Testosterone is normally produced in the human body and serves a variety of roles beyond simply building muscle, as it ranges from cardio protection to preservation of the brains functions (beyond its roles in sexuality and fetus formation). In line with its importance and similar to most hormone or signaling molecules in the human body, it has a whole system of enzymes and regulation steps which attempt to prevent testosterone levels from getting too high or too low.
This tight regulation is partly why many supplemental testosterone boosters do not work over the long term, and if we use D-aspartic acid as a notable example its ingestion is able to increase a rate limiting step (increased activity of the StAR enzyme) and increase testosterone but is eventually regulated at another step (increased activity of D-amino acid oxidase or DAO, which degrades D-aspartic acid) eventually returning to the levels of testosterone seen before supplementation. The only reliable and potent way to manipulate this pathway is, well, injecting testosterone straight.
Despite the difficulties in increasing testosterone to high levels over the long term, if its regulatory pathways are suppressed then they can be suppressed to a small degree for a long period of time and eventually make a significant impact on the person. These ‘deficiencies’ are usually from suboptimal or outright deficient intake of some important nutritive factors, and while dietary protein or fatty acids deficiencies (which do impair testosterone production) are almost unheard of there are some vitamins or minerals which are a bit more commonly deficient.
If you are deficient in these following molecules, but only if you are deficient in these molecules, then supplementation of them will increase testosterone levels (actually normalize them to what they should be, but it is a relative increase).
Zinc is a dietary mineral found in the supplement ZMA, usually recommended as a sleep aid and testosterone booster. Zinc deficiencies are brought on by diets very low in meat and animal products, or from excessive sweating during exercise. Testosterone levels decline during a zinc deficiency. Supplementation of zinc is able to increase testosterone, but only to normal levels. Dosing more zinc than needed to correct a deficiency will not raise testosterone levels past normal levels.
You can see if you are deficient in zinc by either getting a blood test or, if either unwilling or unable to get a blood test, measure your dietary zinc intake. Use a free program such as Wolfram Alpha (A website that can be described as google bred with a TI-84, catered towards the overanalytic) and input your dietary intake for three days where at least one day is a weekend day. If you are continually consuming less than 50% of the RDA or less than 75% of the RDA while also being highly active (since zinc is lost in sweat) then supplementation or at least modification of the diet to hit 100% would be advised.
Supplementation of a low dose of zinc (5-10mg of any form except L-threonate) is unlikely to be harmful, and that particular form is advised against due to it being near perfect absorption. In the context of dietary minerals, ‘near perfect’ absorption is actually a horrible idea in many cases since high doses of any mineral can per se be potent oxidants and the poor absorption of high doses is actually a way the body regulates intake to prevent damage.
Magnesium is the other half of the ZMA supplement, and can also be lost through sweat. Moderate to severe deficiency states are associated with lower testosterone production. Supplementing magnesium can increase testosterone to normal levels. Like zinc, supplementation of extra magnesium will not raise testosterone levels above normal.
To find out if you are deficient in magnesium, you can either measure levels of zinc in your red blood cells (erythrocytic magnesium) or if you are not willing or able to get a blood test you can measure your food intake over the course of three days (including one weekend day) or a whole week and try to calculate your dietary magnesium intake; for this purpose, you can again use Wolfram Alpha for free (albeit a bit tedious) calculations.
If you routinely consume less than 50% of the RDA for magnesium there is a pretty high chance for deficiencies, and if you consume less than 75% of the RDA and are athletic (due to mineral losses in sweat) then supplementation may be advised. Similar to zinc, if you are able to manipulate your food intake so you are above these levels then supplementation is not required and lowballing magnesium supplementation (100-200mg of any form but L-threonate) is unlikely to be harmful.
Vitamin D is known to have a myriad of health benefits. It can also support proper testosterone levels. An individual can have sub-optimal levels of Vitamin D, but not be deficient. Overweight men supplementing 3,000 IU of vitamin D daily have experienced an increase in circulating testosterone levels. Vitamin D may also be linked to testosterone production.
Unlike the above two minerals, you are likely deficient in Vitamin D except for instances where you love the outdoors near the equator or consume over a kilogram of salmon daily (or perhaps have a high dose of vitamin D through consumption of cod liver oil) and supplementation would be recommended.