Tue. May 24th, 2022

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image: Exercising at the beginning of a fast can improve health outcomes, according to a new BYU study.
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Credit: BYU Photo

With Thanksgiving just a day away – and the most anticipated meal of the year already underway – the last thing most people are thinking about right now is fasting. (Of course, maybe Thursday night, a few of us will feel like fasting for a day.)

But even with a short Thanksgiving break, many people turn to fasting for weight loss and improved metabolic health, whether it is a regime of time-limited eating, periodic fasting or “monk fasting”. Now, a new Brigham Young University study published in the journal Medicine & Science in Sports & Exercise finds that intense exercise at the start of a fast can help maximize the health benefits of temporarily giving up food.

“We really wanted to see if we could change the metabolism during fasting through exercise, especially how fast the body goes into ketosis and makes ketones,” said BYU Ph.D. student Landon Deru, who helped design the study for his thesis.

Ketosis occurs when the body runs out of glucose – its first preferred fuel – and begins to break down stored fat into energy and produce chemicals called ketones as a by-product. In addition to being a healthy source of energy for the brain and heart, ketones fight diseases such as diabetes, cancer, Parkinson’s and Alzheimer’s.

For the study, the researchers asked 20 healthy adults to complete two 36-hour fasts while remaining hydrated. Each fast began after a standardized meal, the first fast started without training and the second with a challenging treadmill workout. Every two hours while awake, subjects performed hunger and mood assessments and recorded their levels of B-hydroxybutyrate (BHB), a ketone-like chemical.

Exercise made a big difference: When participants exercised, they averaged ketosis three and a half hours earlier in fasting and produced 43% more BHB. The theory is that the initial exercise burns through a significant amount of the body’s glucose, resulting in a faster transition to ketosis. Without exercise, participants hit ketosis about 20 to 24 hours inside fasting.

“For me, the hardest time for fasting is the period between 20 and 24 hours, so if I can do something to stop fasting within 24 hours and get the same health results, it’s beneficial,” said study co-author Bruce Bailey, a BYU exercise professor of science. “Or if I fast for my usual 24 hours but start training, I will get even more benefits.”

However, there are a few caveats to the proposed strategy.

“If you eat carbs or eat a big meal before fasting, you may not be able to reach ketosis for several days even if you exercise, so you should eat moderately before fasting,” Bailey said. “We also do not know the ideal frequency for fasting. There are definitely certain people who should not fast, such as those with type 1 diabetes, and it is obviously harmful to fast 24/7. But for most people, fasting once or twice a week for 24 hours or more is perfectly safe and healthy. “

The study, which required participants to run on a treadmill for an average of 45 to 50 minutes, also did not determine an ideal amount or type of training for each person. Overall, however, researchers believe that the more energy a person can burn, the better.

“You can get a pretty good estimate of how many calories you burn in most exercises, and the more carbohydrates you burn (without overdoing it or harming yourself), the better you set the stage for starting ketosis early in your fast. , “said Deru.

It is also important to note that exercise, according to participants’ reports, did not appear to aggravate hunger or affect mood during fasting.

“Everyone will be a little more grumpy when they fast, but we found that you will not get worse with exercise – with exercise you can get these extra benefits and be exactly the same amount of gnawing as you would be if you did not train, ”said Deru.


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