Protein Supplementation Does Not Maximize Adaptations to Low-Volume High-Intensity Interval Training in Sedentary, Healthy Adults: A Placebo-Controlled Double-Blind Randomized Study

Reljic D, Zieseniss N, Herrmann HJ, Neurath MF, Zopf Y

Question

Does a single 40 g whey protein supplement after each low-volume HIIT session enhance cardiorespiratory and cardiometabolic adaptations compared with an isocaloric placebo in sedentary healthy adults?

Summary

In sedentary but otherwise healthy adults, an 8-week supervised low-volume cycling HIIT program improved VO2max, workload, and blood pressure whether participants took 40 g whey protein or a maltodextrin placebo after each session. Protein did not add measurable benefit when daily protein intake was already adequate. Training adherence was high, exercise enjoyment was favorable, and no LOW-HIIT-related adverse events occurred.

Methodology

Previously sedentary healthy adults recruited by newspaper and internet advertisements.
47 participants.
Electronically braked cycle ergometer.
Work intervals: 1 minute.
Recovery: 1 minute low-intensity cycling.
Intensity: 80-90% HRmax in weeks 1-2, then 85-95% HRmax; participants adjusted cadence/resistance to reach target HR.
14 minutes in weeks 1-2 and 24 minutes in weeks 3-8, including 2 minute warm-up and 3 minute cool-down.
3 sessions per week.
8 weeks; 24 prescribed sessions.
Placebo-controlled double-blind randomized trial
VO2max, Blood pressure, Blood markers, and Enjoyment and supplement tolerability were tracked.

Outcomes

VO2max and workload

Both groups significantly improved relative and absolute VO2max and maximal workload, with no between-group difference.

VO2max increased PRO-HIIT +2.8 mL/kg/min, p=0.003; PLA-HIIT +3.5 mL/kg/min, p<0.001; time effect p<0.001.

Improved

Blood pressure

Both groups reduced systolic and diastolic blood pressure, with no added protein effect.

PRO-HIIT -7/-3 mmHg, p<0.05; PLA-HIIT -8/-5 mmHg, p<0.001; time effects p<0.001.

Improved

Blood markers

LOW-HIIT affected selected markers, but changes differed by group and did not show a consistent protein advantage.

Time effects for fasting glucose p<0.001, LDL p=0.006, ALT p=0.026, GGT p=0.005; GGT decreased in PRO-HIIT, LDL and ALT decreased in PLA-HIIT.

Mixed

Protein supplementation

A single 40 g whey protein dose after each session did not enhance adaptations compared with placebo.

No significant between-group differences in any outcome changes.

No clear change

Insights

A 14-24 minute cycling LOW-HIIT session can be enough to improve fitness and blood pressure in sedentary healthy adults when supervised and heart-rate guided.
Post-session whey protein should not be marketed as necessary to maximize LOW-HIIT adaptations for people already consuming adequate protein.
Enjoyment and intention to continue were favorable, supporting low-volume cycling HIIT as a feasible consumer format with appropriate screening.

Limitations

Normal diet and physical activity were not tightly controlled outside short recording windows.
Food and physical activity records were self-reported.
Protein dose was fixed rather than body-mass adjusted.
No direct markers of mitochondrial adaptation or muscle capillarization were measured.
Three participants used contraceptive pills, though distribution was similar across groups.

Safety

No adverse events related to LOW-HIIT occurred.
Protein supplement caused mild stomach pain, bloating, nausea, or burping in a few participants; no placebo complaints were reported.
Participants were screened for major health problems and sessions were supervised by certified sports or physiotherapists.