It’s True: A Low-Calorie Diet Could Tank Your Gut Health

·3 min read
Photo credit: taranchic - Getty Images
Photo credit: taranchic - Getty Images


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  • Low-calorie diets could tank your gut health, new research suggests, leading to GI issues such as diarrhea and colitis.

  • Consuming enough calories throughout the day is important not only for the health of your gut bacteria, but to fuel your active lifestyle as a cyclist as well.

Cyclists know that what you eat has an obvious impact on your gut health, but how much you consume may be a factor, too, according to a new study in the journal Nature. As it turns out, more is indeed better.

Researchers studied 80 older women whose weight ranged from slightly overweight to severely obese, and split them into two groups for 16 weeks. Half the participants followed a medically supervised meal replacement plan with shakes that totaled about 800 calories per day. The other half was a control group that maintained their usual habits and weight for those months. Gut bacteria analysis was done for all participants before and after the study period.

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After four months, the number and type of microorganisms in the control group were the same as they’d been at the start. But there were significant and problematic changes for the low-calorie diet group.

For participants in the low-calorie group, the bacteria adapted to absorb more sugar molecules as a way to survive, creating an imbalance that promoted the increase of harmful bacterial strains. There was a particularly large increase in a type called Clostridioides difficile, also known as C. diff., which can cause diarrhea and colitis (inflammation of the inner lining of the colon). The Centers for Disease Control noted that this bacteria can become chronic even when treated regularly.

This effect from low-cal eating is not surprising, according to Kristin Gillespie, R.D., dietitian and nutritional counselor. She told Bicycling that the quantity of food we consume is part of what keeps beneficial gut bacteria nourished.

“The type and amount of foods we consume also feed our gut bacteria, which is why our diet plays such a large role in the composition and health of our gut microbiome,” she said.

Although weight regain wasn’t a part of the study, previous research highlights that can be a big problem as well. When you lose weight quickly—which can happen when you slash calories—and then put that weight back on after returning to a more sustainable calorie amount, those pounds aren’t composed in the same way. What you lose is a degree of fat and muscle mass, but what you put back on tends to be mainly fat, said Gillespie. With multiple spins on this yo-yo, it can be problematic for body composition because you’ll keep losing muscle mass along the way, she added.

Obviously, 800 calories a day is far from enough, especially if you’re an active cyclist. But how much do you really need? Even 1,500 calories daily may be shortchanging yourself.

While your exact number will depend on numerous factors, including your gender, age, weight, and activity levels, when you’re aiming to maintain your weight, active men should consume up to 3,000 calories a day, while active women should consume up to 2,400 calories a day, according to the 2015 to 2020 edition of the Dietary Guidelines for Americans. However, it’s worth investing some time to find out how many daily calories work best for you, which may include consulting with a dietitian.

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