Effects of exercise intensity and nutrition advice on myocardial function in obese children and adolescents: a multicentre randomised controlled trial study protocol

The researchers planned to compare three interventions:

**HIIT + nutrition advice:** 40-minute sessions (10 min warm-up, 4 × 4-minute high-intensity intervals at 85–95% of peak heart rate, interspersed with 3-minute active recovery periods at 50–70% peak heart rate, plus 5 min cool-down), performed 3 times per week, combined with individualised nutrition counselling.

**MICT + nutrition advice:** 44-minute sessions of continuous exercise at 60–70% of peak heart rate, performed 3 times per week, combined with the same nutrition counselling.

**Nutrition advice only:** Individualised nutrition counselling alone, without any supervised exercise.

The primary outcome was **myocardial function**, specifically **peak systolic tissue velocity (S′)** measured by tissue Doppler echocardiography — a measure of how well the left ventricle contracts. Secondary outcomes included vascular function (flow-mediated dilation of the brachial artery), visceral and subcutaneous fat, heart structure, body composition, cardiorespiratory fitness, autonomic nervous system function, blood lipids, insulin sensitivity, inflammation markers, physical activity levels, and dietary intake.

The study also included a **lean, healthy control group** of 100 children and adolescents matched for age, who were measured at a single time point for comparison purposes only — they did not receive any intervention.

The study planned to recruit **100 obese children and adolescents** (aged 7–16 years) from two centres: Trondheim, Norway (Norwegian University of Science and Technology) and Brisbane, Australia (University of Queensland). Obesity was defined as body mass index (BMI) at or above the **95th percentile** for age and sex.

Exclusion criteria included: hypertension (blood pressure above the 95th percentile), any history or evidence of heart disease, abnormal resting or stress echocardiography indicating unsafe participation, chronic diseases (asthma, kidney disease, diabetes), current smoking, orthopaedic or neurological disorders limiting exercise ability, diagnosed attention deficit hyperactivity disorder, and use of steroid medications.

An additional **100 lean, healthy children and adolescents** (age-matched) were recruited as a cross-sectional comparison group, measured once at baseline.

The primary outcome — **myocardial function** — was assessed using **tissue Doppler echocardiography**, specifically measuring **peak systolic tissue velocity (S′)**. This is an ultrasound-based technique that tracks the movement of heart muscle tissue during contraction. Lower S′ values indicate impaired heart muscle contractility.

Secondary measurements included:

**Vascular function:** Flow-mediated dilation (FMD) of the brachial artery — an ultrasound measure of how well the artery dilates in response to increased blood flow, reflecting endothelial health.

**Body composition:** Dual-energy X-ray absorptiometry (DXA) for total body fat, lean mass, and bone density; MRI or CT for quantifying visceral and subcutaneous adipose tissue.

**Cardiac structure:** Standard echocardiography to measure heart chamber sizes and wall thickness.

**Cardiorespiratory fitness:** A maximal exercise test on a cycle ergometer or treadmill with gas exchange analysis (VO₂peak).

**Autonomic function:** Heart rate variability analysis from ECG recordings.

**Blood biochemistry:** Fasting blood samples for lipids (total cholesterol, LDL, HDL, triglycerides), insulin, glucose, inflammatory markers (C-reactive protein, cytokines), and oxidative stress markers.

**Physical activity:** Accelerometry (worn for 7 days) and self-report questionnaires.

**Nutrition:** Dietary recalls or food diaries, analysed for energy and macronutrient intake.

**Pubertal status:** Self-reported Tanner staging (a standardised scale of physical development during puberty, ranging from stage 1 to 5).

**Study design:** This is a **multicentre, parallel-group, randomised controlled trial (RCT)** with three arms. It also includes a non-randomised, cross-sectional comparison with lean controls.

**Randomisation:** Obese participants were randomly assigned to one of three groups: (1) HIIT + nutrition advice, (2) MICT + nutrition advice, or (3) nutrition advice alone. The randomisation method is not fully detailed in the protocol, but the authors state it was computer-generated and stratified by centre (Norway vs Australia) and sex.

**Blinding:** This is an **open-label trial** — participants and exercise supervisors knew which group they were in. Outcome assessors (those measuring echocardiography, blood tests, etc.) were blinded to group allocation where possible, but the protocol acknowledges that complete blinding is difficult in exercise trials.

**Duration:** The intervention lasted **3 months** of supervised training (Phase I), followed by a **9-month home-based maintenance period** (Phase II). During Phase II, participants were re-randomised to either monthly supervised exercise or fully home-based exercise. Final measurements were taken at **12 months** from baseline.

