Digestive Enzyme Activity Units Explained: HUT, DU, SKB, GDU, FIP and USP

July 28, 2026|Barry Lees

Digestive enzyme weight compared with measured activity units HUT, DU, SKB, GDU, FIP and USP

Direct answer: Milligrams measure how much an enzyme preparation weighs; activity units report its measured activity under a defined laboratory assay. To compare labels, match the same enzyme, activity method and serving basis. HUT cannot be compared with GDU, and SKB should not be converted to DU without valid product-specific information.

Commercial disclosure: We sell a Digestive Enzymes formula and print weights plus supplier-declared activity figures. This guide also explains where our own declaration could be more specific.

Why enzyme weight is not enough

An enzyme preparation is not pure enzyme alone: it can contain carrier material and can vary in concentration. Two preparations weighing 10 mg may therefore have very different measured activity. Weight is still useful for identifying the amount of preparation, but it is not a substitute for activity.

An activity unit is tied to an assay—a defined substrate, temperature, pH, time and calculation. That is why labels use different units for different enzymes and why numbers from different methods should not be casually converted.

The safe comparison rule: same enzyme + same activity unit + same serving basis. If any of those three changes, the headline numbers may not be comparable.

What does FCC mean?

FCC refers to the Food Chemicals Codex, a compendium maintained by the United States Pharmacopeia containing identity, quality and assay standards for food ingredients. “FCC” is not one universal activity unit that makes every enzyme comparable. It identifies the standards framework; individual enzymes still use method-specific units such as DU, HUT, ALU, CU or LU.

A label should ideally state both the unit and, where relevant, the standards system used.

The main units found on digestive-enzyme labels

Unit	Usually associated with	What it tells you
HUT	Protease	Proteolytic activity using a haemoglobin/tyrosine-basis method
DU	Amylase	Dextrinising activity under the stated method
SKB	Amylase	Activity under the Sandstedt–Kneen–Blish method
FIP	Lipase	Lipase activity under a pharmacopoeial method
GDU	Bromelain	Gelatin-digesting activity
PU or USP units	Papain or other proteolytic preparations	Activity defined by the named assay; check which PU method is meant
ALU / FCC lactase units	Lactase	Lactase activity; relevant to the authorised GB lactose-digestion claim

HUT: protease activity

HUT stands for Haemoglobin Units on a Tyrosine basis. It is used for certain protease preparations. A product declaring 7,500 HUT per serving declares more HUT activity than one declaring 600 HUT per serving, provided both use the same recognised method and serving basis.

Do not add a GDU or PU figure to HUT to create a “total protease score”. Those units come from different tests and are not additive.

DU and SKB: two amylase methods

DU means Dextrinising Units. SKB refers to the Sandstedt–Kneen–Blish method. DU and SKB both relate to amylase activity, but they are method-specific and should not be treated as interchangeable without a validated product-specific conversion.

If one label gives 2,250 SKB and another gives 3,000 DU, you cannot decide which is stronger from those figures alone. Ask for a valid conversion or an activity result expressed using the same method.

FIP and “lipase units”

FIP is a defined activity method used for lipase. Some labels instead use FCC LU or simply say “units”. A bare “units” declaration is less useful because the assay is not identified.

Our current label states 330 lipase units per capsule but does not identify the assay. That prevents a rigorous comparison with a product declaring FIP or FCC LU until the manufacturer’s technical specification confirms the method.

GDU: bromelain activity

GDU means Gelatin Digesting Units and is used for bromelain. Compare GDU with GDU at the same serving size. Do not compare it directly with FCC PU, HUT or milligrams.

Bromelain weight can still be shown, but the same milligram amount can have different activity depending on the raw material.

Papain: USP units and PU

Papain labels may use USP units or a PU designation. Because “PU” can be used in different contexts, the standards method should be clear before comparing products. “0.3 USP units” and “1,500 PU” are not safely comparable simply because both refer to papain activity.

Lactase: the unit with a specific GB claim threshold

The authorised claim is: “Lactase enzyme improves lactose digestion in individuals who have difficulty digesting lactose.” It may be used only for a food supplement providing at least 4,500 FCC units, with instructions to consume it with each lactose-containing meal. Consumers must also be told that lactose tolerance varies and that they should seek advice about the role of lactase in their diet.

This is an important correction to the idea that digestive enzymes have no authorised claims at all. The claim is specific to qualifying lactase products; it does not authorise general bloating, indigestion or gut-health claims for every enzyme blend.

Per capsule or per daily serving?

Activity numbers are meaningful only with their serving basis. For example:

3,000 DU per capsule becomes 9,000 DU across three capsules.
3,000 DU per three-capsule serving equals 1,000 DU per capsule.

Always check whether a competitor table uses one capsule, the normal daily serving or the maximum permitted intake.

Our current declaration

Preparation per capsule	Weight	Declared activity
Amylase	45 mg	2,250 SKB
Protease 4.5	10.8 mg	540 HUT
Lipase	6.6 mg	330 units; assay not named on the current label
Protease	4.78 mg	23.9 HUT
Bromelain	3 mg	6 GDU
Papain	0.15 mg	0.3 USP units
Protease 3	0.03 mg	48 HUT

At the label direction of one or two capsules with each main meal, multiply every figure by the number of capsules taken with that meal. Do not mix that total with a competitor’s per-capsule figure.

A six-step comparison

Match the same enzyme.
Match the same activity unit and standards method.
Convert both products to the same number of capsules or meal-serving basis.
Keep weight and activity in separate columns.
Check whether the declared value is guaranteed through shelf life or only at manufacture.
Check source, allergens, directions and non-enzyme ingredients separately.

Related guides

How this guide was checked

Our figures were checked against the current product label on 2 August 2026. Standards context was checked against USP’s Food Chemicals Codex information. The lactase claim and conditions were checked against the GB Nutrition and Health Claims Register and retained legislation. Competitor examples were checked against current official product pages.