Fish Sauce vs Soy Sauce: Why Thai Recipes Use Both
Because they carry different molecules. Soy sauce is glutamate-rich. Fish sauce brings glutamate and inosinate — a nucleotide that tastes of almost nothing alone but multiplies glutamate's umami roughly eightfold when the two meet. A recipe asking for both isn't double-salting. It's stacking two compounds that amplify each other.
They're not doing the same job
Open a Thai recipe and you'll often find both bottles listed. It reads like redundancy — two brown salty liquids, surely one would do.
They're made from entirely different things by entirely different processes, and the molecules they deliver aren't the same.
| Fish sauce | Soy sauce | |
|---|---|---|
| Made from | Anchovies and salt | Soybeans and wheat |
| Process | Salted and fermented 12–24 months; the liquid is drawn off | Koji mould, then months to years in brine |
| Umami compounds | Glutamate + inosinate | Glutamate |
| Flavour beyond salt | Savoury depth, background meatiness | Roasted, malty, faintly sweet |
| Colour | Amber, thin | Brown to near-black, thin to syrupy |
| Contains gluten? | Usually not — check the label | Usually yes (tamari is the wheat-free version) |
| Vegetarian? | No — vegan versions exist | Yes |
That third row is the one that matters, and it's the one no recipe ever explains.
Umami isn't additive. It's multiplicative.
Three molecules produce or shape umami, and they don't behave the way you'd assume.
Glutamate is an amino acid and it produces umami directly. It's in soy sauce, tomatoes, aged cheese, seaweed, garlic.
Inosinate (IMP) and guanylate (GMP) are nucleotides. Inosinate is concentrated in meat and fish; guanylate in dried mushrooms, shiitake especially. On their own, these two taste of almost nothing. Put either one next to glutamate and something strange happens.
The umami doesn't add. It multiplies. In humans, the response to a glutamate-and-inosinate mixture is roughly eight times larger than the response to glutamate alone — and the effect is more sustained too, lingering rather than flashing.
Which reframes the whole question. Fish sauce isn't a saltier soy sauce. It's a source of the other category. Adding it to a glutamate-rich pot doesn't increase savouriness by some increment; it changes what the glutamate already in there is capable of.
One tom yum, three umami compounds
Here's what makes this more than theory. I went through the ingredient list for the tom yum taught in Shawnie Kelley's live Thai class on P2Bridge, and it hits all three categories — without ever mentioning that it does.
- 3 tablespoons fish sauce — glutamate and inosinate
- 2 tablespoons soy sauce — glutamate
- 6 ounces cremini or shiitake mushrooms — guanylate
- Tomato and garlic — more glutamate
- Shrimp — more inosinate
Glutamate, inosinate and guanylate in one pot. Most working recipes hit one category, good ones hit two. Hitting all three is why a proper tom yum tastes like considerably more than the sum of a sour broth with some prawns in it.
One honest note on the mushrooms: her list offers cremini or shiitake, and they aren't equivalent here. Dried shiitake carry substantially more guanylate. If you want the synergy at full strength, take the shiitake — and use the soaking liquid.
Nobody designed that recipe from a molecular diagram. It's a traditional dish that arrived at this combination through several centuries of cooks tasting things. The chemistry is the explanation, not the origin.
What each one actually tastes like
Chemistry explains why they work together. It doesn't tell you what you're tasting, and the two sauces sit in genuinely different places.
Fish sauce smells, out of the bottle, like something has gone wrong. It is pungent and unmistakably fishy and it puts people off permanently. That smell has very little to do with what it does once it's in a hot pot with aromatics and acid. Cooked, it reads as a savoury backbone — a background meatiness that makes a broth taste like it simmered for hours.
My advice, which I'd have ignored myself once: don't judge it by the bottle. Nobody drinks it neat, and the finished dish doesn't taste of fish. If your tom yum tastes thin and you're wondering what's missing, it's usually this.
Soy sauce is roasted, malty, a bit sweet under the salt — the wheat and the long fermentation produce browning compounds and a rounder, less pointed flavour. It fills in rather than sticking out.
In the Thai balance of salty, sour, sweet and spicy, both live in the salty corner, but with different accents. Fish sauce brings depth. Soy brings roundness. That's the culinary answer sitting on top of the chemical one, and we pulled that balance apart in tom yum vs panang.
