Threonine: Food Sources, Supplement Claims, and Safety

What is threonine?

Threonine is one of the nine essential amino acids. Your body cannot make enough of it, so your diet must supply it. Cells use threonine to build proteins, including proteins in muscle and the lining of the gut. It is different from L-theanine, a tea compound, and from magnesium threonate, a magnesium supplement; their similar names do not make their effects interchangeable.

The dietary need is clear. Whether taking threonine in addition to enough food protein improves immunity, gut health or exercise outcomes is a separate question.

What threonine does in your body

Threonine is incorporated into proteins as your body builds and renews tissues. It is also used in mucins, proteins that help form the protective mucus layer along the gut. In animal research^[1], changing dietary threonine changed intestinal mucin production. That helps explain interest in threonine and gut health, but it does not give us a human dose that improves digestion or treats gut disease.

You also need the other eight essential amino acids to make complete proteins. Extra threonine cannot substitute for a generally low-protein diet or for missing amino acids.

The short version

Daily food amount: The National Academies^[2] sets an adult recommended amount of 20 mg per kilogram of body weight per day. At 70 kg (about 154 lb), that is 1.4 g across the whole diet.

Supplement evidence: Human trials have explored large doses in narrow settings. A four-week trial tested tolerability in healthy men, while older neurological trials tested treatment in patients. Neither establishes a useful daily dose for a healthy person.

First check: Look at ordinary protein foods before an isolated amino-acid label. Plant foods can contribute meaningful threonine too.

Why do people take threonine?

Some trials concern amyotrophic lateral sclerosis (ALS), a progressive disease that damages the nerves controlling muscles. Those clinical studies are separate from supplementation in healthy adults.

Claimed benefit	Dose / form studied	Research verdict	Evidence strength
More muscle or recovery	No established isolated-threonine regimen	Nutritional necessity does not establish an extra dose for training	Insufficient
Gut lining or immunity	Mechanistic and animal work; no clinical immune-benefit regimen established here	A biological role is not a measured human supplement benefit	Insufficient
Reduce neurological stiffness	4.5–6 g/day in small spasticity trials	Some stiffness-scale changes; uncertain practical value	Limited
Slow ALS progression	2 g/day alone for a year; 4 g/day with vitamin B6 for six months	No clear functional benefit; a combination trial raised a breathing-function concern	Insufficient

The most persuasive reason to care about threonine is that it is essential in the diet. Supplement advertising often moves from that fact to a claim that more will improve a particular body function. The studies below test parts of that second claim.

How much threonine is in food?

These are calculated from USDA FoodData Central SR Legacy records^[3] for the stated edible portions and rounded to 0.01 g. Actual amounts vary with the food and serving.

Food and preparation	Source	Serving	Threonine
^[4]	Meat	3 oz (about 85 g)	1.11 g
^[5]	Fish	3 oz (about 85 g)	1.06 g
^[6]	Plant	½ cup (about 126 g)	0.99 g
^[7]	Meat	3 oz (about 85 g)	0.83 g
^[8]	Plant	1 cup (about 198 g)	0.64 g
^[9]	Dairy	1 cup (about 244 g)	0.33 g
^[10]	Egg	1 large (about 50 g)	0.30 g
^[11]	Plant	1 oz (about 28 g)	0.28 g
^[12]	Plant	1 oz (about 28 g)	0.17 g

For a 70 kg (about 154 lb) adult, the tofu and lentil servings together provide about 1.63 g threonine, above the 1.4 g daily reference before other foods.

How can I tell if I'm low on threonine?

Too little threonine may result from inadequate total protein intake or a highly restricted diet, but does not have distinctive symptoms. If your intake is limited, review a few days of food and the full set of protein sources. If illness, swallowing difficulty or a prescribed diet limits protein, a clinician or dietitian can assess the whole nutrition pattern.

What should I check on a label?

An L-threonine supplement lists the free amino acid per serving. A protein powder or essential-amino-acid blend may contain threonine alongside other amino acids, but its total protein or total EAA grams are not threonine grams. Look for a separate threonine line and the number of capsules or scoops needed to reach the stated serving.

