Dear Doctor: So many bad things seem to be associated with night shifts -- first an increased breast cancer risk, now an increased risk of obesity. Why is that? Is there any way to lessen the risks?

Dear Reader: You're right -- when it comes to the night shift, there's an ever-growing array of pretty bad outcomes. Research has linked this work schedule to a higher risk of coronary artery disease, a higher incidence of diabetes, and increased risk for several types of cancer, depression, social isolation and all manner of sleep disorders. Now, not that surprisingly, obesity has been added to the list. And although the ill effects of night shift work are diverse and wide-ranging, they all have something in common. That is, a significant and ongoing disruption of the body's innate time-keeping system -- circadian rhythms.

We humans have evolved to be awake and active during daylight, and to spend the dark hours in rest and sleep. During the 24(ish) hours that make up a circadian cycle, the shift from daylight to nighttime and back effects changes in our hormones, body temperature, brain function and other metabolic processes. Also involved are biological clocks -- sets of proteins that interact in cells throughout our bodies. These biological clocks are responsible for circadian rhythms -- and regulate their timing.

All of which is why, when a work schedule forces us to stay awake in the dark and sleep during daylight, it's not only our sleep cycles that get drastically altered. Eating habits, digestion, body temperature, the ebb and flow of hormones, and a host of other vital bodily functions are affected as well. Research has shown that the disruption to patterns of sleep and activity not only suppresses the production of melatonin, it may also affect the regulatory mechanisms of genes that play a role in the development of tumors.

It's possible for a worker to become accustomed to the night shift, but his or her body pays a steep price. And it's not just the traditional night shift that is bad for your health. Ongoing research shows that even a moderate deviation from the traditional 24-hour sleep/wake cycle, as in a swing shift or jobs that require rotating day and night hours, takes a physical toll. In the United States, this affects up to 25 percent of the populace, with the biggest challenge faced by workers whose schedules change week to week. Whether they compensate for those time shifts by powering through a sleepless stretch or by trying to switch to a morning bedtime, the outcome is akin to living in a constant state of jet lag.

For those who can manage it, getting into an unbroken 24-hour cycle for sleep and mealtimes can help, sleep experts say. Some workplaces understand the challenges of the night shift and will schedule workers for months at a time. The key is to control exposure to daylight, which regulates circadian rhythms. Be sure to spend time in bright light during a night shift. Wear sunglasses on the trip home from work. Use blackout curtains, sleep masks, ear plugs and sound machines in the bedroom. And on weekends, spend time with friends and family, but try to keep deviations from the schedule to a minimum.

(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: Much is said about calcium and vitamin D, but what about magnesium? As I understand it, that's a crucial part of the bone-building equation. Should people be taking magnesium supplements?

Dear Reader: The element magnesium is necessary for bone health, yes, but also for normal heart function and nerve conduction and to maintain a proper calcium balance in the body. Notably, although it's found in leafy vegetables, nuts, fish and shellfish, soy and cereals, magnesium levels in our foods appear to be slowly decreasing. One reason may be the overutilization of fields, leading to depletion of magnesium in the soil. A survey conducted in 1999 and 2000 found that 79 percent of American adults do not meet the recommended daily allowance for magnesium.

Magnesium, which is stored in the bone, is very similar in atomic structure to calcium, and deficiencies can lower the body's calcium levels. This happens in several ways. First, low levels of magnesium impair the release of parathyroid hormone (PTH), which raises calcium levels when they fall too low. Second, low magnesium levels actually block the function of PTH. And third, low levels decrease the formation of the active form of vitamin D, also needed for healthy bones. Further, low magnesium causes an inflammatory process in the bone, which leads to bone breakdown.

Animal studies show that rats fed magnesium-deficient diets, even in the short term, have more brittle and fragile bones than animals fed a normal diet. This happens because of decreased bone formation and increased bone breakdown. In humans, deficiencies can develop among people who don't absorb the element due to gastrointestinal disease, chemotherapy or alcoholism.

Accordingly, data from the well-known Framingham Heart Study found that people with higher dietary intake of magnesium had greater bone density. As for whether a magnesium supplement will help decrease the risk of osteoporosis, however, the data are definitely not conclusive.

Although a 2010 study found that postmenopausal women who took 1,830 milligrams of extra magnesium daily for 30 days had fewer markers for bone breakdown than those who didn't take extra magnesium, most studies assess the impact of only dietary magnesium.

A notable 2016 study combined data from 12 different studies (involving 109,564 people), assessing the risk of bone density and fracture in relation to magnesium intake. The authors found a slight increase in hip bone density among those with higher magnesium intake, but no increase in spinal bone density.

If you have a good dietary intake of magnesium and if you don't drink heavily or have intestinal or kidney disease, you're unlikely to benefit from taking a magnesium supplement. The best thing you can do for your bones is to eat a diet high in magnesium and calcium; get outside so your body can produce vitamin D; and exercise.

(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: Before I had a double hip replacement at 47, I was an avid daily runner, participating in several marathons per year and any charity event possible. Everything I've read says not to run with hip replacements. Do I really have to cut running out of my life?

Dear Reader: The advice regarding running and hip replacements relies on basic physics. The more stress you put on the new prosthetic joint, the more wear-and-tear can occur, and the sooner you may be a candidate for revision surgery to replace the device. And although the current wisdom is that a modest, even moderate, amount of running can be OK in some cases, the life you have described, with as many races as possible (and, we imagine, plenty of training in between) is not a good fit for someone with artificial hip joints.

In a total hip replacement, the surgeon removes the damaged or diseased bone and cartilage and replaces them with a prosthetic joint. People consider the surgery when medication, lifestyle changes or walking supports fail to have an effect on pain or impaired mobility. From its introduction in 1960, hip replacement has undergone a sustained evolution in surgical techniques, as well as the materials and technologies that go into making the artificial ball-and-socket joint.

However, the data regarding the effects of regular running after a total hip replacement haven't kept pace.

A few studies have looked into what happens when individuals with a total hip replacement continue their running regimens. The results suggested that running could be OK, but there is disagreement among scientists as to whether the sample sizes in those studies were adequate, and whether the subjects were followed for a long enough period of time.

A larger study published several years ago, in which hip replacement patients took part in a diverse range of physical activities, including running, uncovered some potential problems. Though not widespread among the participants, these included fractures, dislocation, loosening of the prosthesis and mechanical wear that resulted in the production of metal ions, which passed into the bloodstream. The long-term effects of this type of exposure are not yet understood.

But here's the good news. Although running isn't recommended for people with total hip replacements who would be new to the sport, it's a different story for experienced runners. Those individuals who had been regular runners before the surgery and who have completed their rehab period can take part in a moderate amount of running. Needless to say, there are some caveats.

-- Before you run a single step, get the OK from your surgeon and your physical therapist. This is non-negotiable. You've put a lot of effort, time and money into your new hips.

-- Take your time and get back into shape with low-impact exercises like swimming and stationary cycling.

-- Be aware of your stride. Twisting of any kind can cause the ball of the prosthesis to separate from the hip bone. Choose flat, straight surfaces that won't tax the joints.

-- Stick to the distance and frequency that your medical team allows.

(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.)