To determine the effect of physical activity on blood lipids in adolescents.

MEDLINE was searched using relevant keywords, and the bibliographies of previously identfied studies were handsearched.

Study designs of evaluations included in the review

Randomised controlled trials (RCTs), non-randomised controlled trials, prospective observational studies and cross-sectional observational studies.

Specific interventions included in the review

Physical activity.

Participants included in the review

Male and female participants from several different ethnic backgrounds, aged from 9.4 to 23 years.

Outcomes assessed in the review

Blood cholesterol (TC), triglyceride (TG), high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C), very low-density lipoprotein cholesterol (VLDL-C), maximum oxygen uptake (VO2 max), and apolipoproteins (Apo A1, Apo A, Apo B).

How were decisions on the relevance of primary studies made?

Since only a small number of controlled trials were located, all were included. All cross-sectional studies of trained versus untrained patients from which the necessary data could be extracted, were included. Reports of quantified habitual physical activity in relation to lipid profile were all reviewed and collated.

The authors use narrative discussion to highlight the appropriateness of different study designs and the validity of the results. The authors do not state how the papers were assessed for validity, or how many of the authors performed the validity assessment.

The authors do not state how the data were extracted for the review, or how many of the authors performed the data extraction.

How were the studies combined?

The studies were combined by a narrative review and in a summarising table. Different designs were treated separately.

How were differences between studies investigated?

The authors do not state how differences between the studies were investigated.

Four RCTs (205 participants), 4 non-randomised controlled trials (161 participants), 1 prospective observational study (12 participants) and 21 cross-sectional observational studies (4,114 participants) were included.

Training studies: controlled trials provide no evidence of a beneficial effect of physical activity.

Trained versus untrained participants: cross-sectional studies evaluating male and female participants as a single group show that physical activity has a positive effect on reducing HDL-C, LDL-C, TG and the HDL-C/TC ratio, but does not have a beneficial effect on TC. In studies which distinguish between the sexes, there is only evidence of a positive effect in lowering HDL-C.

Measures of fitness: cross-sectional studies found that in both male and female participants, physical activity has a beneficial effect on HDL-C but not on TC, LDL-C, TG, nor the HDL-C/TC and TC/HDL-C ratios.

Habitual physical activity: cross-sectional studies found that in females, physical activity reduces HDL-C but not TC, LDL-C, TG, nor the HDL-C/TC and TC/HDL-C ratios. In males there is no evidence of a positive effect on any of these outcome measures. Studies evaluating both male and female participants together show only a beneficial effect on HDL-C, as shown for females when considered alone.

Obese participants: controlled trials found that in males, physical activity results in a lowering of HDL-C but not of TC or TG. In females there is a positive effect on HDL-C and TG but not on TC.

Diabetic participants: controlled trials showed that when male and female participants are analysed as a single group, there is a beneficial effect on LDL-C but not on TC, HDL-C or TG. Data for the individual sexes are not available.

Familial links: in males and females, when analysed as a single group in a controlled trial, physical activity has a positive effect on TC, HDL-C and the TC/HDL-C ratio (other outcomes were not assessed). Cross-sectional studies found that habitual physical activity in males and females has no beneficial effect on any of the outcome measures. In fit males there is a beneficial effect on HDL-C and TG but not on TC or LDL-C; in females there is no evidence of any positive reduction in any of the outcome measures.

The most convincing evidence for a beneficial effect of physical activity on blood lipids is apparent in cross-sectional studies. The results of longitudinal studies are unimpressive, but the studies are generally lacking in rigour. Empirical dose-response data relating the effects of physical activity to blood lipids and lipoproteins in adolescents are nonexistent. Recommendations for an appropriate exercise prescription must, therefore, be based on adult data and expert opinion. Emphasis should be placed on promoting an increase in the amount of adolescents' habitual physical activity.

This was a well-described review that highlighted the lack of research in the subject area. The individual trials were well described, but the diversity of the study designs employed hinders global conclusions.

This is a critical abstract of a systematic review that meets the criteria for inclusion on DARE. Each critical abstract contains a brief summary of the review methods, results and conclusions followed by a detailed critical assessment on the reliability of the review and the conclusions drawn.