Threonine

Threonine is an essential amino acid with a hydroxyl (-OH) group on its side chain — a feature it shares with serine. That hydroxyl group gives threonine an important job in the gut: it’s where sugar chains attach to mucins, the proteins that form the mucus layer lining the digestive tract. Mucins contain unusually large amounts of threonine, and animal studies show that the gut uses a large share of dietary threonine to make them — linking threonine to the gut barrier and the environment in which the microbiome lives. The same hydroxyl group also makes threonine units in proteins common “switch” sites in cell signalling, including in the pathway leucine uses to trigger muscle building. Threonine is less prominent in sports nutrition than leucine or tryptophan, and — like them — it’s covered by eating enough protein.

Classification

• Type: Essential amino acid (EAA) — can’t be made by the body; must come from the diet
• Side chain: Contains a hydroxyl group — threonine is the only essential amino acid with one; serine (non-essential) and tyrosine also have hydroxyl groups
• Metabolism: Classed as both glucogenic and ketogenic. In humans, most threonine is broken down by threonine dehydratase into alpha-ketobutyrate, which is converted via propionyl-CoA into succinyl-CoA and can be used to make glucose. A second route, which produces glycine and acetyl-CoA, is important in many animals but plays only a small role in people.
• Limiting amino acid: In grain-based animal feeds, threonine is often the second-limiting amino acid after lysine, which is why it’s widely added to pig and poultry feed. For human adults, cereals provide enough threonine relative to requirements — lysine is the main shortfall in grain-based diets.

Key Mechanisms

Mucin production and the gut barrier:

The digestive tract is lined with mucus, produced by goblet cells in its lining. Mucus is made mainly of mucins — large proteins covered in long sugar chains. These sugars attach to the hydroxyl groups of threonine and serine units in the mucin protein (a process called O-linked glycosylation).

Mucins contain an unusually high proportion of threonine — in some regions of the protein, it can make up a third or more of the amino acids. In studies of piglets, the gut used a large share of dietary threonine before it reached the rest of the body, much of it for mucin production. How closely this applies to humans is less well studied, but the gut is clearly a major user of threonine.

The mucus layer serves as:

• A barrier: trapping bacteria, toxins, and particles before they reach the gut lining
• Lubrication: helping food move smoothly through the gut and protecting the lining from friction
• A habitat with a buffer zone: in the colon, mucus forms two layers — a dense inner layer that normally keeps bacteria away from the gut lining, and a looser outer layer where many gut bacteria live and feed on the mucin sugars
• Part of the immune defense: secretory IgA antibodies are carried in the mucus, where they help neutralize microbes

The gut lining renews itself every few days, and mucus is secreted continuously, creating a steady demand for mucin production. In animal studies, threonine-deficient diets reduced mucin production and weakened the gut barrier. In people eating enough protein, threonine supply isn’t known to limit mucus production.

Cell signalling switch sites:

Threonine’s hydroxyl group is a common target for protein kinases — enzymes that switch proteins on or off by attaching phosphate groups to serine, threonine, or tyrosine units. Examples include:

• mTOR signalling: when leucine activates mTORC1, one of its key actions is attaching a phosphate to S6 kinase 1 at a specific threonine unit (threonine 389), switching on protein synthesis
• Akt: the enzyme Akt, central to insulin signalling and glucose uptake, is activated partly by phosphorylation at threonine 308
• Cell division: many of the proteins that control the cell cycle are regulated by threonine phosphorylation

These threonine units are built into the proteins themselves, so dietary threonine doesn’t control these switches — eating more threonine doesn’t increase signalling.

Threonine and glycine — less than often claimed:

Threonine is often described as a major source of glycine, which the body needs for collagen, glutathione, creatine, and bile acids. That’s true in many animals, but in humans, the main enzyme for this route (threonine dehydrogenase) is encoded by a non-functional “pseudogene”, and studies in adults suggest the glycine route accounts for only a small fraction — roughly a tenth — of threonine breakdown. People get most of their glycine from food and by making it from serine.

