In Defense of the Holman Index: Defining Fatty Acid Deficiency

Abstract

Essential fatty acid deficiency (EFAD) is a rare but serious condition with significant consequences including delayed growth and development, decreased immune response and reproductive dysfunction, among others. EFAD is of particular concern in vulnerable populations such as preterm infants and those receiving long-term parenteral nutrition (PN). As essential fatty acids (EFAs) must be supplemented in the diet due to the inability to synthesize these endogenously, EFAD develops secondary to inadequate EFA intake. The Holman Index, defined by the ratio of Mead acid to arachidonic acid (triene: tetraene (T:T)) in the plasma, has historically served as the method for diagnosis, with the threshold diagnostic value at ≥0.20. This index is derived from the body’s natural metabolic response to EFA deprivation, increasing synthesis of Mead acid, and thus remains broadly applicable across various populations. Concerns regarding the established ratio and alternative use of absolute fatty acid values and profiles have been raised that question the utility of the Holman Index. Although recent developments in fatty acid profiling have allowed for increased precision in measurement and development of population-specific reference ranges, reliability of this data in diagnosing EFAD is controversial given variability amongst different studies and population dietary confounders. Data from animal and human studies have demonstrated that the Holman index has continued to reliably detect EFAD even in the era of new lipid emulsions and technological advancements. The Holman Index remains a vital tool in the diagnosis and monitoring of EFAD, offering consistency and early detection capacity in at-risk populations.

Introduction

Fatty acids are organic compounds consisting of hydrocarbon chains that serve a multitude of important functions. They act as important structural components of cell membranes, building blocks for complex lipids, ligands for nuclear receptors in gene expression, and modulators of cellular signaling through G-protein-coupled receptors [1]. Specific fatty acids, omega-6 linoleic acid (18:2n-6, LA) and omega-3 alpha-linolenic acid (18:3n-3, ALA), were discovered in the 1920s in a rat model to be essential for growth and reproduction [2]. These fatty acids were later classified as essential fatty acids because humans lack the enzymes to produce them naturally, therefore requiring dietary supplementation. However later studies have shown that it is largely their downstream metabolites, arachidonic acid (ARA), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), that act as the necessary precursors of important cellular growth factors and inflammatory mediators [3].

EFAD develops when individuals are not sufficiently provided with adequate amounts of EFAs, particularly in those with limited fat reserves. Clinical manifestations of this disease include impaired growth, eczematous dermatitis, alopecia, increased susceptibility to infections, delayed wound healing, and reproductive dysfunction [4]. Preterm infants are especially vulnerable due to low fat stores, absence of full third-trimester placental transfer of omega-3 and omega-6 FAs, high metabolic rate, and a reduced ability to synthesize ARA and DHA from their precursor EFAs. EFAD in these babies can lead to improper development of the brain and retina, leading to long-term neurologic and visual impairments. Early recognition and prompt nutritional intervention are therefore critically essential in this patient population to minimize the sequelae of EFAD [5].

While rare, EFAD was of significant concern with the advent and administration of early lipid-free parenteral nutrition (PN) in infants and adults. Upon recognition of EFAD in these patients, intravenous lipid emulsions (ILEs) were introduced as a source of EFAs and have since become crucial components of modern PN. The earliest ILEs were largely composed of soybean oil (SO) due to its high content of EFAs. Alternative ILE formulations were subsequently developed which partially or fully replaced SO with other lipid sources such as fish oil and olive oil [4–6]. Later investigations recognized the associated risks of soy allergies, proinflammatory n-6-derived eicosanoids, and intestinal failure-associated liver disease (IFALD) with these earlier ILE products, resulting in changes in clinical practice and supporting the use of newer formulations over the original SO compositions [6,7].

The Holman index has historically been utilized to diagnose biochemical EFAD. This index is specifically defined by the ratio of Mead acid (20:3n-9; triene) to ARA (20:4n-6; tetraene) (T:T ratio), and not with any other trienes or tetraenes in plasma. The Holman index should be calculated only from plasma fatty acid profiles, as that is how its diagnostic utility was established. In particular, the cutoff of 0.2 indicating deficiency is not applicable to other samples (e.g. erythrocytes or isolated plasma phospholipids), which may have different baseline fatty-acid compositions, and thus applying the plasma threshold in these contexts may lead to misinterpretations of EFAD status. The various enzymes in the metabolic pathways for polyunsaturated fatty acids (PUFAs) act in preferential order of affinity from omega-3 > omega-6 > omega-9 [1], as seen in Figure 1. Established by Holman [8], the index is based on the above pathway: that in the absence of adequate intake of essential fatty acids, the body compensates by synthesizing Mead acid from the non-essential omega-9 oleic acid, reducing ARA production from omega-6 precursors, such as linoleic acid. This metabolic adaptation results in a rise in the Mead acid: arachidonic acid ratio, with a threshold of ≥ 0.20 as diagnostic of EFAD. This threshold was determined using packed-column gas-liquid chromatography to measure plasma fatty acids from individuals with normal EFA metabolism [9]. While clinical manifestations of EFAD do not typically appear until Mead acid: arachidonic acid ratios exceed 0.40, elevations in the index can be detected as early as one week after EFA restriction, making it a valuable tool for identifying disease before signs and symptoms emerge [5,8].

