Lactation
Animal studies
ECS involvement
In dairy cows the plasma levels of AEA and 2AG increase 2.2-2.4-fold during parturition and early lactation. In the paraventricular nucleus (PVN) this was accompanied by increased NAPE-PLD, MAGL and CB1 levels and decreased FAAH levels, suggesting increased AEA tone and decreased 2AG tone in the PVN may increase food intake and regulate energy metabolism (Kuhla et al., 2019).
In mice, stress during lactation (by subcutaneous saline injection) increased the amount of epididymal fat and induced insulin resistance in adult mice. In addition, a significantly increased abundance of CB1R and adipophilin in epididymal fat and liver was observed, together with elevated circulating levels of leptin and corticosterone (Fonseca et al., 2019). The authors suggest increased CB1 may induce fat production and insulin resistance.

Human studies
ECS involvement
Human milk contains both arachidonic acid (omega 6) and docosahexaenoic acid/DHA (omega 3) which serve as precursors for endocannabinoid production (Al, Gm, and P-Y 2019), suggesting these are essential nutrients for infant development.
Another group found that human milk contains AEA, PEA, OEA, DHEA, eicoapentaenoylethanolamine, eicosenoylethanolamine, 2AG, 2OG, 2PG, docosahexaenoylglycerol, eicosapentaenoylglycerol, eiconenooylglycerol, arachidonic acid, docosahexaenoic acid, and eicosapentaenoic acid. Only docosahexaenoylglycerol was different across transitional and mature milk (p ≤ 0.05)(Gaitán et al., 2018).
Food intake is partially regulated by OEA, SEA and PEA, which act as satiety signals. In 4-month old infants with high body weight, OEA, SEA and PEA levels were lower than in infants with lower body weight. As OEA, SEA and PEA are present in breast milk these factors may influence infant growth (Bruun et al., 2018).

Plant cannabinoids
In milk from 50 breastfeeding women, THC was detectable in 34 (63%) of the 54 samples up to ∼6 days after last reported use; the median concentration of ∆9-THC was 9.47 ng/mL (range: 1.01-323.00). Five samples had detectable levels of 11-hydroxy-Δ-9-tetrahydrocannabinol (range: 1.33-12.80 ng/mL) or cannabidiol (range: 1.32-8.56 ng/mL). The sample with the highest concentration of cannabidiol (8.56 ng/mL) did not have measurable ∆9-THC. Cannabinol was not detected in any samples. The number of hours since last use was a significant predictor of log ∆9-THC concentrations (-0.03; 95% confidence interval [CI] -0.04 to -0.01; P = .005). Adjusted for time since last use, the number of daily uses and time from sample collection to analysis were also significant predictors of log ∆9-THC concentrations (0.51; 95% CI 0.03 to 0.99; P = .039; 0.08; 95% CI 0.00 to 0.15; P = .038, respectively)(Bertrand et al., 2018).

Literature:
Bertrand, K.A., Hanan, N.J., Honerkamp-Smith, G., Best, B.M., and Chambers, C.D. (2018). Marijuana Use by Breastfeeding Mothers and cannabinoid Concentrations in Breast Milk. Pediatrics.
Bruun, S., Gouveia-Figueira, S., Domellöf, M., Husby, S., Neergaard Jacobsen, L., Michaelsen, K.F., Fowler, C.J., and Zachariassen, G. (2018). Satiety Factors Oleoylethanolamide, Stearoylethanolamide, and Palmitoylethanolamide in Mother’s Milk Are Strongly Associated with Infant Weight at Four Months of Age-Data from the Odense Child Cohort. Nutrients 10.
Fonseca, L., Castillo, V., Aguirre, C., Silva, P., Ronco, A.M., and Llanos, M. (2019). Stress during Lactation Induces Insulin Resistance Associated with an Increase in Type 1 cannabinoid Receptors in Liver and Adipose Tissue. J. Nutr. Metab. 2019, 2806519.
Gaitán, A.V., Wood, J.T., Zhang, F., Makriyannis, A., and Lammi-Keefe, C.J. (2018). endocannabinoid Metabolome Characterization of Transitional and Mature Human Milk. Nutrients 10.
Kuhla, B., Kaever, V., Tuchscherer, A., and Kuhla, A. (2019). Involvement of plasma endocannabinoids and the hypothalamic endocannabinoid system in increasing feed intake after parturition of dairy cows. Neuroendocrinology.
Salem, N., and Van Dael, P. (2020). Arachidonic Acid in Human Milk. Nutrients 12.