Okay here is the first reference in regards to how to administer Yohimbine effectively:
Med Hypotheses. 2002 Jun;58(6):491-5.
Pre-exercise administration of yohimbine may enhance the efficacy of exercise training as a fat loss strategy by boosting lipolysis.
McCarty MF.
Pantox Laboratories, California 92109, USA.
The natural alpha-2 antagonist yohimbine promotes sympathetic activity by central as well as peripheral mechanisms, and yet in moderate doses dose not usually raise heart rate, increase blood pressure, or induce anxiety (in contrast to sympathomimetic drugs such as ephedrine). Administered prior to exercise, it boosts lipolysis and serum FFA levels both during and following exercise; blockade of adipocyte alpha-2 adrenoreceptors makes at least a modest contribution to this pro-lipolytic activity. These considerations suggest that pre-exercise administration of yohimbine will lower the respiratory quotient during and following exercise, thus promoting fat loss. Since yohimbine can potentiate postprandial insulin secretion, its bariatric benefits should be greatest if administered on a schedule that minimizes postprandial yohimbine activity. A possible synergism of yohimbine and caffeine should be explored. Pre-exercise yohimbine administration has the potential to down-regulate the lipoprotein lipase activity of visceral adipocytes, increase lipolysis in refractory gynoid fat depots, and improve the impaired lipolytic response to exercise in the elderly.
PMID: 12323115 [PubMed - indexed for MEDLINE]
Since that really didnt tell us a whole lot from the abstract I read the full write up and found this piece which is more useful:
Since pharmaceutically significant plasma levels of yohimbine
(or its active metabolites) are sustained for 3±4
hours following an oral bolus dose (12), it would appear
advisable to administer yohimbine on a schedule such
that food is not ingested for at least 4 hours after each
dose. Thus, if yohimbine were administered prior to
morning exercise, it would be desirable to avoid food
ingestion for several hours after the exercise ± in other
words, to skip or at least substantially postpone breakfast.
This approach would be most favorable for achieving fat
oxidation, and may be feasible for the many people who
experience an absence of hunger for hours following
significant morning exercise with HT.
There are some concerns that you can get a tolerance after a while which is true to some degree:
In a study involving chronic administration of this
drug (12mg daily for 14 days), Berlan and colleagues
noted that the acute lipolytic response to yohimbine was
blunted but not abolished after chronic use (23).
So that tells us that it's optimal to use Yohimbine in a fast state prior to cardio. Plasma levels tend to peak around the 2 hour. So I'd say dosing the Y about 1.5 pre-fasted cardio would be optimal.
I suppose you could eat right after but if you want optimal fat oxidation to occur then waiting 4-6 hours to eat after dosing is the best way to go about it. It makes me think that fat loss would be extremley good if you utilized the intermittent fasting diet PLUS fasted cardio (the leangains approach to IF'ing that is...).
U of Memphis actually did a study with VPX Meltdown (primary ingredients = yohimbine, caffeine, synephrine) that showed increased fat oxidation. It's hard to say with the added ingredients in meltdown but the study and measurements were also taken after an overnight fast. So I definitely dont think Yohimbine is worth taking if you have food in your stomach.