Limonene has its name derived from 'Lemon', the food source that is most abundant in limonene and among the first sources discovered. It is found in:
Lemons (as well as limes)
Essential oils from Orange where it acts as an aroma
Oils from mandarin and grapefruit
Teas derived from Phlomis (Lamiaceae)
Limonene is known as a monoterpene molecule (having one isoterpene in its structure) as well as a cyclic terpene (having a six-carboned ring structure). Alongside Limonene, a well-researched compound is its metabolite 'Perillyl Aldcohol'.
Limonene is colourless hydrocarbon compound that smells of oranges. Despite the scent, it is at highest levels in the rinds of lemons, where it acquires its name from.
Limonene is rapidly and almost wholly absorbed in the GI tract after ingestion, and then gets divided to various body tissues after first pass metabolism in the liver where it may be subject to metabolism into carveol metabolites or perillyl metabolites by CYP2C enzymes of which inter-species differences exist.
After a 1.6g dose, between 52-83% of the dose is excreted in 48 hours and no build up of the compound is seen 21 days after cessation. The half-life of D-limonene in humans is estimated to be between 12 and 24 hours.
The primary metabolites of limonene in humans are perillic acid, dihydroperillic acid, and limonene-1,2-diol. These metabolites are glucuronidated by the liver and excreted via the urine.
Serum limonene and metabolite levels are able to be elevated via food consumption. A 30-40 oz. serving of 'Mediterranean Style' lemonade (made with the whole lemons) contains 447-596mg limonene.  The termination half-life of perillic acid is shorter than that of D-limonene (in the range of 0.82-1.84 hours) and spiked serum levels of perillic acid 2.08 to 13.98 μM.
In those with Non-alcoholic Fatty Liver disease (onset by diet), supplementation with D-limonene has been shown (in rats) to reverse the hepatic fatty acid buildup as well as downstream effects caused by the hepatic impairment.</di|authors=Victor Antony Santiago J, Jayachitra J, Shenbagam M, Nalini N|journal=Eur J Nutr]
At this moment in time, no scientific consensus exists on D-limonene's role in adipocytes. Select anecdotes, however, suggest it may induce weight loss in high doses.
It, as well as the metabolite perillic acid, do build up in adipocytes over time, which ensures it can actually get to fat cells. Beyond that, it might exert fat-burning effects via suppression of insulin secretion in response to nutrients at pancreatic cells or perhaps vicariously through possibly being an adenosine A(2A) agonistic ligand.
It seems to exert most carcinogenic effects by prevention of protein prenylation at advanced stages of tumorogenesis, and by increasing levels of glutathione in the earlier stages as a preventative measure. Some Phase 1 enzymes of the liver are also affected positively.
Limonene is a fairly safe compound to supplement, and has a Generally Recognized as Safe (GRAS) listing for being a food additive.
It has no significant toxicity in humans. Past research indicated it may accelerate formation of tumors, however this result was found in rats and has since been shown to be of no significance to human metabolism.