neurons. The aluminum ion can affect normal cellular reactions in the brain, cause brain inflammation and lead to neurofibrillary tangles and amyloid deposits that are the hallmark of Alzheimer's disease. In fact, aluminum chloride in combination with the sugar D-galactose injected in rats and mice induces many of the brain changes and behavioral changes seen in Alzheimer's disease.

In one important study, researchers studied the postmortem brains of 12 people with familial Alzheimer's disease in England. They found elevated aluminum content in all but one of the brain samples. The researchers also found aluminum concentrated near neurofibrillary tangles and amyloid deposits, which have been linked to Alzheimer's. They couldn't conclude that aluminum was the cause of the dementia; it may be that degenerative brain diseases concentrate and retain aluminum within the brain. Still, a connection seems clear.

The findings are alarming. Aluminum is more ubiquitous today than ever before. Aluminum hydroxide, aluminum sulfate and aluminum chloride are used in drinking water treatment and waste water management, and we use aluminum foil and aluminum in our food preparation and storage. Considering such exposure, it's important to ask questions about the potential impact.

A 15-year study in France assessed the aluminum levels in drinking water and the rate of Alzheimer's disease in multiple areas of southern France. The concentration of aluminum in the water varied from 0.001 to 0.514 mg/liter depending on location. Of the 1,677 people over the age of 65 who completed the study, 461 were diagnosed with dementia. Those whose aluminum intake was more than 0.1 mg per day had 2.26 times the risk of dementia compared with those whose intake was less than 0.1 mg per day.

All of that said, neither I nor medical science overall can provide a hard and fast answer to your question about the risk of aluminum exposure. It is heartening, however, to know that water boards measure the amount of aluminum in drinking water at a time when the correlation between human exposure to high levels of aluminum and dementia is still largely unknown.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: What's the difference between probiotics and prebiotics? I'm really interested in how gut bacteria impact health, but I'm not sure I always understand the terminology.

Dear Reader: We agree with you that the information emerging about gut bacteria is fascinating. And as with any area of research, the language used to discuss it can be confusing.

Probiotics are live microorganisms that are meant to have health benefits for the person who ingests them. They work by joining the many trillions of bacteria already living within our bodies, primarily in the gut. Collectively known as the human microbiome, these beneficial strains of bacteria harbor a diversity of genes that dwarfs that of the human genome.

Thanks to the rapid evolution of genetic sequence technology, researchers are learning new information about the human microbiome every day. Thus far, probiotics show promise in the areas of immune function and various digestive and bowel disorders. Probiotics have also been used to help preterm infants acquire a beneficial range of intestinal flora, which can prevent colonization by adverse bacteria.

You can ingest probiotics in food or in supplement form. When it comes to food-based probiotics, the common denominator is fermentation. Foods like yogurt, kefir, apple cider vinegar, pickles, kombucha, sauerkraut, kimchee, miso and certain soft cheeses are all sources of different types and varying amounts of probiotics.

The idea is that by ingesting these microorganisms, the portion of them that survive the acids of the stomach will take up residence in the gut. Once there, though, they need their own food source to grow and thrive. And that's where prebiotics come in.

Prebiotics are indigestible carbohydrates that act as food for probiotics. While some foods offer a greater percentage of prebiotic material than others, when you eat a diet high in a variety of vegetables, fruit and leafy greens, you are inevitably giving your gut bacteria plenty to eat.

You'll find a motherlode of prebiotics in foods like leeks, asparagus, Jerusalem artichokes, bananas, onions, garlic, artichokes, jicama and honey. Also beneficial is something known as "resistant starch," which is the indigestible portion of starch. It can't be broken down by the small intestine, so it moves to the colon, where it is fermented by the microbiota.

Whole cereal grains, many seeds and green bananas are good sources of resistant starch. So are cooked potatoes and rice, but only when they have cooled. Just as with probiotics, you can go the supplement route for prebiotics if you choose.

If you decide you'd like to add probiotic supplements to your diet, we suggest you check with your family doctor for guidance. Although regulated by the FDA, probiotic supplements are treated as a food and not a medication. That means that manufacturers don't have the burden of proof that their products will live up to the claims printed on the label. Your doctor can help you figure out which supplements to choose, and the best dose for you to start with.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)