Supplements are imperfect and their effectiveness varies. Many of them come in multiple forms which are absorbed differently by the body. Dosage can vary by which form of a supplement you use, and some can be more effective than others for particular people depending on the specifics of their genes and current bodily chemistry.
The vitamins we eat are commonly part of a complex molecule – basically attached to something else. And there are multiple different things a given vitamin can be attached to. To actually use vitamins in our body, we generally have to process them with some chemical reactions to change their form. We don’t usually directly eat the form we use. Deficiency can be due to not converting the vitamin to another form within your body correctly, rather than due to inadequate intake. In that case, large doses of the vitamin don’t seem like the right solution, but they do sometimes help.
Thiamin is available in a hydrochloride form (HCL) or mononitrate form, which are water soluble. With those forms, the amount your body actually uses is low. If you inject it, you need around 140x lower dose than if you eat it as a pill.
TTFD is a synthetic version of thiamine which is fat soluble. Your body can use a lot more thiamine from the same dose compared with HCL. TTFD is based on allithiamine which is in garlic. It’s particularly good at getting more thiamine into the brain. Lonsdale studied TTFD and seems to consider it the most advanced or best synthetic, fat-soluble thiamine for general use.
Benfotiamine is a fat soluble thiamine that’s sold by more companies than TTFD, but it doesn’t enter the brain in the same way. You need to ingest lower doses than HCL but higher than TTFD to get around the same amount of thiamin actually used by your body.
I haven’t found very good information about dosing equivalents, but maybe TTFD is 10x more potent than HCL and benfotiamine might be 5x. That’s very rough. I think it depends on your body chemistry and no trivial conversion factor will really be accurate. The good news with dosage is that thiamin and many other nutrients have pretty high limits before doing harm from overdosing. You don’t have to target a really narrow window for the correct dose. This makes sense because our diets vary from day to day, and it’d be good for our bodies to be able to handle 10x or even 100x the daily needs of a nutrient in one meal. A lot may be wasted when you eat a bunch at once, but at least our bodies are pretty good at not getting poisoned by eating too much of a nutrient.
Synthetic, fat-soluble thiamin was mostly invented in Japan where they’ve taken nutrition more seriously, at least regarding some of the B vitamins. Japan and Europe recommend having higher levels of B12 in the blood than the US does, and Japan studied beriberi a lot after having a lot of trouble with it (white rice can provide a lot of calories without nutrients). People say that swallowing B12 isn’t very effective, and it’s better to have a tablet that you dissolve under your tongue.
It’s probably a good idea to eat a nutrient-dense diet with meat and a variety of plants, emphasizing whole foods.
I read that thiamin in food is destroyed by heat. I don’t know how much cooking destroys how much thiamin, but it’s apparently somewhat fragile. I don’t know how to effectively get a lot of thiamin from eating (non-fortified) foods.
We definitely don’t have full knowledge of what nutrients we should eat and in what forms. When you eat real food, you can get important things that are missing from our supplements. It seems to be pretty common that food versions of nutrients are more effective at the same dose because of the forms they come in, though not always (TTFD is synthetic but extra effective per amount, though it is based on an extra-effective food version in garlic, and maybe it’s less effective than real garlic). Effectiveness per amount doesn’t necessarily matter much btw. If you’re using a less effective form, you can just take more. There are presumably some potential downsides and upsides to that, and downsides seem more likely than upsides, but I think it often just doesn’t matter much and is fine.
(This is not medical advice. Do your own research.)