How many essential amino acids do humans need?
Open a modern biochemistry or nutrition textbook, and the answer is straightforward: nine essential amino acids.
Yet a Japanese biochemistry textbook published in 1974 gives a different answer. It lists only eight essential amino acids for adults.
Was the textbook wrong? Or did scientists discover something important after it was published?
The answer leads to an interesting chapter in the history of nutritional biochemistry, involving histidine, nitrogen balance, and a landmark experiment published in 1975.
In this article, we revisit a real 1974 biochemistry textbook and examine how our understanding of essential amino acids changed over the following decades.
1. A Biochemistry Textbook from 1974
While browsing older scientific books at the Toyonaka Campus library of Osaka University, I came across Seikagaku Nyumon [Introduction to Biochemistry] by Yo Imai, published by Asakura Shoten. The first edition appeared on June 10, 1974, and the copy examined here is a 1977 reprint.
Chapter 9 discusses amino acid metabolism, including protein digestion, intestinal absorption, nitrogen metabolism, and the metabolic pathways of individual amino acids.
Much of the biochemical explanation remains familiar to modern readers. The textbook describes transamination, deamination, the urea cycle, and the entry of amino acid carbon skeletons into central metabolic pathways.
However, one statement in Section 9.1.2, concerning amino acid absorption, immediately stands out.
The textbook identifies eight essential amino acids for adult humans.
They are isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
Modern references, however, recognize nine.
The missing amino acid is histidine.
2. What Are the Nine Essential Amino Acids?
Amino acids are the molecular building blocks of proteins. Humans use 20 standard proteinogenic amino acids to synthesize proteins, but not all of them can be produced in sufficient quantities by the body.
An essential amino acid is one that must be obtained from the diet because the body cannot synthesize enough of it to meet physiological requirements.
Modern nutritional references recognize nine essential amino acids for humans.
| Amino acid | Three-letter code | One-letter code | 1974 adult list |
|---|---|---|---|
| Histidine | His | H | Not included |
| Isoleucine | Ile | I | Included |
| Leucine | Leu | L | Included |
| Lysine | Lys | K | Included |
| Methionine | Met | M | Included |
| Phenylalanine | Phe | F | Included |
| Threonine | Thr | T | Included |
| Tryptophan | Trp | W | Included |
| Valine | Val | V | Included |
The modern list is also provided by the US National Library of Medicine through MedlinePlus.
Notice that eight amino acids appear in both lists. Only histidine differs.
Why was histidine treated differently?
3. Why Was Histidine Not Always Considered Essential for Adults?
Historically, the essentiality of an amino acid was established through nutritional experiments. Researchers examined whether humans or experimental animals could maintain normal growth, nitrogen balance, and physiological function when a particular amino acid was omitted from the diet.
Histidine presented a difficult case.
Its importance during growth was recognized, but its requirement in adults was less clearly established. Adult humans can maintain certain physiological functions for some time despite limited histidine intake, making short-term deficiency experiments potentially misleading.
In addition, the human body contains substantial histidine-containing compounds, including proteins and hemoglobin. Changes in these pools can complicate the interpretation of dietary deficiency experiments.
Consequently, older nutritional classifications sometimes treated histidine differently for adults and growing children.
This historical context helps explain why a textbook published in 1974 could list eight essential amino acids for adults.
However, the scientific picture was changing around that time.
4. The Landmark 1975 Study: Is Histidine Essential for Adults?
In May 1975, Joel D. Kopple and Marian E. Swendseid published a study in the Journal of Clinical Investigation entitled Evidence that histidine is an essential amino acid in normal and chronically uremic man.
DOI: 10.1172/JCI108016.
The study investigated whether adult humans required dietary histidine to maintain normal physiological function.
The researchers examined seven adult men: four healthy individuals and three patients with chronic uremia.
Participants received three dietary conditions in sequence. First, they consumed a protein-containing diet. They then received a semi-synthetic amino acid diet lacking histidine. Finally, histidine was restored to the amino acid diet.
The histidine-deficient period lasted approximately five weeks on average.
This design allowed the investigators to observe physiological changes during histidine depletion and determine whether these changes improved following histidine repletion.
5. What Happened When Histidine Was Removed?
The results were striking.
During the histidine-deficient period, nitrogen balance gradually became negative. Plasma histidine concentrations decreased substantially, and muscle histidine concentrations also fell.
The investigators reported an average reduction of approximately 82% in plasma histidine and 62% in muscle histidine.
Hematocrit also declined, indicating changes affecting red blood cell production or maintenance.
Several participants developed symptoms, including feeling unwell and skin abnormalities.
When dietary histidine was restored, multiple abnormalities improved. Nitrogen balance became positive in six participants, circulating and muscle histidine concentrations increased, and hematological indicators showed recovery.
The researchers concluded that histidine was an essential amino acid in both healthy adults and adults with chronic uremia.
Importantly, this experiment did not merely measure whether histidine was present in the bloodstream. It examined the physiological consequences of removing histidine from the diet and then restoring it.
That distinction is central to nutritional science: an amino acid is essential because adequate physiological function depends on its dietary supply, not simply because it exists in human tissues.
6. What Is Nitrogen Balance, and Why Does It Matter?
Nitrogen balance was one of the major measurements used in the 1975 experiment.
Because amino acids contain nitrogen, researchers can estimate the relationship between dietary protein intake and the body's nitrogen losses.
