What are metabolic enzymes, and how do they differ from digestive ones? It comes down first to where they work: digestive enzymes cut up your food in the digestive tract, like a cutting workshop, while metabolic enzymes assemble and transform matter inside each of your cells, like a factory. At Biovie, we built this page on six scientific sources, from the major review on pancreatic enzymes to the EFSA opinion on lactase.
Same nature, same name, two jobs. Enough to get your wires crossed. An enzyme is a protein that speeds up a chemical reaction without wearing out: picture a carpenter in blue overalls who walks out of the workshop in the evening with his tools intact, ready to start again the next day. So if you wonder "what are metabolic enzymes?", the answer is not about what they are, it is about the place where they work.
I have been interested in enzymes since the early 2000s, when I discovered living food at a time when almost everything was written in English, with no YouTube and no Facebook (only a handful of us in France were looking into the subject, UFOs, basically). My engineering training at ISTOM left me with a reflex: take a mechanism apart before believing in it. That is what I suggest we do here, piece by piece, all the way to a question that is rarely settled: can a swallowed enzyme become a metabolic enzyme?
What are metabolic enzymes, and how do they differ from digestive enzymes?
Digestive enzymes, produced by the salivary glands, the stomach, the pancreas and the intestine, cut food into small molecules, outside the cells. Metabolic enzymes, for their part, never leave home: they work inside the cells, where they chain together thousands of reactions that turn these molecules into energy and building material.
The table below sets digestive enzymes vs metabolic enzymes side by side, criterion by criterion.
| Criterion | Digestive enzymes | Metabolic enzymes |
|---|---|---|
| Where they act | Digestive tract, outside the cells | Inside every cell |
| Who makes them | Salivary glands, stomach, intestine and above all the pancreas | Each cell, from its own genes |
| Role | Cut food into molecules small enough to be absorbed | Transform and assemble the absorbed molecules |
| Examples | Amylase, pepsin, lipase | Cellular respiration enzymes, liver enzymes |
| Order of magnitude | A few major families | 13,417 reactions in the Human1 model |
| If you swallow them | May act in the digestive tract, then are digested | Digested like proteins; only a trace may reach the blood |
If you had to keep just one line of this table, it would be the first one. The place tells you the job.
A word, in passing, on the word "enzyme" itself, which gets thrown around a bit everywhere: it is a catalyst, that is, a molecule that makes a reaction go much faster without being consumed by it. If you want to go back to basics, I have covered what an enzyme is in a dedicated article.
Digestive enzymes: scissors that work outside your cells
In the biological sense, the inside of the digestive tract is outside the body. Think of a bead threaded on a string. The string runs right through it, and yet it never entered the bead. Your food does the same, and until it has been cut fine enough to cross the intestinal wall, it stays outside.
Mouth, stomach, pancreas, intestine
It all starts in the mouth, where salivary amylase attacks the starch in a mouthful of rice (chew a grain of rice for a long time and it ends up tasting slightly sweet, that is amylase at work). The stomach takes over with its acidity and pepsin, which starts on the proteins. The bulk of the job is then done in the small intestine, because the pancreas produces most of the enzymes that cut proteins, fats and starch (Whitcomb and Lowe, 2007).
Three families: amylases, proteases, lipases
Each has its target, each has its tool. Amylases cut starch into smaller sugars: we speak of dietary amylase for the one that acts on the meal, whether it comes from saliva or from the pancreas. Proteases cut proteins down to amino acids. Lipases, finally, deal with fats (here are the different types of fats they come across).
When I'm asked what the different types of enzymes are in digestion, I answer with these three families. I still owe you one honest detail: the Whitcomb and Lowe review points out that a large part of this knowledge comes from animal studies, re-examined since then thanks to the human genome.
We are not what we eat but what we absorb, I am convinced of it, and this cutting up is its first condition.
