Dear Doctors: I always thought that your body ages gradually as you get older, kind of like your car does. But now I’ve been hearing that we actually go through a couple of spurts where we age really quickly. Can you please explain where this idea comes from?

Dear Reader: You’re referring to a study that made quite a splash when it was published last summer. Researchers at Stanford Medicine upended the idea that we age at a steady and measured pace throughout the course of our lives. Instead, the results of their research suggest that our bodies undergo two periods in which significant changes that take place at the molecular level add up to a sudden acceleration in aging. The first occurs at age 44 and the second at age 60. Not surprisingly, the idea that aging isn’t directly tied to chronology is still making headlines months later.

Working with scientists in Singapore, the researchers wanted to understand why the risk of certain diseases and health conditions appeared to spike at specific time points. In middle age, people often begin to get muscle injuries and see a marked increase in body fat percentage. In older age, they begin to lose muscle mass and develop an increased risk of cardiovascular and neurodegenerative disease, kidney disease and Type 2 diabetes.

To learn the reasons for these patterns, the research team recruited 108 adults ranging in age from 25 to 75 to take part in a multiyear study. The participants were asked to donate blood, stool, skin, nasal and other biological samples every few months. The researchers then analyzed age-related changes taking place in more than 135,000 different proteins, microbes in the gut, mRNA and other molecules. They found that at about age 44, and again around age 60, these molecules undergo rapid changes that correlate to the specific health risks associated with those ages.

For example, when you reach your mid-40s, a marked shift in your ability to metabolize alcohol occurs. That’s why the night out that barely fazed you in your 20s leaves you in rough shape in middle age. Other shifts include the ability to metabolize caffeine and changes to lipid metabolism and skin and muscle tone. In your 60s, caffeine metabolism takes another hit. So do the immune system and, again, muscle and skin. Interestingly, the data also suggest that some of the midlife changes that have been attributed to hormonal shifts in women during perimenopause are actually taking place at a molecular level and in both sexes.

So, how does this translate to daily life? At the most basic level, it underlines the importance of sticking to a healthful diet throughout your life. As you reach middle age, adjust your diet to anticipate fat gain and implement exercise routines to prevent muscle injuries. Caffeine drinkers should make appropriate changes. Ditto for alcohol use. Older adults can prepare for an accelerated rate of muscle loss, known as sarcopenia, by engaging in age-appropriate weight-training programs. The study also emphasizes the importance of regular health screenings, particularly at these pivotal times of life.

(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'm pretty good about eating a healthy diet. The exception is each month during my period, when all I seem to want is potato chips and ice cream. Is there something going on with my body, or is this just a good excuse for a few cheat days each month?

Dear Reader: Eating a healthful diet means putting a daily limit on the sweet treats and salty carbs that can wreak havoc on weight management, blood pressure and blood sugar. That kind of sustained discipline makes the occasional cheat day a welcome break. But when it comes to the intense food cravings that can occur during the menstrual cycle, years of research suggest that the many changes taking place in the body during that time are a significant factor. These include rising and falling levels of estrogen, progesterone, cortisol, serotonin and insulin. Levels of ghrelin and leptin, the so-called hunger hormones, also seesaw during the menstrual cycle. Ghrelin boosts feelings of hunger, whereas leptin promotes satiety.

Now, a small study from researchers in Germany, which was published last year in the journal Nature Metabolism, suggests that brain chemistry plays a significant role in period cravings. Specifically, the cycle of hormonal shifts that take place during a woman’s menstrual cycle cause the brain to become more sensitive to changes in insulin levels. The hormone insulin, which is produced by the pancreas, regulates blood sugar. It is also able to cross the blood-brain barrier. The body’s response to insulin has an effect on hunger and cravings.

The researchers conducted two experiments. In the first, they used a device known as a glucose clamp to measure changes to insulin sensitivity in the brains of 11 young women as they went through the phases of a menstrual cycle. They found that during the follicular phase, which is when the egg is being prepared to leave the ovary, the brain becomes more sensitive to insulin. Insulin sensitivity is associated with good blood sugar control and a stable appetite. But during the luteal phase, which is when the egg travels to the uterus and waits to be fertilized, insulin sensitivity in the brain measurably decreased. That suggested that this phase of menstruation, which is when food cravings typically occur, is marked by insulin resistance. This is a metabolic state that prevents the cells from absorbing glucose from the bloodstream, which in turn can cause an increase in hunger.

In the second experiment, conducted with a different group of 15 young women, the researchers again measured changes to insulin sensitivity throughout the phases of a menstrual cycle. The women underwent MRIs, which verified the original findings.

The idea that insulin resistance in the brain can have an effect on appetite is intriguing. It not only offers an explanation for the food cravings that many women struggle with during their periods, but it also points to new avenues of research in obesity and diabetes. However, as is true of all small studies, this is just a beginning. More research is needed to confirm these findings and to further explore their effects on the body.

(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 father is 74 years old. He wants to stay healthy, so he has started taking a fitness class at his local community center. One of the exercises they do is to walk backward. The instructor says it’s good for your body, and also for cognition. Do you know if this is actually true?

Dear Reader: The health benefits of walking are well-established. Make it a regular part of your life, and walking helps with strength, endurance and balance; bolsters cardiovascular health; aids in blood sugar control; helps maintain a healthy weight; contributes to improved mood and emotional health; reduces anxiety and tension; and helps you get a good night’s sleep. Increasing distance, speed and frequency increases the benefits. And as your father has been told in his fitness class, so does a change of direction.

Known as retro walking, the practice of walking backward engages the same groupings of muscles as does forward motion. That is, the hip flexors and adductors, gluteal muscles, quadriceps, hamstrings, calf and foot muscles. However, when walking backward, each of the muscles involved must work in a different way and put forth different degrees of effort. The mechanics of maintaining balance are similarly challenged.

Researchers who study the biomechanics of walking backward found this change in direction has multiple benefits. It can strengthen the muscles in the back; increase strength and flexibility in the hamstrings; strengthen the ankles; improve balance, posture and coordination; and even ease lower back pain. Due to a decreased range of motion in the knees and hips, retro walking can be kinder to those joints. There is also evidence that walking backward can have a beneficial effect on cognitive health.

Several studies have found that when someone walks backward, activity in the prefrontal cortex increases. That is the region of the brain associated with problem-solving, logic and making decisions. In one particularly intriguing study, participants who simply visualized the act of walking backward, but were actually sitting still, improved their scores on a memory test.

The precise connection between retro walking and improved cognition is not yet clear. However, researchers suspect that the increased complexity of the physical aspects of the process somehow support the data processing centers of the brain.

When it comes to adding retro walking to your regular fitness regimen, the data shows that as little as 10 to 15 minutes per day can be beneficial. Safety is obviously an important consideration. When moving backward, it can be difficult to anticipate and compensate for obstacles. Glancing backward to assess the terrain can throw off balance, which can add an extra element of risk.

Walking backward on a treadmill gives you a stable base, allows you to set a steady and controlled speed and lets you track your distance and progress. Another option is working with a partner, each taking turns going forward and backward. If working alone, a hallway that is clear of rugs or obstacles can be an option. To maximize benefits and avoid injury, it is important to plan ahead and ease into the process.

(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.)