The 20 mg/kg/day recommendation is for total intake from food and any supplements. Do not confuse L-threonine with L-theanine or magnesium threonate when comparing labels or research.

Drawbacks and safety limits

A safety study in which men received the different doses in a randomized order enrolled 30 healthy men and tested 0, 3, 6, 9 and 12 g/day for four weeks per condition. Six men withdrew; depending on the condition, 20–22 were included in the analysis. No serious adverse events were recorded. Mean liver-enzyme and muscle-enzyme readings were higher at 9 g/day than placebo, though that difference did not recur as statistically significant at 12 g/day. That irregular pattern is one reason not to translate the study into a promise of safety at every dose.

The authors proposed 12 g/day as a four-week dose at which they did not observe an adverse effect in these healthy men. It is not an official upper limit, a benefit dose, or proof about long-term use, women, children or people with illness. The study was funded by the company that supplied the amino acid. The National Academies did not set a tolerable upper limit for amino acids because the evidence was insufficient.

Do extra doses help immunity or the gut?

The human studies discussed here do not show that isolated oral threonine reduces infections or improves a gut outcome. A safety trial's white-blood-cell count was a laboratory check, not a test of whether people became less likely to get sick. For these purposes, the useful next step is to check overall protein adequacy; there is no established extra-threonine target to compare a product with.

What happened in the neurological trials?

In a six-month ALS trial^[13], 95 people were randomized and 77 completed treatment. One group received 4 g threonine plus 160 mg pyridoxal phosphate, a form of vitamin B6, daily; another received a mixture of the branched-chain amino acids leucine, isoleucine and valine; a third received placebo. Strength and daily-function results did not show a significant benefit. Breathing capacity declined about 2.5 times faster in both amino-acid groups than in the placebo group. The authors said chance could explain this finding but could not rule out harm. Because the threonine arm also took pyridoxal phosphate, the result cannot be assigned to threonine alone.

The concern about that combination should not obscure the separate trial of threonine alone. In a 1992 randomized, double-blind ALS trial^[14], 15 people completed a year of 2 g/day or placebo. Functional decline did not differ significantly between groups. This small result supplies no clear evidence that isolated threonine slowed the disease.

Stiffness changed more clearly than everyday function

Researchers also asked whether threonine could help spasticity—excess muscle stiffness caused by nervous-system disease—because it might increase glycine, another amino acid involved in signals that reduce nerve-cell activity. In an 18-person trial in familial spastic paraparesis^[15], each patient received threonine and placebo during different treatment periods—a design called crossover. Threonine at 4.5 or 6 g/day for two-week periods improved some examiner-rated severity measures. The patients' and physicians' overall assessments did not show a useful benefit, and the authors concluded that the changes were not clinically valuable. Blood and spinal-fluid threonine rose, but glycine did not: the proposed increase in glycine did not occur.

A second small crossover trial in spinal spasticity^[16] tested 6 g/day against placebo. A response meant at least a 10% reduction in the Ashworth score, which measures muscle tone. The comparison favored threonine on that measure. This positive stiffness finding does not by itself tell us how much easier walking, dressing or other activities became.

An earlier uncontrolled study^[17] gave six patients with inherited spasticity syndromes 500 mg/day threonine for a year and reported partial improvement. Without a comparison group, that observation cannot separate treatment effects from other changes over time; the later controlled trials are more informative.

Together, these studies leave a narrower conclusion than either “threonine does nothing” or “threonine treats neurological disease”: some stiffness measurements improved, meaningful overall benefit was inconsistent, and the ALS trials did not show slower functional decline.

The bottom line

Threonine is essential, and both animal and plant protein foods supply it. At 70 kg (154 lb), the adult daily reference is about 1.4 g from the whole diet. Gut and immune claims move beyond what human supplementation studies here can show. The four-week study gives a narrow safety observation, while the neurological trials do not establish a useful self-treatment. Check food intake and the actual threonine line before treating a supplement's dose as necessary.

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