The findings of all studies meeting the inclusion criteria are presented in Sects. 4.1 (aerobic exercise), 4.2 (resistance training) and 4.3 (combined aerobic and resistance training). In total, 13 original research articles and two review articles were included in this review. Care was taken to ensure that information was not ‘counted’ twice—that is, as an original research investigation and as part of a review. The characteristics and findings of all included studies are also presented in Table 1.
Aerobic exercise involves cardiorespiratory endurance exercises such as jogging, running and cycling . Leon and Sanchez  conducted a meta-analysis of 51 interventions involving 12 weeks or more of aerobic exercise (n = 4,700). It was reported that, on average, HDL cholesterol increased by 4.6 % while triglyceride levels fell by 3.7 % and LDL cholesterol fell by 5 %. Total cholesterol remained unchanged, although the HDL:LDL cholesterol ratio improved considerably, suggesting that the increased intensity and structure normally associated with aerobic exercise has a more consistent impact upon triglycerides and LDL cholesterol than moderate levels of physical activity. Studies subsequent to or not included in that meta-analysis are reported below.
It was suggested in the introduction that HDL cholesterol is the component of the lipid profile that is most likely to improve as the result of physical activity. This is supported by evidence relating to aerobic exercise presented by Banz et al. , who reported a 13% increase in HDL cholesterol (from 29.8 to 33.7 mg/dL, p < 0.05) following a relatively short 10-week protocol of training three times weekly at 85 % of the maximal heart rate (HRmax) [from the second week onwards] for 40 min on ski-style exercise equipment. The authors reported that HDL cholesterol was the only component of the lipid profile that improved. Nybo et al.  reported that the total:HDL cholesterol ratio was the only component of the lipid profile that improved significantly (decreasing from 3.41 to 2.92, p < 0.05) by 150 min of exercise weekly at 65 % of the maximal aerobic capacity (VO2max) in previously untrained participants. That investigation compared a prolonged (150 min/week) aerobic exercise protocol with an intense interval running protocol (40 min/week) [n = 36]. No improvements in the lipid profile were reported following the intense interval programme. Those authors consequently suggested that the training volume, as opposed to the training intensity, is the key to improving the lipid profile, and that there may be a relationship between body fat (which decreased only in the prolonged exercise group) and cholesterol levels, whereby a volume sufficient to elicit changes in fat mass is required to favourably alter the lipid profile.
When the intensity of aerobic exercise is increased during continuous effort, the effects upon HDL cholesterol appear to become more consistent. Dunn et al.  investigated the effects of a 6-month aerobic exercise training programme, which progressed from 50 to 85 % of maximum aerobic power for 20–60 min three times weekly, and reported significant decreases in total cholesterol (−0.3 mmol/L, p < 0.001) and in the total:HDL cholesterol ratio (−0.3, p < 0.001). In this case, the intervention period was relatively long and the intensity was relatively high. In a 16-week study, LeMura et al.  reported significant reductions in plasma triglycerides (from 1.4 to 1.2 mmol/L, p < 0.05) and increases in HDL cholesterol (from 1.4 to 1.8 mmol/L, p < 0.05) after training three times weekly at 70–75 % HRmax for 30 min for the first 8 weeks, progressing to four times weekly at 85 % HRmax for 45 min thereafter. The data suggested that shorter-term interventions will be effective also if the training volume is high enough. Increasing the frequency of training to four times weekly may have elicited the additional benefits seen by LeMura et al. in comparison with those observed by Banz et al. (with three training sessions weekly). Further, LeMura et al. observed a 13 % reduction in the body fat percentage (from 26.4 to 22.9 %, p < 0.05), suggesting that the additional volume of training generated an additional metabolic response—a parameter not reported by Banz et al.
Kraus et al.  investigated the impact of increasing the volume and intensity of aerobic exercise upon the lipid profiles of 111 sedentary overweight participants, all with mild to moderate dyslipidaemia. Participants were allocated to either 6 months in a control group or 8 months in one of three aerobic exercise groups. The three aerobic exercise groups were high-intensity/high-volume aerobic exercise (jogging for the calorific equivalent of 20 miles/week at an intensity of 65–80 % of the peak aerobic capacity (VO2peak), high-intensity/low-volume aerobic exercise (jogging for the calorific equivalent of 12 miles/week at an intensity of 65–80 % VO2peak) and moderate-intensity/low-volume exercise (walking for the calorific equivalent of 12 miles/week at an intensity of 40–55 % VO2peak). It was reported that the high-intensity/high-volume training combination resulted in the greatest improvements in 10 of 11 lipid variables (LDL cholesterol decreased from 130.1 to 128.2 mg/dL, p < 0.05; HDL cholesterol increased from 44.3 to 48.6 mg/dL, p < 0.05; triglycerides decreased from 166.9 to 138.5 mg/dL, p < 0.05). These data suggest that in relation to aerobic exercise, both total energy expenditure and intensity are factors in lipid reduction.
