Information on bioavailability of bioactive compounds is paramount in understanding their intake, digestion, absorption, metabolism, and excretion (80). That will, in turn, enable traceability of bioactive compounds and their derivatives in animal products (i.e., meat and milk). Bioavailability entails describing or quantifying the specific nutrient or bioactive available at a target organ/blood circulatory system level (81, 82). Bioavailability is an integral process that has five steps which include release from feed matrix in the gastrointestinal tract (GIT) (i.e., liberation), absorption, distribution, metabolism, and excretion.

To date, there is no literature on how digestion of hemp by-products in ruminants affects bioavailability of terpenes, cannabinoids, lignans, and polyphenols (Figure 1). Overall, terpenes and CBD are volatile compounds easily released from the feed matrix, absorbed and excreted unchanged (10, 83). Terpenes digestion and absorption begins during mastication and rumination and continues throughout the GIT (83). Glucuride conjugates of CBD were detected in urine and feces as the second most abundant component next to intact CBD in animal studies (10). These findings could imply that their metabolism during and/after uptake follows conjugation (phase II) processing with hydrophilic compounds such as glycine and glucuronic acid in the liver (28, 83). Less of phase II reaction of substances might occur in the gut or blood, but the majority occurs in the liver and bile due to abundance of enzymes involved in this reaction (28).

Metabolism pathways for terpenes and CBD are believed to be the same since the two compounds and their derivatives possess similar physical and chemical properties (27, 84). Previous studies demonstrated the transfer of dietary terpenes to ruminant meat (85–87). Since terpenes are fat-soluble, adipose tissue is the main depot assessed when studying between-diet effects in ruminants (87, 88). Cannabinoid conjugates were observed in major tissues of Large White pigs injected with THC (200 μg/kg) (84). Cannabinoids and their derivatives have also been detected in milk, feces, and urine of lactating ewes injected with THC as well as in fecal and urine samples of suckling lambs (89). Some studies concluded that cannabinoids are eliminated from the body after a short period (<48 h), even with chronic exposure (10, 84).

Kuhnle et al. (90) reported lignans in beef (6–16 μg/100 g wet weight) and lamb (4–17 μg/100 g wet weight). Ruminal biohydrogenation leads to the conversion of dietary lignans to enterolactones, which are absorbed from the gut and deposited in tissues including milk (91) and meat (90). Manipulation of dietary phenolic compounds has been confirmed to change their contents in meat (92, 93). However, not all phenolics found in the diet are incorporated in similar amounts in meat. Moñino et al. (92) observed that among the 11 major phenols identified in rosemary containing diets fed to lambs, only 3 (rosmarinic acid, carnosol, and carnosic acid) significantly increased with dietary inclusion levels of rosemary. Some of the phenolics in the diet are lost in feces or biotransformed before urinary excretion (92, 94).

The α-tocopherol content of meat from steers fed HSC diets was reported to be 2.55 mg/100 g lipid (95). Dietary α-tocopherols are non-degradable in the rumen (96), hence, availed in the small intestines for absorption and assimilation into adipose tissue and cell membranes to exert antioxidant activities (97, 98). Overall, bioefficacy of bioactive compounds is closely related to the amount released from the feed matrix, absorbed, and assimilated into tissues (80, 99). In this case, response/efficacy of bioactive compounds in meat derived from ruminants fed hemp by-products diets is closely related to their bioavailability. Understanding bioavailability of hemp by-products bioactive compounds is, therefore, essential in establishing their optimum inclusion levels in ruminant diets that efficiently improve meat production and quality.