Metabolic enzymes: thousands of workshops in every cell
Once the intestinal wall has been crossed, the small molecules pass into the blood, then into the cells, and there begins a completely different job. Picture an assembly line where carpenters in blue overalls each make a single gesture before passing the piece to their neighbour. A metabolic pathway is exactly that: a sequence of reactions in which every link has its own enzyme, and the product of one becomes the raw material of the next.
How many workstations in this factory? The reference model of human metabolism, Human1, lists 13,417 reactions, 4,164 unique metabolites and 3,625 genes (Robinson et al., 2020). The figure is dizzying, but the engineer in me insists on saying where it comes from. This figure comes from a computer model that merges several earlier models: nobody counted the enzymes one by one. See it as a city directory, where every line would be a reaction.
So what are metabolic enzymes for, and what are the functions of enzymes in the body more broadly? The answer fits in two verbs: transform and build. Those of cellular respiration draw energy from nutrients, those of the liver transform many molecules brought by the blood. The body makes its metabolic enzymes itself, from its genes.
Many of them need a partner, a cofactor: often a mineral, or a molecule derived from a vitamin, which slots into the enzyme like a key in the ignition. Without the key, the engine is there. It just does not turn.
Can a swallowed enzyme become a metabolic enzyme?
The starting reasoning is disarmingly simple. An enzyme is a protein, and the stomach and the pancreatic proteases cut up proteins without asking for their papers: a lactase or a piece of lentil goes through the same cutting. A swallowed enzyme is therefore largely digested, like any protein on the plate. Largely, not entirely. A trial carried out in 19 healthy men measured bromelain, an enzyme from pineapple, in their blood after oral intake (Castell et al., 1997). At 3 g per day, the researchers found up to 5,000 pg/ml in the plasma after 48 hours, with part of the activity preserved. Their conclusion speaks of a "low but significant" intestinal passage of undegraded proteins.
Low, how low? A picogram is a thousandth of a billionth of a gram. So those 5,000 pg/ml amount to a few millionths of a gram per litre of plasma, for 3 grams swallowed every day: a trace, measurable but tiny.
The authors also estimated a half-life of 6 to 9 hours, that is, the time after which half of the quantity present has disappeared. A queue that would shrink by half with every slice of time, in short.
Of course, this result is solid, and it is also limited: it shows that a small fraction can cross the intestinal wall, nothing more. It does not show that this enzyme reaches your cells, still less that it takes the place of a metabolic enzyme, which the cell makes itself from its genes. On the route swallowed enzyme, blood, cell, the data stop at the blood. Beyond that, nothing, and the difference between digestive enzymes and metabolic enzymes remains intact.
What studies say about digestive enzymes as supplements
If a swallowed enzyme does not replace a metabolic enzyme, it can on the other hand work where digestive enzymes work: in the digestive tract, at mealtime. That is where the most solid data are found.
First case, lactase. In 2009, EFSA concluded that lactase contributes to the breakdown of lactose in people whose lactase production is insufficient, with 4,500 FCC units with each meal containing lactose (EFSA, 2009). FCC is a unit of activity, not a weight, and that makes sense: you judge a carpenter by the planks he cuts, not by his build. This benchmark is to be read as part of a varied and balanced diet and a healthy lifestyle.
For alpha-galactosidase, a randomised placebo-controlled trial brought together 8 healthy volunteers around a meal of 420 g of cooked beans (Di Stefano et al., 2007), which is a big, well-filled soup plate. At 1,200 GalU, exhaled hydrogen and the severity of flatulence went down. Eight healthy people is not many, and nothing can be concluded from it for those who suffer from digestive disorders.
That leaves the medical case of pancreatic insufficiency, when the pancreas no longer produces enough enzymes: enzymes of animal origin remain the reference there, on prescription and with medical follow-up. A non-systematic review considers plant and microbial enzymes promising (Ianiro et al., 2016).
And if the question on your mind is rather "should you take digestive enzymes?", I answer it in a dedicated article. At Biovie, it is in fact this family, the better studied of the two, that we chose to offer you first, with Enzymes Assimil.