Light soy, dark soy: the split most cooks miss
There's a second distinction hiding inside "soy sauce," and most Western kitchens own one bottle without knowing there are two.
Denise Fan's pot sticker class on P2Bridge uses three separate soy entries, at deliberately different amounts:
- 1 teaspoon light soy sauce in the pork filling
- 1 tablespoon light soy sauce in the dipping sauce
- ½ tablespoon dark soy sauce in the dipping sauce
That's not a cook being fussy. The two bottles do different jobs.
Light soy sauce is thinner and saltier. It's the seasoning bottle — the one you reach for when a dish needs salt with a savoury edge. Confusingly, "light" refers to colour and viscosity, not sodium; light soy is the saltier of the two.
Dark soy sauce is aged longer, thicker, often with molasses added, and noticeably less salty. It's there for colour and a faint caramel sweetness. Use it as your only soy and the dish comes out under-seasoned and oddly dark.
Notice too what her dipping sauce doesn't contain: any fish sauce at all. Light soy, dark soy, rice wine vinegar, sugar, garlic, sesame oil, chilli. It's a Chinese-style sauce built entirely on the glutamate side — which is the clean counter-example to the Thai approach and a useful thing to taste back to back.
Taste the soy-only version for yourself
Denise Fan teaches pork and cabbage pot stickers with a dipping sauce built on light and dark soy — the same sauce family, no fish sauce, and a good way to feel where the difference actually sits on your palate.
We covered the folding and filling side of that class in pot stickers and dumplings.
A century of cooks who were right before the science
The molecules are recent. The technique isn't.
Kikunae Ikeda identified glutamate as the source of umami in 1908, working from kombu seaweed. One of his students, Shintaro Kodama, found inosinate in dried bonito flakes a few years later. Guanylate came last — Akira Kuninaka isolated it from shiitake broth in 1960, and it was Kuninaka who discovered the synergy effect itself.
The story of that discovery is better than it needs to be. Kuninaka tasted inosinate and found it weak. He tasted glutamate and found it much stronger. Then he tasted the inosinate again without rinsing his mouth — and got a wallop of umami, because the inosinate had met the glutamate still on his tongue.
Meanwhile Japanese dashi — kombu plus katsuobushi, glutamate plus inosinate — had been made that way for centuries. So had Thai cooks been reaching for both bottles. Nobody could explain it. Everybody could taste it.
That's the ordinary way cooking works: technique arrives first, and the explanation shows up a few hundred years later to confirm what the cooks already knew.
Both of these are salt
Easy to forget when you're thinking about flavour compounds. These are salt-delivery systems as much as seasonings, and they should be counted as salt when you season the rest of the dish.
Denise's published nutrition panel puts the pot sticker recipe at 3,870mg of sodium across the whole batch. That's not a criticism of the recipe — it's a dumpling recipe with three soy additions and a salted cabbage step, and it's meant to be shared. It's just worth knowing the number is there.
The practical upshot: if a recipe calls for both fish sauce and soy sauce, taste before adding any further salt. It very likely doesn't need it.
Substituting one for the other
| Situation | What to do | What you lose |
|---|---|---|
| No fish sauce, have soy | Use soy at roughly the same volume | The inosinate. Dish tastes flatter — add dried shiitake or a little miso to compensate |
| No soy, have fish sauce | Use less, maybe two-thirds | The roasted malty note. The funk sits further forward |
| Vegetarian or vegan | Vegan fish sauce, or soy plus dried shiitake and a little seaweed | Less than you'd think — seaweed and mushrooms cover both categories |
| Gluten-free | Tamari instead of soy sauce | Nothing meaningful. Check the fish sauce label too — some brands add wheat |
| Want maximum depth | Use both, plus dried shiitake | Nothing. This is the full stack |
On the vegetarian question specifically: Shawnie's own class list offers vegan fish sauce as a documented substitution, and tofu in place of the shrimp. That's a working instructor's answer rather than a workaround someone invented on a blog, and it's the version I'd trust.
You can absolutely make a decent tom yum with soy sauce alone. It just won't have the same bottom end, and once you've tasted the two side by side you'll notice the gap every time.