In simplified form:
Nitrogen balance = nitrogen intake − nitrogen loss.
A positive nitrogen balance indicates that nitrogen intake exceeds measured loss, while a negative balance indicates that more nitrogen is being lost than supplied.
Negative nitrogen balance may occur during inadequate protein intake or increased protein breakdown, although nitrogen balance measurements have methodological limitations.
If an essential amino acid is missing, the body cannot use all other amino acids efficiently for protein synthesis. This can contribute to changes in protein metabolism and nitrogen balance.
The 1975 experiment used nitrogen balance together with biochemical and physiological measurements. The combination of these observations strengthened the evidence for an adult histidine requirement.
7. Did One Study Change the Textbooks Overnight?
No. Scientific understanding rarely changes through a single experiment alone.
The 1975 study provided important evidence, but subsequent research continued to investigate histidine requirements, dose responses, and suitable methods for assessing amino acid adequacy.
For example, a study published in 1981 examined relationships between histidine intake, plasma and urinary histidine concentrations, and nitrogen balance in healthy and chronically uremic men.
That investigation supported the conclusion that histidine is essential in adults. However, it also noted that longer clinical and metabolic studies might be required to determine the precise daily histidine requirement.
See the study indexed in PubMed (PMID: 7241233).
This distinction is important. Establishing that an amino acid is essential and determining exactly how much of it is required are related but different scientific questions.
Over time, histidine became incorporated into the standard list of nine essential amino acids used in modern human nutrition.
8. The Difference Between Historical Knowledge and Scientific Error
Looking back at a 1974 textbook, it is tempting to say that the author simply made a mistake.
But this interpretation misses the more interesting point.
The textbook reflects the nutritional understanding commonly taught during its period. The later classification of histidine illustrates how experimental research can refine an established scientific framework.
The eight amino acids listed in the old textbook did not become nonessential. Rather, evidence supporting the adult requirement for a ninth amino acid became stronger.
In other words, the definition of an essential amino acid did not fundamentally change.
Our evidence about which amino acids satisfy that definition improved.
This is an important feature of scientific progress. New findings do not always replace entire theories. Sometimes they change just one item in a list.
9. What Else Has Changed Since 1974?
The basic biochemical roles of amino acids were already well established in the 1970s. The textbook describes protein digestion, amino acid absorption, transamination, deamination, and the relationship between amino acid metabolism and the tricarboxylic acid cycle.
These fundamental pathways remain recognizable today.
Modern biochemistry has added considerably more detail, particularly concerning membrane transporters, molecular regulation, amino acid sensing, and tissue-specific metabolism.
For example, amino acid availability influences cellular signaling systems that regulate protein synthesis and metabolism. Such molecular mechanisms extend far beyond the broad nutritional classifications found in older textbooks.
Yet the central question remains the same: how does the body obtain, transform, and utilize the amino acids required for life?
The comparison between 1974 and modern biochemistry therefore reveals both continuity and change.
10. What Can We Learn from a 50-Year-Old Textbook?
When we examined the glycolysis chapter of the same textbook, we found that the major biochemical pathway had remained largely unchanged, even though estimates of ATP production and our understanding of metabolic regulation had evolved.
The essential amino acid list tells a different but complementary story.
Here, a small difference in nutritional classification leads directly to an important experiment published just one year after the textbook's first edition.
A modern reader sees nine essential amino acids. A reader in 1974 might have learned eight for adults.
Between those two statements lies a history of experiments, changing evidence, and the gradual refinement of nutritional requirements.
The most interesting lesson is not that old textbooks contain outdated information. It is that old textbooks allow us to see scientific knowledge at a particular moment in its development.
11. Explore Amino Acids with BioChemCalc
Essential amino acids are only one part of amino acid biochemistry. Each of the 20 standard amino acids has a characteristic molecular structure, side chain, and chemical behavior.
If you would like to explore their structures, names, and one-letter and three-letter codes, try our interactive 20 Amino Acids Structure Quiz.
You can also read our previous article, Has Glycolysis Changed in 50 Years?, which compares carbohydrate metabolism in the same 1974 textbook with modern biochemistry.
Conclusion
Modern nutritional science recognizes nine essential amino acids for humans, including histidine.
A Japanese biochemistry textbook published in 1974 listed eight essential amino acids for adults, reflecting an earlier stage of nutritional understanding.
The following year, Kopple and Swendseid published experimental evidence supporting the essentiality of histidine in adults. Subsequent studies strengthened this conclusion and investigated dietary requirements in greater detail.
The difference between eight and nine essential amino acids may appear small. But it illustrates something fundamental about science.
Scientific knowledge advances not only through revolutionary discoveries, but also through careful experiments that refine what we thought we already knew.
References
- Imai Y. Seikagaku Nyumon [Introduction to Biochemistry]. Tokyo: Asakura Shoten; 1974. First edition published June 10, 1974. The copy examined was reprinted May 25, 1977.
- Kopple JD, Swendseid ME. Evidence that histidine is an essential amino acid in normal and chronically uremic man. Journal of Clinical Investigation. 1975;55(5):881–891. DOI: 10.1172/JCI108016.
- Effect of histidine intake on plasma and urine histidine levels, nitrogen balance and N-tau-methylhistidine excretion in normal and chronically uremic men. 1981. PubMed PMID: 7241233.
- US National Library of Medicine. Amino Acids. MedlinePlus Medical Encyclopedia. https://medlineplus.gov/ency/article/002222.htm.