Exercise, Timing, and Fat Reduction

    Obesity continues to rise globally, with the World Health Organization reporting that more than 1 billion people are now living with obesity. Modern life makes weight gain easier—long work hours, high stress, processed foods, and more time sitting all play a role. And while traditional calorie-cutting diets can work, many people find them hard to maintain because of hunger, limited nutrients, and declining motivation over time.

    For this reason, researchers have been looking at more flexible dietary approaches, including intermittent energy restriction. One practical option is intermittent meal replacement (IMR), where a few meals each week are replaced with low-calorie, nutrient-balanced shakes, soups, or bars. These typically provide 200–400 calories and high-quality protein that supports fullness and helps protect muscle during weight loss.

    Exercise plays an equally important role. Moderate-intensity continuous training (MICT)—such as brisk walking or cycling for 20–60 minutes—burns calories, improves metabolism, and is easier to sustain than high-intensity workouts. However, studies show that some women feel hungrier after exercise, which can lead to overeating and offset calorie burn. A structured, protein-rich meal replacement after exercise may help minimize this effect and support better fat loss.

Study 1: Exercise + Intermittent Meal Replacement

    One study investigated whether combining exercise with IMR could enhance weight-loss results. Forty overweight women were randomly assigned to either MICT alone or MICT plus meal replacement (MICT + IMR) for eight weeks.

    Both groups selected three nonconsecutive days per week to perform a 40-minute MICT workout. The MICT + IMR group consumed a protein-enriched nutritional shake within 30 minutes post-exercise as a replacement for dinner, while the MICT group ate their usual dinner.

    On non-exercise days, both groups followed a calorie-restricted diet of 1,000–1,500 kcal/day, designed to reduce unhealthy, high-calorie foods while keeping regular eating patterns intact.

    All exercise sessions were supervised by an exercise physiologist. Using the Apple Watch Series 8 to monitor heart rate, each workout included:

·      5-minute warm-up at 50% HRmax

·      30-minute aerobic dance session at 60–70% HRmax

·      5-minute cool-down at 50% HRmax

    Maximum heart rate (HRmax) was estimated with the standard formula:

HRmax = 220 – age.

Results

    The women in the MICT + IMR group had greater improvements:

·      Weight loss: –3.70 kg vs. –1.17 kg in the exercise-only group

·      Fat loss: –2.25 kg vs. –1.19 kg

·      Better fasting blood sugar and insulin levels, suggesting improved metabolic health

    After the 8-week program, the MICT group showed modest but meaningful improvements. They lost 1.17 kg of body weight (p = 0.016), 1.19 kg of body fat (p < 0.001), and their BMI dropped by 0.45 kg/m² (p = 0.015). Body fat percentage also decreased by 1.24% (p < 0.001). Fat loss occurred in both the limbs and trunk, while lean body mass and muscle mass stayed about the same.

In the MICT + IMR group, the results were larger. Participants lost 3.70 kg of body weight (p < 0.001), mainly due to a 2.25 kg reduction in body fat (p < 0.001). They also lost some muscle mass (0.83 kg, p < 0.001). BMI decreased by 1.35 kg/m² (p < 0.001), and body fat percentage dropped by 1.78% (p = 0.008).

    The additional weight loss in the MICT + IMR group came partly from lean body mass loss, suggesting that while this approach is effective for bigger weight loss, exercise alone is better for preserving muscle.

What this means

·      Exercising 3 times a week with a calorie-restricted diet is effective for weight and fat reduction.

·      Replacing regular dinner with a high-protein, low-calorie shake on exercise days can increase weight loss—but may also increase lean mass loss.

·      Exercise alone remains an effective strategy for weight control while preserving muscle.

Study 2: Does Timing of Exercise Matter? Morning vs. Afternoon

    Another study explored whether exercising in the morning or afternoon leads to different health effects in individuals with metabolic syndrome (MetS). A total of 139 adults were randomly assigned to:

·      Morning exercise (AMEX), n = 42

·      Afternoon exercise (PMEX), n = 59

·      Control (no training), n = 38

    Participants completed 48 supervised high-intensity interval training (HIIT) sessions over 16 weeks.

·       AMEX trained between 8:00–9:00 a.m.

·       PMEX trained between 4:00–6:00 p.m.

·       All participants ate a meal at least one hour before exercise

Each training session included:

·      10-minute warm-up at 70% HRmax

·      Four 4-minute intervals at 90% HRmax

·      Three 3-minute active recovery periods at 70% HRmax

·      5-minute cool-down

Results

Compared with the control group, both morning and afternoon exercisers showed:

·      ~0.7% body fat loss (–0.8 kg (AMEX), –0.7 kg (PMEX), +0.6 kg (control), p = 0.002)

·      2.1 cm reduction in waist circumference (p < 0.001)

·      3.8 mmHg decrease in diastolic blood pressure (p = 0.004)

·      VO₂max increase of 3.5 mL/kg/min (p < 0.001)

    These improvements were similar regardless of training time.

But morning exercise produced additional benefits:

·      Larger drop in systolic blood pressure (–4% vs. –1%, p = 0.019)

·      Greater reduction in fasting insulin (–12% vs. –5%, p = 0.001)

·      Better improvement in insulin resistance (–14% vs. –4%, p = 0.006)

·      Bigger improvement in overall MetS Z-score (–52% vs. –19%, p = 0.021)

Key Takeaways from Both Studies

·      Exercise 3 days per week works.

·      Combined with a calorie-restricted diet, it reduces fat and improves metabolic health.

·      Adding meal replacement increases fat loss, but it may also increase lean mass loss. Consider this for short-term goals, but prioritize resistance training for long-term muscle preservation.

·      Morning exercise may offer extra metabolic benefits, especially for improving blood pressure, insulin, and overall metabolic syndrome risk.

Both morning and afternoon exercise improve body composition, so choose the time that best fits your lifestyle—even though exercising in the morning seems to provide additional health benefits, consistency still matters more than timing.

References:

1.     Liu Q, Guo Y, Peng B, Fan D, Wu J, Wang J, Wang R, Liu JM, Wu J, Wang S, Zhao Y. Protein-enriched intermittent meal replacement combined with moderate-intensity training for weight loss and body composition in overweight women. Sci Rep. 2025 Apr 11;15(1):12485. doi: 10.1038/s41598-025-96486-6. PMID: 40216877; PMCID: PMC11992100.

2.     Morales-Palomo F, Moreno-Cabañas A, Alvarez-Jimenez L, Mora-Gonzalez D, Ortega JF, Mora-Rodriguez R. Efficacy of morning versus afternoon aerobic exercise training on reducing metabolic syndrome components: A randomized controlled trial. J Physiol. 2024 Dec;602(23):6463-6477. doi: 10.1113/JP285366. Epub 2023 Nov 28. PMID: 38015017; PMCID: PMC11607890.

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