Dear Doctors: What is the latest understanding, from a medical standpoint, about the safety of tattoos? I read about someone with a tattoo who received burns on his chest during an MRI. I also heard about tattoo ink interfering with X-ray results. Are these possible outcomes or simply urban legends?

Dear Reader: Tattoos, in which pigments are inserted into the top layer of skin to create a permanent mark or design, date back thousands of years. Historians cite tattooed mummified remains, which radiocarbon dating revealed to be at least 5,000 years old, as the earliest known examples of the practice. Over the millennia, tattoos have variously been used as charms, talismans, status symbols, decorations, declarations of beliefs and also as punishment. Today, with their sometimes-fraught history left behind, tattoos have become a widely accepted form of self-expression. Permanent makeup, which uses the tattoo process to reproduce the effects of cosmetics on lips, eyelids and eyebrows, is also growing in popularity.

Data collected in recent surveys suggest that from one-third to up to half of all adults in the United States have at least one tattoo. This has led to a growing awareness of the potential adverse effects of the practice.

Because the act of tattooing pierces the skin, bacterial or viral infection is a risk. This can occur if the inks are contaminated, if the tattoo artist’s tools or hands are not clean, or if the person fails to take proper care of their fresh tattoo while it is healing. Some people find they are allergic to pigments or other ingredients in the ink and develop an itchy rash with bumpy, scaly or flaky skin. And because the ink is permanent, the body’s response to it can persist, as well.

The tattoo process itself, during which the tattoo needle punctures the skin thousands of times per minute, can lead to the development of scar tissue, raised bumps known as granulomas or a thickening of the skin known as keloids. Medical data shows that about 5% of people who get a tattoo develop an infection, and up to 10% experience short-term complications.

You’re correct that tattoos have been known to interfere with certain types of medical scans, such as an MRI. When pigments in tattoo inks include metal ores, it is possible for them to interact with the magnetic field of the scanner. Surgeons doing biopsies have noted that nearby lymph nodes are sometimes stained with tattoo ink that has been absorbed and then carried off by immune cells. It’s possible that, if ferrous particles are present, the migrated ink can affect the appearance of a lymph node in an X-ray. However, these types of complications are quite rare. A hospital in Germany, where scans were performed on thousands of patients with tattoos, found that it occurs in less than 1% of cases.

But you bring up a good point: Tattoo inks are not regulated in the U.S., and manufacturers are not required to disclose their contents. In addition to researching the skill and safety of a specific tattoo artist, learning as much as possible about the inks that are used is a good idea.

(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: I’ve been doing keto for a few months to lose weight. I’ve almost reached my goal, and I’m ready to go back to eating more carbs. Is there a good way to switch from keto back to a regular diet? I’m worried that when I eat carbs again, I’ll gain back the weight I lost.

Dear Reader: When someone follows a keto diet, it means that they are severely limiting their intake of carbohydrates. These are the building blocks from which the body manufactures glucose, which is its preferred source of fuel. With carbohydrates mostly removed from the diet, the body is forced to switch to Plan B, which is burning fat for energy. This is a metabolic process known as ketosis, which gives the keto diet its name.

In a ketogenic diet, carbohydrate intake drops to about 10% of total food intake. The remaining 90% of the diet is made up primarily of either fat or protein. The proportions depend on which type of keto plan you are following. There’s a high-fat and moderate-protein approach, which currently is the most popular form of keto. Another way to put the body into ketosis is with a high-protein and low-fat diet, which many people know as the Atkins, paleo or Zone diets. Whichever version of keto you choose, it’s the limits on carbohydrates that lead to the fat-burning state of ketosis.

Keto can be a good way to jump-start weight loss, but it has its drawbacks. With such strict limits on carbohydrates, eating a nutritionally balanced diet takes careful planning and sustained effort. And whenever you follow a restrictive diet for a period of time, returning to “normal” eating can be daunting. For the best results, it’s a good idea to make the switch gradually. Plan on taking several weeks to slowly add more carbohydrates -- and also more calories -- back into your daily diet.

It’s also important to pay attention to the two different categories of carbs -- simple and complex. Complex carbohydrates have more nutrients than simple carbs. They are also higher in fiber. Because they are made up of long chains of sugar molecules, they take longer to digest. Complex carbs are found in whole, unprocessed foods, including vegetables, whole grains, legumes, nuts, seeds, leafy greens and fruits.

Simple carbs, by contrast, are short chains of sugar molecules that absorb quickly. They include table sugar and sugar syrups. Simple carbs deliver calories but lack nutrients. They can cause a spike in blood sugar and leave you feeling hungry. Fruits, vegetables and other whole foods do contain simple carbs. However, the effect is mitigated by their higher fiber content.

As you move away from keto restrictions, steer clear of simple carbs. This includes sugar, soda, candy and other sweets, and highly processed foods. Instead, concentrate on a diet that is rich in lean proteins, healthy fats and complex carbs. A good model to follow is the Mediterranean diet. Call on the same discipline that allowed you to stick to keto, and choose high-quality carbs, lean proteins and healthy fat.

(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: I heard a story on the news about a connection between your gut microbiome and whether you like to exercise. Is that really true? If it is, I’d like to know how the scientists figured that out. Is there a way to change your gut to make you like working out?

Dear Reader: You’re referring to the results of intriguing new research that, not too surprisingly, has found its way into the headlines.

According to the study, which was published late last year in the journal Nature, certain bacteria living in the guts of mice appear to amp up their desire to be physically active. But it’s not the microbes themselves that make the mice want to exercise. Rather, they turned out to be a crucial part of a complex metabolic pathway associated with the release of dopamine, a neurotransmitter produced in the brain. Among dopamine’s numerous functions is a key role in pleasurable reward and motivation.

The study itself is fascinating. The researchers began with a question: What motivates the desire to be physically active? In search of an answer, they looked at the exercise habits of 200 mice. When placed in cages outfitted with treadmills and running wheels, the mice voluntarily exercised at different rates and for varying durations. While some ran long and often, others exercised only sporadically. Tests found no measurable differences in either the genetic makeup or the metabolisms of the high- and low-exercising groups of mice.

But when the scientists began to study the makeup of the rodents’ gut microbiomes, significant differences emerged. They administered a range of antibiotics to the mice, each of which selectively targeted certain gut bacteria. When the researchers measured activity levels after the gut bacteria had been eliminated, they found they had been cut in half. And when bacteria were systematically restored to the guts of the mice, only two types, Eubacteria and Coprococcus, were linked to a voluntary increase in exercise. But why?

Further study uncovered a link between those two bacteria and a metabolic process that leads to increased levels of dopamine in the brain. As we mentioned before, this neurotransmitter is associated with pleasure and reward. The takeaway was that the presence of those two types of bacteria allowed the mice to get a good jolt of dopamine, which led to the enjoyable physical sensation of a runner’s high.

It’s important to remember that this was a rodent study, and the results don’t translate directly to humans. But because our tiny cousins have biological and genetic characteristics that closely resemble our own, the results are promising.

The researchers have announced plans to look for similar metabolic pathways in humans, in which the presence of certain bacteria in the gut make exercise pleasurable. In the meantime, there are steps we all can take to enhance and preserve gut diversity. Eat a diet rich in a variety of fresh vegetables, fruit and leafy greens. Steer clear of added sugars and highly processed foods. And, even if you don’t naturally crave it, exercise.

(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.)