[Espi]

Indeed, B12 supplementation by injection is quite popular among bodybuilders. Less so among 'lay people', hence why sublingual tablets are so good. It works almost equally well and is less painful/intimidating.

Meanwhile an interesting abstract was posed on BR.com by Lyle. May explain why my metabolism hasn't dropped yet.

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Clin Endocrinol (Oxf). 2007 Jan;66(1):49-57.
Short-term fasting-induced autonomic activation and changes in catecholamine levels are not mediated by changes in leptin levels in healthy humans.

* Chan JL,
* Mietus JE,
* Raciti PM,
* Goldberger AL,
* Mantzoros CS.

Division of Endocrinology, Diabetes, and Metabolism, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, MA, USA.

Objective In animal models, the adipocyte-secreted hormone leptin increases energy expenditure by increasing sympathetic outflow but its role in humans remains to be elucidated.
We evaluated whether inducing hypoleptinaemia (with and without administration of leptin at replacement doses) for 3 days would influence catecholamine levels and sympathetic and parasympathetic activity in healthy humans.

Methods We studied six normal-weight subjects in the General Clinical Research Center (GCRC) under three conditions:
baseline fed state (control study) and
two 72-h fasting studies (to decrease leptin levels), with administration of either placebo or
replacement-dose recombinant methionyl human leptin (r-metHuLeptin)
in a randomized, double-blind fashion.
In each condition,
24-h urinary catecholamine levels,
heart rate and
heart rate variability (HRV),
a standard tool for assessing cardiac autonomic modulation, were measured.

Results Study parameters remained stable during the control condition and the baseline assessment of all three studies.
In response to 72-h fasting, which decreased serum leptin levels by 80%, 24-h urinary norepinephrine and dopamine levels and heart rate increased while cardiac vagal modulation decreased (all P < 0.05).

Replacement-dose r-metHuLeptin to keep leptin levels within the physiological range during fasting did not alter fasting-associated changes in heart rate, catecholamine levels or cardiac vagal tone.

Conclusions The findings of this controlled, interventional study indicate that changes in heart rate, catecholamine levels and cardiac vagal modulation associated with 72-h fasting are independent of regulation by leptin.
Thus, changes in leptin levels within the physiological range do not seem to play a role in regulating autonomic function during short-term starvation in healthy humans.