The Impact of HIIT on Women’s Endurance and Body Fat
In this edition of “Highway to Health – Studies in Brief,” we explore new research findings: A meta-analysis shows how high-intensity interval training (HIIT) affects endurance and body fat in trained women. And a clinical study from England demonstrates that the degree of food processing is crucial for how effectively the body breaks down fat–with some surprising results.
HIIT Boosts Endurance and Reduces Body Fat in Physically Active Women
What was studied? A systematic review with meta-analysis evaluated 15 studies examining how high-intensity interval training (HIIT) affects the fitness and body composition of physically active women. The 304 participants were on average 25 years old and trained regularly.1 The training programs lasted from 2 to 16 weeks, usually three times a week. The goal was to determine whether HIIT is effective for this group—and whether age or training duration influences the results.
Results: The VO2max, a key measure of endurance performance, improved slightly but noticeably. The so-called VO2peak—a similar value describing maximum performance—showed a significant increase. Body fat percentage also decreased significantly, considered a small but relevant effect. However, body weight and total fat mass changed little. The analysis also suggested that HIIT can provide secondary benefits such as increased muscle strength and endurance. Neither age nor training duration had a noticeable impact on the results.
Significance: HIIT is an effective method for already physically active women to improve endurance and reduce body fat percentage—with minimal time investment. This form of training can be efficiently integrated into existing programs and offers a time-saving alternative to traditional endurance training.
Also interesting: Only Takes 12 Minutes! What the Cooper Test Says About Your Endurance Performance
Unprocessed Foods Support Fat Loss More Than Industrially Processed Foods
What was studied? A clinical study in England examined whether the degree of food processing makes a difference—even if they meet the same nutritional guidelines.2 Fifty-five overweight adults (average age 43) were given two different diets, each for eight weeks: one based on largely unprocessed foods such as fresh vegetables, whole grains, or meat; the other on industrially processed products like breakfast cereals, ready meals, or protein-enriched snacks. Both diets adhered to the United Kingdom’s official recommendations for healthy eating. Participants could eat as much as they wanted.
Results: While participants lost weight on both diets, the unprocessed diet led to nearly twice as much weight loss. Body fat, especially visceral fat, also decreased more significantly. Blood lipid levels improved: the unprocessed diet lowered triglycerides more, while the less favorable LDL cholesterol decreased slightly more with the processed diet. Appetite, cravings, and the desire for sweets or savory foods decreased more with the unprocessed diet. Gastrointestinal issues were common with both diets but more frequent with processed foods—such as constipation or heartburn.
Significance: The results show that the degree of food processing independently affects weight, fat loss, and eating behavior—regardless of nutrient content. Even industrially produced products with seemingly healthy compositions performed worse. The study authors recommend paying attention not only to fat, sugar, and salt but also to the degree of processing—through adjusted dietary recommendations.
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Dicken SJ, Jassil FC, Brown A, Kalis M, Stanley C, Ranson C, Ruwona T, Qamar S, Buck C, Mallik R, Hamid N, Bird JM, Brown A, Norton B, Gandini Wheeler-Kingshott CAM, Hamer M, van Tulleken C, Hall KD, Fisher A, Makaronidis J, Batterham RL. Ultraprocessed or minimally processed diets following healthy dietary guidelines on weight and cardiometabolic health: a randomized, crossover trial. Nat Med. 2025 Aug 4. doi: 10.1038/s41591-025-03842-0. Epub ahead of print. PMID: 40760353. ↩︎
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