Time of day may make small difference in workout results

  • A recent study found women who exercise before 8:30 a.m. burned more body fat than those who worked out in the evening.

    A recent study found women who exercise before 8:30 a.m. burned more body fat than those who worked out in the evening. Stock Photo

By Dr. Eve Glazier and Dr. Elizabeth Ko
Posted11/27/2022 7:00 AM

Q: I'm a 33-year-old woman in good health. The problem is that I got into some bad habits during the pandemic and gained weight. I'm back on track with diet, and now I am adding exercise. I remember reading that the time of day women exercise makes a difference. Is that true?

A: We think you're referring to the results of a study that made headlines last spring. The findings are intriguing in that they suggest the optimal time of day to exercise may be different for women than it is for men.


The 12-week study, which was published in the journal Frontiers in Physiology, included results from 27 women and 20 men between the ages of 25 and 55. All were in good health, had comparable body mass indexes and were already physically active when the study began. The participants were randomly divided into two groups. Each group followed the same exercise regimen, which included stretching, endurance exercises, resistance training and interval sprints. They also all ate the same diet, and at the same times of day.

The only difference between the two groups was the time of day at which they were asked to exercise. The morning group worked out for an hour and ended their sessions by 8:30 a.m. The evening group also exercised for one hour, but between 6 and 8 p.m. Over the course of the study, the researchers took regular blood pressure readings and measured changes in body fat percentage. An assessment of each person's strength, aerobic capacity and flexibility was made at the start and the conclusion of the study.

At the end of 12 weeks, each of the participants saw an increase in athletic performance, as well as improvements to measures of general health, including body fat percentage and blood pressure. But the results included a surprise. It turned out the women in the group who completed their one-hour exercise sessions before 8:30 a.m. burned more body fat, particularly around the abdominal region, than did those whose workouts took place in the evening. The opposite proved to be true for the men. Those whose exercise sessions took place between 6 and 8 p.m. burned more body fat than their early-morning counterparts.

Meanwhile, the women with the late-day exercise schedule saw greater gains in upper-body strength. They also reported improvements to mood, as well as to hunger and satiety, that were not seen in the morning group. For the men, improvements to mood occurred no matter when they exercised. These results bolster the idea that time of day has an impact on exercise for women and men. However, this is a small study and can't be considered conclusive. With such a fascinating outcome, we can count on seeing future research into the topic. What we do know for sure, thanks to numerous studies over the years, is that regular exercise is beneficial to both physical and mental health. That means the best time to exercise is whenever it fits into your day.

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Q: My boyfriend has been lifting weights at his gym. He's working on getting stronger, and one of the trainers there suggested he start taking creatine as a supplement. I've never heard of that before. What is creatine, and what is it made out of? How does it affect your body?

A: Creatine is a naturally occurring compound that our bodies use to help power the skeletal muscles. About half of our daily requirement is produced by the liver and kidneys. The other half is derived from dietary sources, primarily red meat, seafood and chicken, and to a lesser extent dairy products.

Creatine is stored in the skeletal muscles, then used to help power high bursts of physical activity. It appears to be particularly effective in boosting anaerobic performance, including weightlifting and resistance training. It can also help the muscles and muscle groups that are used during a specific type of exercise to grow larger and stronger.

Numerous studies have shown that adding a creatine supplement to the daily diet can enhance an individual's natural store of the compound. This can lead to modest, but still measurable, improvements to both athletic performance and endurance when engaged in high-intensity exercise. Research into creatine supplements also suggests they may play a role in helping to prevent use-related muscle injuries, support or even speed post-exercise recovery, and help the body to regulate temperature during exercise. All of this has led to creatine quickly becoming one of the most popular dietary supplements among athletes and others hoping to improve physical performance.


Another interesting aspect of research into creatine is the discovery that the supplement doesn't produce the same effects in all populations. Instead, studies have found that people who are younger and in good health derive the greatest benefits. Also, these benefits, including muscle growth, were seen only when the individual followed a targeted and sustained training program.

Outside of the gym, creatine may have some useful clinical applications. Studies are looking into whether supplementation with creatine can help to slow disease progression in people living with Parkinson's or Huntington's diseases, aid in recovery from spinal cord injury, ease the effects of fibromyalgia and perhaps play a role in blood-glucose management, including in people living with diabetes.

If your boyfriend eats a well-rounded diet that includes a wide range of animal-based sources of lean, high-quality protein, he is quite likely getting enough creatine. However, because increases in intramuscular creatine concentrations can benefit athletic training, many athletes find it to be an attractive option. When used appropriately, creatine supplementation has proved to be safe and well-tolerated by individuals who are healthy and in good physical condition. Some people report experiencing water retention. Others, particularly those using it in large amounts, have occasional gastric distress. As with all supplements, it's important to use a high-quality product, and to follow the guidelines for use. We always think it's a good idea to check with your health care provider or a licensed dietitian before adding any type of supplement to your daily diet.

• Dr. Eve Glazier is an internist and associate professor of medicine at UCLA Health. Dr. Elizabeth Ko is an internist and assistant professor of medicine at UCLA Health. Send your questions to askthedoctors@mednet.ucla.edu.

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