Immune function:

Antibodies (immunoglobulins) contain relatively large amounts of threonine, and the gut mucus layer — a major user of threonine — is the body’s first line of defense in the gut. In animal studies, threonine deficiency impaired antibody production and gut immune defenses.

Breakdown and energy:

Threonine’s main breakdown route in humans produces propionyl-CoA, which is converted into succinyl-CoA by an enzyme that needs vitamin B12 — the same step used in breaking down isoleucine, valine, and methionine. Succinyl-CoA enters the Krebs cycle, where it can be used for energy or to make glucose — mainly relevant when protein is being broken down for fuel.

Recommended Intake

Threonine needs are met comfortably by any diet with enough protein from varied sources. There’s no evidence that athletes need extra threonine beyond what their higher protein intake already provides.

Found In (per 100g)

Threonine is plentiful in animal proteins, and most plant proteins provide it well too. In USDA data, most food proteins are about 3–5% threonine, well above the World Health Organization’s adult reference of 2.3%; eggs, dairy, soy, and beans are near the top of that range. Even cereals — whose protein is about 2.7–3.8% threonine in USDA data, with wheat at the low end — provide more than the reference. All complete protein supplements — whey, casein, egg white, soy, and pea — provide threonine well above requirements at standard serving sizes.

Helps With

• Producing mucins, the main proteins of the gut’s protective mucus layer
• Gut barrier function, as part of the mucus layer
• Immune function, including antibody production
• Building and repairing muscle and other proteins, as one of the nine essential amino acids
• Growth and tissue repair, especially in children
• Normal liver fat metabolism — in classic rat studies, threonine-deficient diets caused fatty liver

Threonine for Athletes and Active People

Threonine’s main relevance to athletes is its role in the gut — but for athletes eating enough protein, threonine itself isn’t something that needs attention.

Exercise and gut permeability: hard or prolonged exercise — especially running, and especially in the heat — diverts blood away from the gut to the working muscles. This can make the gut lining more permeable, let bacterial toxins (lipopolysaccharides) into the bloodstream, and trigger inflammation and digestive symptoms such as nausea, cramping, and diarrhea. The mucus layer is part of the gut’s defense, but there’s no evidence that extra threonine prevents exercise-related gut problems. Better-supported strategies include training the gut to handle carbohydrate during exercise, using drinks or gels that combine different carbohydrates, staying hydrated, managing heat exposure, and avoiding anti-inflammatory painkillers (NSAIDs) before hard sessions.

Collagen and connective tissue: because the threonine-to-glycine route is minor in humans, threonine isn’t a meaningful source of glycine for collagen. For connective tissue support, focus on adequate protein, vitamin C, and copper; gelatin or collagen taken with vitamin C before training is an option with emerging evidence (see our vitamin C page).

mTOR signalling: leucine-activated mTORC1 works partly by phosphorylating threonine units in its target proteins, such as threonine 389 in S6 kinase 1. These threonines are part of the proteins’ structure, so this isn’t a reason to take threonine supplements — but it’s a good illustration of why complete protein, which provides all the essential amino acids, works better for muscle building than leucine alone.

Plant-based athletes: unlike lysine, threonine isn’t a concern for people eating varied plant proteins — grains, legumes, soy, and seeds all provide enough. Including legumes and soy foods daily matters for lysine, not threonine.

Threonine Deficiency

True threonine deficiency is uncommon in people eating enough protein. Low intake happens mainly as part of an overall protein shortfall:

• Severe protein-energy malnutrition
• Prolonged very low-calorie diets
• Malabsorption conditions

Effects of threonine deficiency — most of what’s known comes from animal studies:

• Reduced mucin production and a weaker gut barrier
• Impaired immune function, including reduced antibody production
• Poor growth
• Fatty liver, seen in classic studies of threonine-deficient rats

Excessive Doses of Threonine

No Tolerable Upper Intake Level has been established for threonine, and high-dose threonine supplements are rarely used.

