Battle of the sexes: Which is better for you, high- or low-intensity exercise?

1. Introduction

Cardiovascular disease (CVD) accounts for more than 17 million deaths per year;¹ however, morbidity statistics alone underestimate the scale of the problem because a significant proportion of the population live with CVD. Obesity and type 2 diabetes mellitus (T2DM) have significant influence on CVD initiation and progression, and obesity is independently associated with CVD, particularly among women.² Obesity levels are rising rapidly in adults, but of particular concern is its swift rise in children and adolescents,³ who are at risk of remaining overweight into adult life⁴ in which long-term health consequences are most likely to manifest.⁵ In its simplest form, obesity arises because of an imbalance between energy intake and energy expenditure, with excess triglycerides being stored in adipocytes, which can increase in both size and number.⁶ Obesity results in numerous potential adverse effects such as sleep apnea, increased incidence of osteoarthritis,⁷ and psychological impact,⁸ but adipocytes also act as an endocrine organ secreting hormones and cytokines such as retinol-binding protein 4 (RBP-4) and adiponectin. In obesity, adiponectin levels decrease and RBP-4 levels increase, the effects of which enhance insulin resistance, thus increasing T2DM risk.⁹ Individuals with T2DM have a several-fold increased risk of CVD, and many of the risk factors for obesity, T2DM, and CVD overlap highlighting the interlinking complexity of these pathologic processes: poor diet, age, gender, ethnicity, smoking, dyslipidemia, hypertension, and physical inactivity, to name a few.10, 11, 12

2. Physical inactivity

Physical activity (PA) is an effective way of managing weight, and public health guidelines suggest individuals perform 150 min of moderate-intensity aerobic exercise per week or 75 min at a vigorous level to lose weight, reduce blood pressure, and decrease cholesterol levels (i.e., sufficiently active);¹³ for greater health benefits, the World Health Organization now recommends 300 min of moderate-intensity exercise per week or 150 min at a vigorous level.¹⁴

Globally, physical inactivity was approximately 23% in 2010, and this physical inactivity is seen in both genders to different degrees. It is well known that females do not exercise as much as males, and this is reflected in the 2010 global inactivity levels, which reported that in the UK, 32% of males and 42% of females do not exercise to the recommended levels.¹⁴ Similar gender differences are observed throughout the world; for example, the United States (male vs. female: 25% vs. 39%); Australia (male vs. female: 20% vs. 28%); and Saudi Arabia (male vs. female: 53% vs. 69%).¹⁴ It is therefore clear that compliance with current recommendations is poor, particularly in women; the reasons are varied, but common explanations include limited available time and motivation.^15,16 Recommendations for PA levels are currently the same for both males and females, which overlooks any potential requirement for different levels based on gender, particularly because males have lower overall health levels than females¹⁴ (e.g., hypertension and CVD), males have higher incidences of hypertension and CVD compared to females. Health status is clearly based on more than just PA levels and includes other modifiable risk factors such as diet and stress and nonmodifiable factors such as being male, which confers additional risk in itself. Despite this, exercise is still a key player in overall health and decreased disease risk.

3. High-intensity interval training (HIIT)

The high-time and low-motivation reasons for a lack of exercise has driven research into the use of HIIT as a possible PA regime that provides rapid positive benefits in health risk factors, such as blood pressure, cholesterol levels, and insulin sensitivity, and also takes up little time.16, 17, 18 In fact, some studies have shown that health benefits can be gained by undertaking 2 weeks of supervised HIIT comprising just 15 min of total exercise,¹⁵ whereas other studies have shown benefits in slightly longer 4- to 6-week programs.¹⁹ HIIT is defined as repeated short-duration exercises at a high-intensity level interspersed with recovery periods and is thus a time-efficient method of exercising. Because HIIT offers an impact on physical health and assists with compliance issues, it is an exciting prospect for use in the treatment, management, and prevention of chronic diseases, which are currently pose significant concern to the global population.

4. Gender responses to exercise

It is well documented that exercise has considerable physiological and psychological health benefits for both men and women; however, it is less clear if there are differing levels of health benefits between the sexes or if different modes of exercise are more beneficial to one gender than the other. Fuel metabolism during exercise places a varying degree of importance on fuel substrate utilization between the sexes. During long-distance exercise at approximately 65% maximal oxygen consumption (VO_2max), women metabolize more lipids, and correspondingly less carbohydrates and proteins, than equally trained and nourished men.²⁰ In part, this may be a result of increased adrenaline levels observed in men, which is a powerful mobilizer of muscle and liver glycogen. Muscle glycogen is an important fuel during exercise, and carbohydrate loading for 4 days in athletes revealed that although male muscle glycogen levels increased by 41%, there was no significant increase in female levels.²¹ Performance was subsequently analyzed in a submaximal endurance cycle at 75% VO_2max; again, males showed a significant increase in performance that females did not, and as previously noted, females metabolized significantly more lipid than males, who metabolized more protein and glycogen during exercise.²¹ Interestingly, it is possible that the lower glycogen usage in females, and thus reduced glycogen depletion, may result in a significant increase in performance time in women, particularly when working in the 75%–85% VO_2max range.²² This is backed up by the observation that females had significantly lower mid and terminal exercise respiratory quotient values, indicating a later occurrence of muscle glycogen depletion.²²

Body size, body composition, and muscle characteristics have also been implicated as potential reasons for gender-based differences in response to exercise.^23,24 Females tend to have a greater proportion of body fat than men, which is stored in the gluteal-femoral region in women compared with the visceral area in men.²⁵ Total cross-sectional muscle area is 60%–85% lower in women than in men, and greater muscle mass activation requires increased need to replenish stores and increased glycogen breakdown turnover.¹⁵ The degree of fitness has been implicated in accounting for gender-based differences in performance; prior training triggers muscular adaptations to improve fuel utilization and therefore more efficient functioning during exercise.²⁶ PA levels should therefore be matched between the sexes when studying the effects of exercise on performance between the sexes.