Learn it live with a chef
Seasoning is the hardest thing to learn from writing. "Add fish sauce to taste" assumes you already know what you're tasting for — and the moment a broth goes from thin to complete is much easier to recognise when someone points it out as it happens.
★★★★★ 5.03 reviews · 12 experiences
Cuisine Specialist · United States · Chef Instructor, food & travel writer · 18+ years, five published books
Leads culinary programs for Columbus State in Ohio. Her tom yum is the full umami stack — fish sauce, soy sauce and mushrooms in one pot — and her ingredient list includes a vegan fish sauce swap for anyone who needs it.
★★★★★ 5.03 reviews
Verified Host · South Africa · 10 years experience · English + Afrikaans
The soy-only side. Her dipping sauce uses light and dark soy at different quantities for different jobs, with no fish sauce at all — the clearest way to learn what each bottle contributes.
More on the site: the full culinary catalogue, upcoming group sessions, class packages, every verified host, or request a custom experience.
Frequently asked questions
Why do Thai recipes use both fish sauce and soy sauce?
Because they deliver different umami compounds. Soy sauce is glutamate-rich, while fish sauce carries glutamate plus inosinate, a nucleotide that multiplies glutamate's umami rather than adding to it. Using both produces a deeper, longer-lasting savouriness than either could alone.
Can I substitute soy sauce for fish sauce?
Yes, at roughly the same volume, and the dish will still taste seasoned. What you lose is the inosinate, so the result tastes flatter and less rounded. Compensate by adding a dried shiitake mushroom, a little miso, or a piece of seaweed to restore some of the missing depth.
What is umami synergy?
The effect where glutamate combined with a nucleotide — inosinate or guanylate — produces far more umami than glutamate alone. In humans the response to a glutamate and inosinate mixture is about eight times larger. The nucleotides produce almost no umami by themselves.
What is the difference between light and dark soy sauce?
Light soy sauce is thinner, saltier and used for seasoning. Dark soy sauce is aged longer, thicker, often contains molasses, and is less salty — it is used mainly for colour and a faint caramel depth. Light refers to colour and viscosity, not sodium content.
Does fish sauce make food taste fishy?
No. It smells strongly fishy from the bottle, but that aroma largely cooks off. In a finished dish it reads as savoury depth and background meatiness rather than fish. If a Thai broth tastes thin and you cannot identify what is missing, a lack of fish sauce is the usual cause.
Is there a vegetarian substitute for fish sauce?
Yes. Commercial vegan fish sauces exist and work well. You can also combine soy sauce with dried shiitake mushrooms and a piece of seaweed, which covers both the glutamate and the nucleotide side. Some live cooking classes list vegan fish sauce as a standard substitution.
Is soy sauce gluten-free?
Usually not. Most soy sauce is brewed from soybeans and wheat. Tamari is the wheat-free version and works as a direct replacement. Fish sauce is typically gluten-free since it is made only from fish and salt, but a few brands add wheat, so check the label.
Which mushrooms boost umami the most?
Dried shiitake. Guanylate concentrates during drying, and shiitake is the richest common source — it is the mushroom Akira Kuninaka used when he isolated the compound in 1960. Fresh cremini contribute some, but noticeably less. If you use dried shiitake, keep the soaking liquid.
Related reading: coconut milk vs coconut cream vs cream of coconut, palm vs brown vs coconut sugar, and Korean banchan and the twelve-dish table.
How we sourced this
The umami science comes from peer-reviewed research on umami receptor mechanisms and from the Umami Information Center, both linked below. The approximately eightfold synergy figure and the Ikeda, Kodama and Kuninaka history are drawn from the peer-reviewed source. We did not cite sauce manufacturers.
Class details, prices, ratings and ingredient quantities were read directly from the live P2Bridge class pages on 24 August 2026. Every quantity attributed to a host appears in that class's published ingredient list. Ratings are shown with review counts — see the P2Bridge reviews page for the underlying feedback. Prices change; check before booking.
What this article does not claim. Sodium figures are quoted from a published recipe panel and are not dietary advice. Glutamate and nucleotide levels vary widely by brand, production method and ageing, so treat the categories as reliable and the magnitudes as approximate. We have not described the contents of classes whose ingredient lists we could not read.
