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Morning versus evening aerobic exercise and atherogenic and fat-distribution indices in older women with type 2 diabetes: a quasi-experimental controlled study

In a quasi-experimental study of older women with type 2 diabetes, 12 weeks of supervised aerobic exercise significantly improved metabolic and adiposity indices compared to a control group, with evening sessions demonstrating superior benefits over morning sessions for specific lipid-related and visceral-fat markers.

Original authors: Zahra Hemati Farsani, zahra Heydari, Shahrzad Habibi Ghahfarrokhi, Fatemeh Safari, Asma Soleimani

Published 2026-10-12

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CC BY 4.0

Original authors: Zahra Hemati Farsani, zahra Heydari, Shahrzad Habibi Ghahfarrokhi, Fatemeh Safari, Asma Soleimani

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

For millions of people, the rhythm of the day is more than just a schedule; it is a biological force that shapes how their bodies process food and energy. In the human body, a complex internal clock, known as the circadian rhythm, governs when we feel alert, when we sleep, and crucially, how our cells handle sugar and fat. When this internal timing system falls out of sync with daily habits, it can contribute to serious health issues, particularly type 2 diabetes, a condition where the body struggles to manage blood sugar levels. This struggle is often worsened by the accumulation of harmful fat around the organs and in the blood, which increases the risk of heart disease. While doctors have long known that physical activity helps manage these risks, a lingering question remains: does the time of day a person exercises change how well their body responds?

A recent study set out to answer this question by looking closely at older women living with type 2 diabetes. The researchers focused on a specific group: postmenopausal women between the ages of 60 and 65. This group is particularly vulnerable because the natural drop in estrogen after menopause can make it harder for the body to control blood sugar and maintain a healthy distribution of fat. The scientists wanted to see if moving a workout from the morning to the evening could offer a distinct advantage in improving the body's metabolic health, specifically looking at how fat is stored and how well the body handles sugar and cholesterol.

To find out, the team organized a twelve-week experiment involving forty-five sedentary women who had been diagnosed with type 2 diabetes. The participants were divided into three groups based on their availability and preference, rather than by random chance. One group performed supervised aerobic exercise, such as brisk walking or light jogging, at 9:00 in the morning. A second group performed the exact same routine at 9:00 in the evening. The third group served as a control, continuing their normal daily lives without adding any new exercise. All the women exercised three times a week, and their health was measured before the study began and again after the twelve weeks were over. The researchers looked at a wide range of indicators, including body fat distribution, insulin resistance, and various markers that predict the risk of heart disease.

The results were clear and significant. Both groups that exercised saw major improvements in their health markers compared to the group that did not exercise. The women who moved their bodies, whether in the morning or the evening, reduced their levels of insulin resistance, which is the body's inability to use sugar effectively. They also saw improvements in how their fat was distributed, with less dangerous fat accumulating around their internal organs. In contrast, the women who did not exercise saw their health markers worsen over the same twelve-week period, with their insulin resistance and risk factors for heart disease increasing. This confirmed that any moderate aerobic exercise is far better than none for this population.

However, the study uncovered a subtle but important difference between the two exercise times. While both groups improved, the women who exercised in the evening showed greater benefits in several key areas related to fat and heart health. Specifically, the evening group experienced a larger reduction in measures of visceral fat, which is the deep belly fat that wraps around organs and drives inflammation. They also saw a more significant drop in indices that predict the risk of clogged arteries and heart disease. The morning group improved, but the evening group's improvements in these specific lipid and fat-distribution measures were statistically superior. The researchers noted that this advantage held true even after accounting for the fact that the evening group started the study with slightly higher risk levels than the morning group.

The study suggests that the body's internal clock may play a role in how it processes exercise. Because the body's ability to burn fat and manage energy fluctuates throughout the day, exercising later in the day might align better with the body's natural metabolic peak for these specific tasks. The researchers were careful to note that their study was not a randomized trial, meaning the groups were not assigned by chance, which limits how definitively they can claim cause and effect. They also acknowledged that the sample size was small and focused only on older women, so the findings might not apply to men or younger people. Nevertheless, the data provides a strong signal that for older women with type 2 diabetes, shifting a workout to the evening could offer a slightly greater shield against the metabolic risks that lead to heart disease.

Ultimately, the research reinforces the vital importance of movement for managing diabetes, showing that even a short period of regular exercise can reverse the natural decline in health that occurs without it. While the study does not prove that evening exercise is a miracle cure, it does suggest that timing matters. For those who can choose, scheduling a walk or a jog later in the day might provide an extra layer of protection for the heart and blood vessels, turning a simple daily habit into a more powerful tool for long-term health.