Possible effects of high supplemental doses:

• Digestive discomfort: nausea and loose stools
• Ammonia: like other amino acids, threonine releases nitrogen when broken down, which is a consideration for people with urea cycle disorders or severe liver disease
• Competition for absorption: very high doses could, in theory, reduce the uptake of other neutral amino acids that share the same transporters

Threonine toxicity isn’t a practical concern at dietary intakes.

Types and Forms

• L-Threonine (free form): the biologically relevant supplement form, rarely used in human nutrition. In the 1990s, it was tested as a treatment for spasticity and for amyotrophic lateral sclerosis (ALS), with small trials finding modest benefits for spasticity at best and no benefit in ALS. Its main use today is in animal feed, where it’s one of the most widely added amino acids because of its limiting role in grain-based livestock diets.
• Complete protein supplements: whey, casein, soy, pea, and EAA blends all provide threonine in proportion to their protein — the most practical “threonine supplement” for anyone who needs more protein.
• Collagen and gelatin: contain only small amounts of threonine; they’re rich in glycine, proline, and hydroxyproline instead.

Which form for your goal:

How to Take Threonine

Through diet: any diet with enough protein from varied sources meets threonine needs comfortably. Threonine doesn’t need specific targeting beyond getting enough protein.

Supplements: standalone threonine supplements aren’t needed for gut health, connective tissue, or performance. For gut health in athletes, the training and fuelling strategies above matter far more. Glutamine, a major fuel for gut lining cells, has been studied for exercise-related gut permeability, with a few small studies showing promising but limited results.

For plant-based athletes: varied plant proteins provide plenty of threonine. The key is total protein and including legumes or soy for lysine.

Interactions

Vitamin B6 (as PLP) is needed by threonine dehydratase, the main enzyme for breaking threonine down in humans. Vitamin B12 is needed to convert the resulting propionyl-CoA into succinyl-CoA. Folate is linked to threonine metabolism through glycine and serine, which take part in the folate-dependent one-carbon cycle. Threonine has no known clinically important interactions with medicines.

Synergy

• Works with serine in mucin production — both provide the attachment points for mucin sugar chains.
• Works with glutamine in gut health — glutamine fuels the cells lining the gut, while threonine is a major building block of the mucus that protects them.
• Works with vitamins B6 and B12 in its breakdown, sharing the propionyl-CoA pathway with isoleucine, valine, and methionine.
• Works with leucine and the other essential amino acids in muscle protein synthesis — as a building block, and through threonine switch sites in signalling proteins.

Antagonistic Interactions

Very high threonine supplement doses may compete modestly with other neutral amino acids for absorption — negligible at dietary intakes.

Threonine breakdown releases nitrogen, so very high intakes could add to ammonia load in people with urea cycle disorders.

Vitamin B12 deficiency impairs the main breakdown pathway for threonine, isoleucine, valine, and methionine, raising methylmalonic acid.

Food values: USDA FoodData Central, SR Legacy — parmesan, hard (FDC ID 170848); chicken breast, roasted (171477); turkey breast, roasted (174516); yellowfin tuna, cooked (172006); pork top loin chop, broiled (167841); cheddar, sharp, sliced (170899)*; sardines, canned in oil (175139); pumpkin and squash seed kernels, dried (170556)*; ground beef, 80% lean, cooked (171797); Atlantic salmon, farmed, cooked (175168); peanuts, raw (172430)*; tofu, firm, calcium-set (172475); egg, hard-boiled (173424); cottage cheese, low-fat, 2% (172182)*; oats, dry (173904); black beans, cooked (173735); chickpeas, cooked (173757); lentils, cooked (172421); white rice, cooked (168878). * SR Legacy values used as fill-ins for the Foundation Foods entries for cheddar (328637), pumpkin seeds (2515380), peanuts (2515376), and cottage cheese (328841), whose amino acid data aren’t included in our tracker. Threonine as % of protein is calculated from the same USDA records; cereal comparisons use SR Legacy data against the World Health Organization’s 2007 adult reference pattern. Requirements from the US National Academies and the World Health Organization (2007).