The significant health benefits that exercise provides are well understood, and it is therefore often prescribed to aid in the prevention and management of chronic disorders.¹⁶ Our lab has previously highlighted the heterogeneous responses individuals have to exercise, with some participants showing positive responses to a variety of health markers, whereas others have no change or show potentially adverse responses.¹⁸ For example, we showed that positive responders had significant improvements in systolic and diastolic blood pressures and VO_2max.¹⁸ Not all individuals show positive responses to exercise; many show no significant change, and others may show an adverse response (e.g., systolic pressure may increase (get worse) or insulin sensitivity decrease).¹⁷

There is a wealth of knowledge on how male and female physiology responds differently to exercise; this includes lipid, protein, and carbohydrate metabolism and hormonal changes (e.g., adrenaline during exercise or estrogen during the menstrual cycle21, 22, 23, 24, 25, 26, 27); however, a key question remains whether there are different health benefits following exercise. One of the best current markers of overall morbidity and mortality is VO_2max,²⁸ which, put simply, is a measure of how effective the body is at using oxygen; it is thus a marker of overall respiratory and cardiovascular (heart and blood vessels) function. Our preliminary data indicates that, following a 6-week HIIT program in healthy 20- to 24-year-old participants, males showed an average increase in VO_2max of 11% (p = 0.037), whereas the change in female VO_2max was not significant (unpublished data). As discussed previously, there are significant differences in body composition between the sexes; males have greater skeletal muscle mass,²⁹ and women have more body fat.²⁵ There is also an overall increased left ventricular end-diastolic volume in males compared with that found in females.³⁰ All of these factors could potentially influence the increased VO_2max we observed in males compared with females. Previous work has demonstrated that both sexes gain significant health benefits when moving from the sedentary state to low levels of exercise³¹ however, when intensity further increases to vigorous levels, as observed in our HIIT program, males gain greater health benefits than females.³² Women appear to benefit from increased duration of low to moderate intensity³³ with limited benefits following higher-intensity exercise.³⁴ Despite this, females have increased health benefits and protection from T2DM and CVD than their male counterparts when undertaking low to moderate aerobic exercise (e.g., brisk walking, cycling, or jogging where a conversation can still be held).³³

There have been several studies looking into the effects of exercise on cancer. For example, reports have suggested that lung cancer risk is not altered following PA in non-smokers but does reduce risk in smokers, and that risk for colon, breast, and endometrial cancers is reduced in those who undertake regular PA.³⁵ In males, prostate cancer risk may be reduced with PA, particularly if activity levels are high.³⁵

Exercise is well known to improve health, energy levels, and mood, for example, through the production of endorphins and the so-called runner's high, which energizes individuals, leading to sharper focus, and through increased cardiovascular health, which improves endurance. Research consistently shows that exercise reduces the incidence of insomnia, resulting in better sleep as well as improved mood.³⁶ Furthermore, the incidence of depression is lower in those who exercise regularly, which is thought to be through increased release of neurotrophic factors that cause new nerve connections to form, thus improving brain functionality and potentially reducing depressive tendencies.³⁷ It is interesting to note that men seem to gain more antidepressant value from vigorous PA, whereas women gain it from lower levels of activity, such as walking, which may enable greater social interaction and bonding.³⁸ This appears consistent with other studies comparing exercise modalities and health between the genders.

5. Conclusion

Low levels of PA and cardiorespiratory fitness are associated with increased risk of CVD, and a large proportion of the population does not meet these recommended levels. There is thus considerable heterogeneity in an individual's ability to improve cardiorespiratory health and metabolic disorder risk such as diabetes mellitus in response to exercise, with responses varied depending on sex, genetics, and the environment. Evidence suggests that females and males respond differently to the type of PA they undertake: females appear to gain greater benefit from low to moderate aerobic exercise, whereas males may acquire greater health status through more intense activity. There are significant differences in the structure and function of males and females: the cardiovascular system of males enables them to perform more vigorous exercise, and therefore males tend to have higher VO_2max values, higher red blood cell counts, lower heart rates, and greater lung function39, 40 females use more lipids compared with males, who use greater proportions of carbohydrates and proteins as energy sources.

All of these differences lead us to the conclusion that one size does not fit all and that exercise prescription should be targeted and not generalized. Although females may not currently undertake as much PA as their male counterparts, perhaps they do not need to; perhaps lower level activity, and a little bit more of it, preferably with friends, will allow them to gain significant health benefits. Males, however, may want to take part in higher-intensity exercise, such as HIIT, in which their cardiovascular system is pushed to adapt and improve while gaining overall health benefits. Of course, we should also be thinking about exercise and health across the life span; just as males and females appear to differ in their requirements, so will the young and the old, the young female and young male, and the old male and old female. So although increasing PA levels is a clear benefit to individuals, it must be done smartly.

Competing interests

The author declares that he has no competing interests.