Clinical trial📅 04/23/2026🤖 AI Research

Ketogenic vs Low-GI vs High-GI: 12-Week RCT in 57 Athletes — Body Composition Identical, but LDL and Training Volume Diverge

A randomized 12-week trial compared three nutrition strategies in endurance athletes. The big surprise: all three diets improved body composition and VO₂max equally — but the type of diet had a critical impact on weekly training volume, perceived effort and LDL-cholesterol levels.

What was studied

57 male endurance athletes (mean age 27.5 ± 4.9 years) were randomly assigned to three groups of 19: a low-glycemic diet (Low-GI), a high-glycemic diet (High-GI), and a carbohydrate-restricted ketogenic diet (LCHF, less than 50 g of carbs per day). The intervention ran for 12 weeks. The researchers measured body composition, maximal oxygen uptake (VO₂max), LDL cholesterol, rating of perceived exertion (RPE) and weekly training mileage.

The headline result

Body composition and VO₂max improved in all three groups across the 12 weeks — the time effect was significant in every group (p < 0.001). However, a significant "time × group" interaction emerged (p < 0.001) on the key training-behavior and metabolic-marker outcomes:

Participants on the high-glycemic diet (refined carbs, high sugar) saw their perceived effort stay flat and actually cut their weekly mileage. Participants on Low-GI and LCHF, by contrast, trained more and felt the same load was easier. The defining feature of the ketogenic diet was a significant rise in LDL cholesterol — something that did not happen in the other two groups.

What this means for you

If you do running, cycling, swimming or other endurance sports, the study points to one thing: any structured diet is likely to improve body composition and aerobic power. But the quality of your training hinges on your carbohydrate choices.

High-glycemic eating (white bread, sweets, processed foods) was associated with heavier perceived effort and lower training volume — even when energy intake was formally "adequate." Low-glycemic sources (legumes, whole grains, non-starchy vegetables) let athletes train more while perceiving the load as lighter. The ketogenic diet produced a similar training response but came with rising LDL cholesterol — a marker of cardiovascular risk. Before going keto, it's worth checking your lipid panel and consulting a doctor.

Practical takeaway: if your goal is to improve endurance and body composition without metabolic risk, a low-glycemic diet looks like the optimal balance in the context of this study.

Important caveats

• Male athletes only. All 57 participants were trained men. The findings may not generalize to women or to untrained people.
• Small sample. n = 57 can detect medium and large effects, but firm recommendations need studies with bigger samples.
• Preprint status. The work has not yet been peer-reviewed; specific conclusions may be revised.
• Short duration. 12 weeks is enough to assess body composition, but the long-term cardiovascular effects of a ketogenic diet require separate study.