It's somewhat rare that there's a nutritional supplement that works to improve performance in low doses and has the opposite effect with higher ones. Caffeine comes to mind as one such example, where the right dose can be a performance and focus enhancer, but too much can turn you into a jittery mess. Bulbine natalensis is another example, where the right dose can boost testosterone but the wrong dose can actually lower it (thereby providing me with hours of laughs at the company who purports to have pioneered a 100:1 extract of the herb).
In laymen's terms, this is the "too much of a good thing" phenomenon, while in the scientific world it is known as an "inverted U-shaped dose/response curve" [Scientists are always overcompensating]. Interestingly, it appears that glycine propionyl-l-carnitine (which most of us simply call GPLC) is another supplement that falls into this category. Take too much and you'll actually reduce your power output, but take just enough and you'll increase it.
Never even heard of the stuff? Check this out before you read the rest of the article:
Here's how the research breaks down:
Forty-five men with experience in weight training were put through a participated in a double-blind, controlled research protocol. The subjects underwent two testing sessions one week apart, an hour and a half after randomly ingesting either 4.5 grams GPLC or 4.5 grams of placebo (in this case cellulose).
The men performed 5 x 10-second Wingate cycle sprints with one minute of active rest between the efforts for two sessions. After session number two, they received 1.5 g, 3.0 g, or 4.5 g GPLC per day for a 28 day period, after which they were retested.
There were no significant effects of condition or significant interaction effects detected for Peak Power (PP) and Mean Power (MP). But there was a dip in performance for sprints three, four and five, which produced 2 - 5% lower values of PP:
While there was a 3-7% lower value of MP with the two higher doses of GPLC (3.0 or 4.5 g per day):
However, the lowest dose of GPLC (1.5 g GPLC/day) netted 3 - 6% higher values of PP and 2 -5% higher values of MP (compared to the placebo group). compared with PL baseline values. The lowest dose group not only displayed a 24% reduction in net lactate accumulation, but also produced a lower average reduction in power - again, besting the higher dose groups - which actually showed a performance decrease!