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Semen contains both albumin (a protein structure) and free amino acids. The proteins come from the prostate, whereas the amino acids come from the seminal vesicles. A review of several studies found the average amount to be 5040mg/100mL, whereas albumin made up 1550mg of the 5040mg. Although an overall percentage is impossible to calculate due to varying specific gravities of semen and protein content, it is close to 50% protein by weight (although no volume).
As the higher end of average ejaculate volume is 10mL for human men, this is approximately 0.5g (500mg) of dietary protein per ejaculate on average.
Other substrates and properties
There are about 2-5mg of fructose per mL seminal fluid and it appears to be higher in men who are more fertile. Fructose is present as a fuel supply for sperm cells, and without fructose infertility would result. Breakdown of fructose via fructolysis (for energy consumption) may result in lactic acid production.
The main three catecholamines (adrenaline, noradrenaline, and dopamine) are also present in semen. Their levels tend to correlate with beneficial semen parameters, such as sperm count, motility and general fertility. This also applies to the neurotransmitter D-Aspartic Acid that is found in semen.
In general, any substance that confers protection to the semen can likely be found in the semen and this tends to include proteins and fructose (as macronutrients) and some antioxidant compounds like uric acid and Zinc
Diet and Vegetarians
It has been 'reported' (anecdotes) that vegetarian men are 'sweeter' than their omnivorous counterparts. Consumption of meat is associated with higher levels of uric acid/urate in the blood and, relative to meat consumption, serum uric acid is lower in vegetarians. Similarly to its effects in the blood, uric acid is also an anti-oxidant found in semen to protect the sperm from oxidative damage and thus it exists in sperm, displaying a correlation with serum levels. Semen also displays a high xanthine content, which is a compound structurally related to Uric Acid. This class, the xanthine-related molecules, is used as a research standard for the oral perception of bitterness, such as 6-n-propylthiouracil (PROP) and Caffeine. It is highly possible that a vegetarian diet results in reduced levels of uric acid in the semen, and the reduction in perceived bitterness allows an enhancement of the perceived sweetness from semen's fructose content (slightly sweeter than table sugar on a gram per gram basis).
There is some scientific basis for the idea that vegetarian men 'taste' better, as the exclusion of meat has the effect of reducing the uric acid concentration in semen. However, a proper intervention study measuring seminal uric acid levels has not been conducted
Some supplements have been noted to alter the taste of semen, although science on this exact claim is somewhat lacking (an ethics board may not approve of such 'blinded' studies). Bromelain is a mixture of compounds from pineapple and possesses volatile aromatics that appear to make semen taste like pineapple; this seems to occur at a cup of pineapple daily. Fenugreek has also been reported to make semen taste like maple syrup taste, probably through the aromatic compound sotoline, which can also make your urine smell like maple syrup when one is supplementing with fenugreek.
Vanilla has been reported to have its aromatics present in semen.
Yes, it is likely that some supplements and foods can alter the taste of semen, but there is a lack of proper research on this topic