Figure 1.

Diagram of essential fatty acid metabolism including elongation, desaturation and β-oxidation reactions leading to the synthesis of EPA, DHA and ARA from ALA and LA in humans. All pathways share the same enzymes with these enzymes preferring ω-3 > ω-6 > ω-9 fatty acids.

Challenges to the Holman Index

The Holman Index has long been the subject of ongoing debate, particularly in regard to the diagnostic threshold for EFAD. Some of the earliest challenges to the index were introduced by Siguel who stated that established threshold of the Mead acid: arachidonic acid ratio was in error by at least a factor of 10, if not greater, and pushed for substantially lower Mead acid: arachidonic acid ratios. This recommendation was based on their own research, which reported the detection of smaller quantities of Mead acid using newer methods [10]. Their Letter to the Editor in response to Gould’s 1994 paper on the effects of cholesterol lowering in coronary artery disease in Circulation argued that Gould was underdiagnosing EFAD by using Holman’s diagnostic criteria rather than the ranges noted in Siguel’s studies. However, upon assessment of Siguel’s Mead acid: arachidonic acid ratio results with special attention to the statistical significance of his findings, Holman’s existing diagnostic EFAD criteria was found to be consistent with Siguel’s own data. Moreover, Siguel’s proposed thresholds were within the range of normal biological variation and failed to identify true deficiency [11,12]. This undermines the diagnostic utility of the ratio by creating false positives and misclassifying healthy individuals as deficient, without any clinical correlation. Siguel also introduced the concept of “essential fatty acid insufficiency,” a condition that they report was associated with abnormal lipid levels, high blood pressure, coronary artery disease and risk of premature death due to heart disease [11]. This proposed condition however lacks clinical validation as compared to the established diagnosis of EFAD. To date, Siguel’s proposed thresholds for diagnostic criteria of EFAD and his new diagnosis of EFA insufficiency have not been adopted into modern medical practice.

Further confusion was later introduced by the work of Teitelbaum and Cober, who proposed an alternative diagnostic cutoff for the Mead acid: arachidonic acid ratio for EFAD. Specifically, their studies focused on a strict diagnostic criterion of EFAD defined as a Mead acid: arachidonic acid ratio > 0.05 based on Mayo Medical Laboratories reference ranges (e.g., normal range for children 32 days old to 17 years old is 0.013–0.050), while the existing criteria based on Holman defines the official diagnosis of EFAD at the cutoff of 0.20 as the upper limit of normalcy from random samples from patients of ages 0–90 years of life [9]. Using this lower cutoff for defining EFAD, their data then reported 8 of 13 infants having developed mild EFAD without any physical manifestations, including one infant having progressed into a “more significant, but nonclinical EFAD”. They also supported the use of absolute values for ALA and LA as a marker of EFAD rather than the Mead acid: arachidonic acid ratio, noting 2 patients whose LA and ALA values were abnormal with ILE reduction but improved with weekly soybean oil ILE administration. However, there is no mention of any clinical consequences in these patients, and again no patients in their study diagnosed with EFAD by their strict criteria of > 0.05 showed any symptoms [13]. It remains unclear how mildly elevated Mead acid: arachidonic acid ratios translate into clinical outcomes in the pediatric population, and it is not well known the threshold at which a biochemical EFAD (by Mead acid: arachidonic acid ratio) begins to produce substantial adverse effects. And so, while their intent may have been to trial a different set of population-specific thresholds to catch early deficiency in vulnerable pediatric patients, they were arbitrary and did not correlate with clinical outcomes. Lowering the threshold must be done cautiously as it may risk overestimating EFAD prevalence in populations where mild and/or transient elevations in the Mead acid: arachidonic ratio may not be clinically meaningful. This may ultimately reduce the reliability and precision of EFAD diagnoses and making any relevant findings difficult to compare across subject groups and studies. Additionally, caution must be taken when evaluating individual fatty acid levels, as low absolute levels of linoleic acid or alpha-linolenic acid in isolation are not always diagnostic of EFAD if the Mead acid: arachidonic acid ratio remains normal. Conversely, a moderately elevated Mead acid alone might occur in certain contexts (such as use of lipid emulsions high in oleic acid) without clinical EFAD. While an argument could be had to consider reclassification of EFAD at a higher threshold value such as 0.40 which was identified to be where clinical signs and symptoms develop, monitoring at a higher value holds greater risk in late diagnoses and treatment plans for patients with possible long-term sequelae [8]. So, although their purpose was likely to advocate for using a lower Mead acid: arachidonic acid ratio for infants to reflect newer normative data to improve sensitivity in detecting EFAD, it also raises concern for over-diagnosis. Therefore, we recommend using the traditional Holman Index threshold (Mead acid: arachidonic acid ratio ≥ 0.2) for now, with careful consideration of age-specific reference ranges. It is important to note however that the normal ranges used by both Holman and clinical laboratories are derived from healthy individuals consuming a typical Western diet and may not always be appropriate in isolation when evaluating an individual who is being artificially fed with PN or in high-risk populations such as preterm infants. However, with years of laboratory and clinical validation, the Holman index remains a reliable baseline for further patient-specific analysis and diagnosis in vulnerable populations, with trends in the Holman index values serving an important role in monitoring and the ultimate prevention of true EFAD.

