excerpts from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7278738/

Most of the body’s daily requirement for CoQ10 is produced within the body. Some CoQ10 is ingested from food (typically 5 mg/day [5]). It is estimated that the daily requirement for CoQ10, from both endogenous bio-synthesis and food sources, is about 500 mg; this estimate is based on a total body quantity of about 2 mg of CoQ10 and based on an average turnover time of 4 days in tissue [5]. This estimate serves as the justification for the size of the dosage (typically 300 mg/day) that is used in many clinical trial studies.

As people mature into adulthood and begin to increase in age, the ability of the body to synthesise its own CoQ10 decreases; optimal human bio-synthesis occurs in the mid-twenties, with a continual gradual decline in tissue levels thereafter [6]. In addition to the normal aging process, CoQ10 levels have also been shown to be depleted in a number of disorders, particularly heart disease [7]. The synthesis of CoQ10 is a complex multistage process (governed by at least 13 genes), requiring a number of amino acids, vitamins, and trace element precursors and cofactors, deficiency of any of which can adversely affect normal CoQ10 production [8]. Nutritional supplementation with CoQ10 therefore provides a mechanism to maintain adequate levels within the body.

...The CoQ10 raw material comes in the structural form of polymorphic crystals [9]. CoQ10 in polymorphic crystalline forms will not be absorbed in the gastro-intestinal tract. CoQ10 can be absorbed only as individual molecules, as described in Section 4 of this article below [10]. The CoQ10 crystals must therefore be dissociated first into individual CoQ10 molecules.

...The first point of note is the difference in bioavailability between samples 01 and 02 of the study. Both samples contained 100 mg CoQ10 in identical ubiquinone form, in a soy carrier oil with identical excipient content and capsule specification; sample 01 (Myoqinon, Pharma Nord, Vejle, Denmark) had been subject to a patented thermal crystal dispersion process whilst sample 02 (from the same manufacturer) had not undergone this procedure. The respective mean AUC and Cmax values in the Lopez-Lluch study were 28.0 mg/L/48 h and 1.07 mg/L for sample 01, and 6.89 mg/L/48 h and 0.33 mg/L for sample 2. Failure to subject crystalline CoQ10 to crystal dispersion therefore reduced bioavailability by approximately 75%.

...Thus, the AUC for CoQ10 in ubiquinol form was approximately twice of that for ubiquinone, which had not been subjected to thermal crystal dispersion (sample 02), but was only 52% of that for ubiquinone that had been subjected to thermal crystal dispersion (sample 01). It should be noted that it is not possible to subject ubiquinol to the same crystal dispersion process as that described for sample 01; this is because of the differential stability of ubiquinone and ubiquinol. These data therefore demonstrate (i) the importance of CoQ10 crystal dispersion, the absence of which reduces bioavailability by some 75%; (ii) the fact that the relative bioavailability of ubiquinone and ubiquinol forms of CoQ10 depends on CoQ10 crystal dispersion status and carrier oil/excipient composition.

... The study therefore concluded that intestinal absorption of CoQ10 is highly variable among individuals, and is independent of the form of CoQ10 (ubiquinone/ubiquinol) administered. This is because the body is limited in how much CoQ10 it can absorb at a given time, and thus if an individual takes several 100 mg capsules together, some of the CoQ10 tends to be wasted
CoQ10 is a lipid soluble substance; therefore, the highest quality CoQ10 preparations have the CoQ10 dissolved in a well-suited carrier lipid (e.g., soy oil or palm oil).

The KISEL-10 study was a clinical trial that enrolled 443 senior citizens living in south-eastern Sweden. The study was designed as a randomized, double-blind, placebo-controlled clinical trial. The elderly study participants—average age of 78 years—took either 200 milligrams of coenzyme Q10 together with 200 micrograms of an organic high-selenium yeast preparation or matching placebos daily for 4 years

The cardio-protective effect of the combined daily supplementation with CoQ10 and selenium endure for several years beyond the period of intervention. A follow-up analysis showed that the significantly reduced risk of death from heart disease still held 12 years after the intervention