Technical Summary: Morning versus Evening Aerobic Exercise and Atherogenic and Fat-Distribution Indices in Older Women with Type 2 Diabetes

Problem Statement
Type 2 diabetes mellitus (T2DM) is a global metabolic disorder characterized by hyperglycemia, insulin resistance, and dyslipidemia, leading to significant cardiovascular morbidity and mortality. The prevalence of T2DM rises sharply in women following menopause due to the decline in estrogen, which exacerbates oxidative stress, inflammation, and insulin resistance. While aerobic exercise is a established intervention for improving metabolic health, the optimal timing of exercise relative to circadian rhythms remains under-investigated, particularly regarding lipid-derived atherogenic indices and fat distribution in older postmenopausal women. Existing literature suggests afternoon or evening exercise may yield superior glycemic control, but few studies have compared morning versus evening training on composite indices of adiposity and cardiovascular risk in this specific demographic.

Methodology
This quasi-experimental controlled study utilized a pre-test–post-test design with a non-equivalent control group.

Participants: 45 sedentary postmenopausal women (aged 60–65 years) with physician-confirmed T2DM were recruited. Allocation to groups was based on availability and scheduling preference rather than randomization.
Groups: Participants were assigned to a Morning Exercise group (09:00 h), an Evening Exercise group (21:00 h), or a non-exercising Control group.
Intervention: The exercise groups underwent 12 weeks of supervised moderate-intensity aerobic training (brisk walking/light jogging), three sessions per week, totaling 36 sessions. Intensity was prescribed using the Karvonen method, progressing from 50% to 60–70% of heart-rate reserve. The control group maintained habitual activity.
Measurements: Thirteen anthropometric and biochemical indices were assessed before and after the intervention. These included standard measures (BMI, waist circumference) and composite indices: Abdominal Volume Index (AVI), Body Adiposity Index (BAI), Waist-to-Height Ratio (WHtR), Visceral Adiposity Index (VAI), Model of Adipose Distribution (MOAD), Conicity Index, Triglyceride–Glucose Index (TyG) and its combinations (TyG–WC, TyG–BMI), Atherogenic Index of Plasma (AIP), Castelli's Risk Indices I and II (CRI-I, CRI-II), and HOMA-IR.
Statistical Analysis: Primary analysis used baseline-adjusted ANCOVA with false-discovery-rate (FDR) control. Sensitivity analyses included linear mixed-effects models, non-parametric tests (Kruskal–Wallis, Mann–Whitney U), and raw change-score comparisons to address potential biases from non-randomization and non-normal data distributions.

Key Results
Forty-two women (14 per group) completed the study. Significant between-group effects were observed for 11 of the 13 indices (AVI, BAI, WHtR, VAI, HOMA-IR, TyG, TyG–WC, TyG–BMI, AIP, CRI-I, CRI-II), with all remaining significant after FDR correction ( $q < 0.001$ ).

Exercise vs. Control: Both morning and evening exercise groups demonstrated significant improvements (reductions) in insulin resistance, visceral adiposity, and atherogenic indices. Conversely, the control group showed significant deterioration in HOMA-IR (+21.8%), VAI (+13.6%), and atherogenic indices over the 12-week period.
Morning vs. Evening: While both exercise timings were beneficial compared to the control, evening exercise produced superior outcomes for specific lipid-related and fat-distribution indices:
- Significant Superiority of Evening Exercise: Evening sessions resulted in significantly greater reductions in BAI, VAI, AIP, CRI-I, and CRI-II compared to morning sessions ( $p = 0.044$ to $p < 0.001$ ).
- Trend Favoring Evening: Change-score analysis further favored evening exercise for TyG and TyG–WC, though these did not reach significance in the primary ANCOVA pairwise comparisons ( $p = 0.077$ and $0.106$, respectively).
- No Timing Difference: No significant difference between morning and evening exercise was found for AVI, WHtR, HOMA-IR, TyG–BMI, MOAD, or the Conicity Index.
Sensitivity Analyses: The superiority of evening exercise for BAI, VAI, AIP, CRI-I, and CRI-II was confirmed across linear mixed-effects models, non-parametric tests, and raw change-score analyses, suggesting the findings are robust to analytical assumptions.

Significance and Claims
The authors conclude that 12 weeks of moderate-intensity aerobic exercise improves adiposity, insulin resistance, and atherogenic indices in older women with T2DM, regardless of whether the training occurs in the morning or evening. However, the study identifies a potential circadian influence, where evening exercise yields greater benefits for lipid-related and visceral-fat indices than morning exercise.

The paper explicitly frames these findings as hypothesis-generating due to the quasi-experimental design and small sample size. The authors acknowledge limitations including the lack of randomization, the narrow demographic (postmenopausal women aged 60–65), and the absence of molecular circadian biomarkers. Consequently, the study calls for adequately powered randomized controlled trials to confirm the timing effect and to explore the underlying mechanisms, such as circadian clock gene expression and mitochondrial function, which may favor lipid oxidation during later-day sessions. The authors emphasize that while evening exercise appears advantageous for specific cardiometabolic markers, the primary clinical takeaway is the substantial benefit of exercise over inactivity, with evening sessions potentially offering an incremental advantage for lipid profiles and visceral fat reduction in this population.

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