O’Donovan et al.  controlled the training volume to directly assess the impact of the training intensity. Sixty-four previously sedentary men were randomly allocated to either a control group, a moderate-intensity exercise group (at 60 % VO2max) or a high-intensity exercise group (at 80 % VO2max). Both exercising groups completed three 400 kcal sessions weekly for 24 weeks. By setting the session volume in calories, the overall training volume was controlled. Participants were instructed to maintain their dietary habits. It was reported that significant lipid profile improvements occurred only in the high-intensity group, with significant decreases (p < 0.05) in total cholesterol (from 6.02 to 5.48 mmol/L), LDL cholesterol (from 4.04 to 3.52 mmol/L) and non-HDL cholesterol (from 4.58 to 4.04 mmol/L).
The evidence suggests that a moderate-intensity exercise programme will be effective in increasing HDL cholesterol. This will have a positive impact upon atherosclerosis (hardening of artery walls through plaque and fat accumulation ) via HDL cholesterol-facilitated removal of LDL cholesterol. To directly reduce LDL cholesterol and triglyceride levels, however, the intensity of aerobic exercise must be increased—something that may not be possible in individuals with a limited exercise capacity or other risk factors.
Theoretically, resistance training (strength-developing exercise utilizing external resistance or one’s own body weight ) may be a more accessible form of exercise for less mobile groups, as well as providing an alternative to aerobic training for more mobile individuals . Prabhakaran et al.  investigated the effect of 14 weeks of resistance training in premenopausal women (n = 24). Resistance training was at an intensity of 85 % of one maximal repetition (85 % 1 RM), where one maximal repetition is the maximal load that can be lifted once for a given exercise . Participants were randomized to either resistance training or to a non-exercising control. Supervised exercise sessions lasted 40–50 min and were completed three times weekly. Significant (p < 0.05) decreases in total cholesterol (from 4.6 to 4.26 mmol/L) and LDL cholesterol (from 2.99 to 2.57 mmol/L) were observed, along with lowered body fat (from 27.9 to 26.5 %). Acute changes in the lipid profile following different intensities of resistance training were examined by Lira et al. . Untrained males (n = 30) were randomized to intensity groups at baseline. Measures of cholesterol were collected at time points of 1, 24, 48 and 72 h following resistance training at intensities of 50, 75, 90 and 110 % (in the later scenario in the eccentric phase only, performance was assisted during the concentric phase). The total training volume was equalized between the groups to ensure that the resistance training intensity was the factor being assessed. Triglyceride clearance at 72 h was significantly (p < 0.05) greater following 50 % 1 RM (−14.6 mg/dL) and 75 % 1 RM (−10.7 mg/dL) than following 90 % 1 RM (+9.5 mg/dL) and 110 % 1 RM (+12.1 mg/dL). Further, increases in HDL cholesterol were significantly greater following 50 % 1 RM and 75 % 1 RM than following 110 % 1 RM (p = 0.004 and 0.03, respectively). The authors concluded that low- to moderate-intensity resistance training results in greater benefit to the lipid profile than high-intensity resistance training, although the mechanisms underlying this difference are unclear. It is speculated that the reduction in total cholesterol is a result of the exchange of cholesterol ester between tissues and lipoproteins to HDL cholesterol (Fig. 1); however, the way this differs between 50, 75, 90 and 110 % 1 RM warrants further investigation.