**Control group:** The lean, healthy control group was measured only once (cross-sectional) and did not receive any intervention. This design allows comparison of obese participants' outcomes against healthy norms, but cannot control for time-related changes or placebo effects.

**Statistical approach:** The primary analysis compared changes in S′ from baseline to 3 months between the three groups using analysis of covariance (ANCOVA), adjusting for baseline values, centre, and sex. The sample size of 100 (approximately 33 per group) was calculated to detect a clinically meaningful difference in S′ of 1.5 cm/s with 80% power at α = 0.05.

**What this design can and cannot prove:**

**Can prove:** Whether HIIT + nutrition advice causes greater improvements in heart function compared to MICT + nutrition advice or nutrition advice alone over 3 months, because randomisation balances known and unknown confounders between groups.

**Cannot prove:** Whether the effects last beyond 12 months, because the maintenance phase involves re-randomisation and unsupervised exercise, which introduces variability. The design also cannot separate the effects of exercise from nutrition advice, since both exercise groups also received nutrition counselling. The lean control group cannot be used to infer causality — it only provides normative reference values.

**Major methodological weaknesses:**

**No blinding of participants or trainers** — this introduces potential bias from expectations, effort, and adherence differences between groups.

**No placebo control for exercise** — the nutrition-only group knows they are not exercising, which may affect motivation and outcomes.

**Re-randomisation at 3 months** for the maintenance phase complicates interpretation of 12-month results.

**Self-reported Tanner staging** is subjective and may introduce error in accounting for pubertal changes.

**The lean control group is cross-sectional** — it cannot track changes over time, so any differences at 12 months could reflect maturation rather than intervention effects.

**High dropout risk** is acknowledged by the authors, which could bias results if dropouts differ systematically from completers.

**Important note:** This is a **study protocol**, not a results paper. The findings described below are from the **pilot study** and **background literature** cited in the introduction, not from the trial itself.

**Pilot study findings (cited by authors):** In a previous pilot trial of HIIT in obese adolescents, the 4×4 HIIT protocol (4 × 4-minute intervals at 85–95% peak heart rate) **almost normalised cardiac function** to levels seen in lean, healthy controls. High compliance was noted, but the pilot had a small sample size and no comparative treatment group.

**Meta-analysis of HIIT vs MICT in adults (Weston et al., 2014):** HIIT produced **nearly double the improvement in cardiorespiratory fitness** compared to MICT: **19.4% increase vs 10.3% increase** in VO₂peak. This was cited as rationale for expecting superior effects in children.

**Prevalence of abnormal myocardial function in obese youth:** Previous studies cited by the authors found **significantly reduced tissue Doppler velocities** (S′) in obese children and adolescents compared to lean, age-matched controls. The exact values are not given in the protocol, but the difference was described as clinically meaningful.

**Vascular function impairment:** Obese paediatric studies consistently show **impaired flow-mediated dilation** (FMD) compared to lean controls, indicating early endothelial dysfunction.

**Treatment success rates in paediatric obesity:** A large European registry study cited by the authors found that **fewer than 10% of participants** achieved significant treatment effects following lifestyle interventions over 2 years of follow-up. Treatment was **least effective in participants older than 12 years**.

**Physical activity deficit:** Obese children spend approximately **100 minutes per day less** being physically active than healthy-weight or overweight children.

Since this is a protocol, no results are available. However, based on the cited pilot study and adult meta-analysis:

The pilot study suggested that 3 months of HIIT could **nearly normalise** heart muscle function in obese adolescents — meaning S′ values moved from impaired levels to within the range of healthy-weight controls. This is a large effect, but the exact magnitude in cm/s was not reported.

In adults, HIIT improved cardiorespiratory fitness by **19.4%** compared to **10.3%** with MICT — roughly a **9 percentage point advantage**. If similar effects occur in children, this would mean a child with a VO₂peak of 35 mL/kg/min might improve to ~42 mL/kg/min with HIIT vs ~39 mL/kg/min with MICT.

The 100-minute daily activity deficit in obese children means that even modest increases in physical activity could close a meaningful gap — for example, adding 20–30 minutes of HIIT per day could reduce this deficit by 20–30%.

**Acknowledged by authors:**

**Growth and maturation confound:** Children grow and go through puberty over the 12-month study period, which naturally changes heart size and function. The authors plan to normalise cardiac measurements for body size, but self-reported Tanner staging may be inaccurate.

**High dropout risk:** Paediatric obesity interventions historically have high dropout rates, which could bias results if those who drop out are less adherent or have worse outcomes.

