Few RCTs have tested the effects of maca on sexual function. This review found limited evidence from four small trials that suggested that maca is effective in improving sexual desire after at least 6 weeks. However, this finding is limited because the studies were underpowered. In particular, two studies found no effect after 2 or 4 weeks of treatment. Evidence from other studies suggests that maca may be effective for sexual dysfunction in patients with ED and postmenopausal women after 12 weeks or 6 weeks, respectively. However, the total number of trials, the total sample size, and the average risk of bias in the primary studies were too limited to draw firm conclusions.
All of the RCTs employed double-blind methods, but none of the included trials reported methods of sequence generation for randomisation and allocation concealment. Trials with inadequate sequence generation and inadequate allocation concealment are likely to show exaggerated treatment effects [25
] and thus limit the reliability of the study results. Although all included RCTs used placebo controls, none reported the success of blinding. None of the studies reported a power calculation, and sample sizes were very small in some of the RCTs (ranging from 8 to 57). One article has a poor description of the outcome and was therefore difficult to interpret [23
]. In this trial, only the mean average of two active groups (high and low dosage of maca) was compared with the control group. One RCT failed to include washout periods between crossover periods [22
]. One of the main problems in crossover trials is the possibility of a carry-over effect. Therefore, this RCT is difficult to interpret. The other RCT has a small sample size and is therefore susceptible to a type II error [24
Four kinds of questionnaires were used for measuring sexual function or sexual desire, including the IIEF-5 [21
], Green Climacteric Scale (GCS) [22
], subjective Likert scale [23
], and Sexual Desire Inventory [24
]. Three RCTs [21
] employed validated questionnaires, while one RCT [23
] did not. Because the GCS is not specified for sexual function, it might not be sensitive enough to detect changes in sexual dysfunction. One trial tested changes in sexual desire compared to a baseline value with a 6-point Likert scale. It is not clear whether the authors used validated scales. It is important that only validated questionnaires are employed; otherwise, the outcome measures used have less established reliability and validity, data derived from them are subject to bias, and comparisons between the results of different studies are problematic.
The extent to which the therapeutic effects of maca depend on the type of maca used and the amount of various constituents in the preparation is unclear. The optimum dose of maca is unknown. Single-dose studies used extract quantities ranging from 1.5 g/d to 3.0 g/d. One excluded RCT compared two dosages of maca, 1.5 g/d and 3.0 g/d, for the management of SSRI-induced sexual dysfunction, but the results of that study failed to show differences between the two doses [20
]. Further studies are required to identify the optimal dose.
One argument for the use of maca for the management of sexual function might be that it causes fewer adverse effects than conventional drug treatments. However, none of the four RCTs described here assessed the adverse effects of maca. This should be tested in future studies.
One could question the validity of our conclusions by pointing to the review method used (reviewing a small number of trials with many limitations). However, the reasons for doing a systematic review include answering questions not addressed by individual studies, settling controversies arising from apparently conflicting studies, and generating new hypotheses [19
The limitations of our systematic review pertain to the paucity of data and the potential incompleteness of the evidence reviewed. None of the three RCTs have been submitted for independent replication. We aimed to identify all studies on the topic. The distorting effects of publication bias and location bias on systematic reviews and meta-analyses are well documented [26
]. None of the RCTs included in our review were fully successful in minimising bias. Collectively, these facts seriously limit the conclusiveness of our systematic review.
Our decision to exclude non-randomised trials might also be criticised. However, we strongly feel that non-randomisation introduces a selection bias that, in turn, would render any results uninterpreptible. The exclusion of RCTs comparing different dosages might be criticised. We feel that such trials would not give objective clinical information of value. Moreover, these studies cannot provide reliable data on the effectiveness of maca. Therefore, we believe the exclusion of such studies was the correct decision.
Regarding an implication for practice, this review identifies limited evidence for the use of maca in improving sexual function in healthy subjects or patients with ED. However, practitioners and clinical decision makers need to be aware that there are too many limitations to draw firm conclusion. Future trials testing the effects of maca should adhere to rigorous trial designs that adequately suit the research question being asked. Such trials should preferably be randomised, control for placebo effects, be double-blinded, adequately conceal allocation, have optimal treatment dosages and sample sizes based on proper sample size calculations, use validated outcome measures, assess other outcomes such as quality of life or partner outcome as well as adverse effects, and include a full description of the actual interventions being tested.