Vatani et al.  examined the effects of various intensities of resistance training on the lipid profile over 6 weeks. Healthy male participants (n = 30) were randomized to either a moderate-intensity resistance training programme (45–55 % 1 RM) or a high-intensity resistance training programme (80–90 % 1 RM). Both groups were supervised during training sessions and attended three sessions weekly. Significant (p < 0.05) reductions in LDL cholesterol (moderate-intensity −13.5 mg/dL vs high-intensity −12.1 mg/dL), total cholesterol (moderate-intensity −12.2 mg/dL vs high-intensity −11.3 mg/dL) and the total:HDL cholesterol ratio (moderate-intensity −0.38 vs high-intensity −0.47) were found in both groups, with no significant differences between the two groups. Significant increases in HDL cholesterol, however, were observed only in the high-intensity group (+5.5 mg/dL). This is perhaps surprising, considering that previous research indicated that increased HDL cholesterol is likely to be the first lipid profile response to exercise, even at low intensities of activity . This study once again demonstrated the limited additional benefit of increasing the resistance training intensity when equalizing the training load by reducing the numbers of sets and repetitions being completed to compensate for the increased weight being lifted. In addition, the authors reported no significant changes in lipoprotein lipase activity following the exercise training intervention—which was surprising, considering the lipid profile changes that were elicited. This would, however, be dependent upon the time interval between the final exercise session and the blood sample collection (normally longer than 24 h), because of the acute response of lipoprotein lipase (increases have previously been shown to only be maintained for 48 h following a 1,500 kcal exercise session and not following a ≤1,300 kcal session —levels unlikely to be attained by this exercise intervention). Thus, although the levels were unchanged at post-intervention testing, lipoprotein lipase should not be ruled out as a mechanism.
Fett et al.  incorporated resistance training into circuit training sessions in which no specific weight was specified but a specific time duration was allocated to each exercise. Sessions lasted 60 min and were completed three times weekly for 1 month and four times weekly for the second month. Significant reductions were reported in total cholesterol (from 203 to 186 mg/dL, p < 0.01) and triglycerides (from 122 to 91 mg/dL, p < 0.05), further adding to the speculation that the volume of movement may be just as important as—or even more important than—the amount of weight lifted.
The evidence presented above demonstrates the effectiveness of both aerobic exercise and resistance training in controlling and improving cholesterol levels through various modes, frequencies, intensities and durations of exercise, in different populations. There is limited literature that has examined the two modalities combined, although a recent review by Tambalis et al.  suggested that although some combination protocols have been effective in lowering LDL cholesterol and increasing HDL cholesterol, others have not.
Shaw et al.  examined the effect of a 16-week moderate-intensity combined aerobic and resistance training protocol in previously untrained but otherwise healthy young men (n = 28). The protocol lasted 45 min and combined aerobic exercise at 60 % HRmax with resistance training (two sets of 15 repetitions) at 60 % 1 RM. It was reported that LDL cholesterol was significantly reduced following aerobic and resistance training (from 4.39 to 3.23 mmol/L, p < 0.05), although the reduction was not significantly different from that achieved by 45 min of aerobic exercise alone (from 3.64 to 2.87 mmol/L, p < 0.05). It can therefore be concluded that no additional LDL cholesterol reduction resulted from combining the two modes of exercise. However, this investigation did demonstrate that resistance training might successfully compensate for reductions in aerobic exercise. Further, the authors suggested that additional physiological systems benefited from resistance training, making it potentially more effective.
Yang et al.  reported a study investigating relationships between exercise, cholesterol and arterial stiffness in obese middle-aged women (n = 40, body mass index >25 kg/m2, age 30–60 years). The experimental protocol consisted of 45 min of aerobic exercise at an intensity of 60–75 % HRmax at 300 kcal per session and 20 min of resistance training at 100 kcal per session five times weekly over a 12-week period. Reductions were observed in total cholesterol (from 5.2 to 4.2 mmol/L, p = 0.655), LDL cholesterol (from 3.2 to 2.6 mmol/L, p = 0.172), triglycerides (from 3.0 to 2.5 mmol/L, p < 0.001) and arterial stiffness measured via the brachial–ankle pulse wave velocity (from 1,286 to 1,195 cm/s, p < 0.001). While no controls were included in this study, these data suggested the potential clinical significance of reductions in cholesterol—that is, a reduction in arterial stiffness, which is all too often associated with heart attacks and strokes.
Ha and So  combined 30 min of aerobic exercise at 60–80 % of the maximal heart rate reserve (maximal heart rate − heart rate at rest) [HRreserve] with 30 min of resistance training at 12–15 repetitions maximum in 16 participants aged 20–26 years for 12 weeks. The intervention significantly reduced the participants’ waist circumference, body fat percentage and blood pressure values, compared with those of non-exercising controls. The lipid profile improved in the exercising condition, with reductions in total cholesterol from 180.29 to 161 mg/dL, LDL cholesterol from 112.14 to 103.57 mg/dL and triglycerides from 97.14 to 50.43 mg/dL, although the changes did not reach statistical significance when compared with values in the controls. The authors suggested that the participants was too young to elicit the clinical and significant effects shown by previous research in predominantly elderly or middle-aged participants.