**Lack of blinding:** Exercise interventions cannot be double-blinded; participants and trainers know group allocation.

**Single time point for lean controls:** The lean comparison group is measured only once, so cannot account for maturation or seasonal effects.

**Additional critical limitations:**

**No true control for nutrition advice:** All three groups receive nutrition counselling, so the study cannot isolate the independent effect of exercise vs nutrition.

**No sham exercise control:** The nutrition-only group does not receive any attention-matched activity, so differences could reflect social contact or structured time rather than exercise physiology.

**Sample size modest for subgroup analyses:** With 100 participants split into three groups (~33 per group), the study is underpowered to detect differences by age, sex, or pubertal stage.

**Industry funding not disclosed:** The protocol does not mention funding sources, so potential conflicts of interest are unknown.

**Generalizability limited:** Participants are from two university cities in high-income countries (Norway and Australia); results may not apply to lower-income settings or different ethnic groups.

**Exercise supervision intensity:** The majority of sessions are supervised, which is resource-intensive and may not reflect real-world adherence.

For someone running their own n=1 experiment (adapted for adults, since this study is in children):

**What to test:**

Compare **HIIT** (4 × 4-minute intervals at 85–95% of maximum heart rate, with 3-minute active recovery between intervals) vs **MICT** (44 minutes of continuous exercise at 60–70% max heart rate) vs **no structured exercise** (but maintaining normal activity).

Add **nutrition advice** to all conditions (e.g., standardised dietary counselling or a consistent meal plan) to control for dietary changes.

**Minimum meaningful duration:**

**3 months** of consistent training (3 sessions per week) is the minimum to see measurable changes in heart function and fitness, based on the study design.

For maintenance effects, continue for **12 months** with reduced supervision.

**What to measure (specific metrics):**

**Primary:** Resting heart rate and heart rate recovery after exercise (as a proxy for cardiac function — not as precise as tissue Doppler, but accessible). Measure heart rate at rest, then after a standardised 3-minute step test or 6-minute walk, record recovery at 1 and 2 minutes.

**Secondary:**

- **Cardiorespiratory fitness:** VO₂max estimate from a submaximal exercise test (e.g., 1-mile walk test or Astrand-Rhyming cycle test).

- **Body composition:** Waist circumference, hip circumference, and body weight measured weekly at the same time of day.

- **Blood markers:** Fasting glucose, insulin, and lipids (if accessible via a home test kit or clinic).

- **Vascular function:** Not easily measured at home, but morning resting blood pressure and pulse pressure can serve as rough proxies.

- **Physical activity:** Step count or active minutes from a wearable device (e.g., Fitbit, Garmin) worn 24/7.

**Key confounds to control for:**

**Growth and maturation:** If you are an adolescent, your heart and body are naturally changing. Track height and weight weekly to normalise measurements.

**Diet:** Keep a food diary for at least 3 days per week to ensure nutrition is consistent across conditions.

**Sleep:** Poor sleep impairs cardiac function and recovery. Track sleep duration and quality (e.g., using a sleep diary or wearable).

**Stress:** Chronic stress elevates cortisol and affects heart function. Use a daily stress rating (1–10 scale).

**Time of day:** Measure all outcomes at the same time of day (e.g., morning, before eating).

**Hydration and caffeine:** Standardise water intake and avoid caffeine for 2 hours before measurements.

**What a positive result would look like:**

**HIIT superior to MICT:** Resting heart rate decreases by 5–10 bpm more with HIIT than MICT; heart rate recovery improves by 10–20 beats in the first minute after exercise; estimated VO₂max increases by 15–20% with HIIT vs 8–12% with MICT.

**Normalisation:** Your cardiac function metrics (resting heart rate, recovery, blood pressure) move into the range of age-matched healthy-weight individuals — e.g., resting heart rate below 70 bpm, blood pressure below 120/80 mmHg.

**Clinically meaningful change:** A 1.5 cm/s improvement in tissue Doppler S′ (if you have access to echocardiography) would be considered meaningful — this translates to roughly a 10–15% improvement in heart muscle contractility.

**Confounds to watch for specifically:**

**Enjoyment and adherence:** HIIT may be more enjoyable (stop–start nature mimics play), but also more intense and potentially intimidating. Track your motivation and enjoyment daily (1–10 scale) to see if adherence differs between conditions.

**Injury risk:** HIIT places higher stress on joints and muscles. Monitor for any pain or discomfort and adjust intensity if needed.

**Overtraining:** If you feel persistently fatigued, have trouble sleeping, or see declining performance, reduce volume or take a rest week before concluding the intervention is ineffective.