Over more recent years, questions have emerged regarding the continued validity in the use of the Holman index to diagnose EFAD in light of newer methodologies. With more sensitive methods for measuring fatty acids, fatty acid profiles have been gaining traction for use in the assessment of fatty acid status. More recent assays have provided reference ranges seen in each population rather than a single threshold value, which are dependent on dietary patterns and fatty acid intake unique to each population [14]. These values are generally inconsistent across the literature due to different methods and units of measurement. This introduces variability to the diagnosis of biochemical EFAD which makes it more challenging to apply these ranges across different groups. Meanwhile the Holman index benefits from the relative increased production of Mead acid to ARA in the body secondary to the deficiency of EFAs, rather than population dietary differences impacting reported EFA ranges [4,15]. Furthermore, being that the Holman index is a ratio-based metric, it is less affected by inter-laboratory variability than absolute fatty acid concentrations, making it friendlier in both reproducibility and comparability across studies. However, there are disease states that are appropriate for diagnosis and/or monitoring with specific ranges or percentages. These include fatty acid oxidation disorders (FAODs) and peroxisomal disorders such as X-linked adrenoleukodystrophy, Zellweger syndrome (cerebrohepatorenal syndrome) and Refsum disease (phytanoyl-coenzyme A hydroxylase deficiency) [16–18]. Although the biochemical natures of these disease processes allow for specific fatty acid concentrations to be used as biomarkers for disease diagnosis, EFAD lends itself to the risk of inappropriate diagnoses given inconsistencies in fatty acid profiling data secondary to dietary intake and laboratory techniques.

With the advent of new ILE formulations, concerns about the potential occurrence of EFAD have resurfaced due to the variations in fatty acid composition. However, with the use of the Holman index in analyzing these new formulations, similar to pure soybean oil ILE, these products are effective at providing sufficient EFAs and their use does not lead to EFAD, despite containing low levels of ALA and LA but high concentrations of the omega-3 and -6 downstream metabolites, DHA and ARA. Murine studies have shown that fish oil-based lipid emulsions (FOLE) which provide DHA, EPA and ARA, do not lead to EFAD despite containing lower levels of LA and ALA. This suggests that supplementation of EFA derivatives may be sufficient to meet essential fatty acid requirements despite altered fatty acid profiles and can be appropriately evaluated using the Holman Index [19]. Furthermore, mice fed formulations with varying DHA:ARA ratios (from 1:1 to 200:1) maintained Mead acid: arachidonic acid ratios ranging from 0.003–0.06, well below the diagnostic threshold for EFAD, and even demonstrated reversal of biochemical EFAD in EFA-poor coconut-oil fed mice with administration of DHA:ARA ratios of 20:1, 200:1, and 100:0 [3]. Nandivada [20] validated this phenomenon in human studies, demonstrating that children receiving long-term FOLE monotherapy showed no biochemical or clinical evidence of EFAD despite emulsion composition, further strengthening the validity in the use of the Holman index for diagnosis of EFAD over the use of fatty acid profiles and reference ranges.

Another common concern brought up regarding the Holman index focuses on the Mead acid: arachidonic acid ratio being based on the omega-6 pathway, accounting for the rise of Mead acid relative to ARA. As such, it does not directly account for the omega-3 pathway, which ultimately generates the downstream metabolites DHA and EPA. However, it is important to note that both omega-3 and omega-6 compete for the same enzymes, and their combined deficiency results in the compensatory production of Mead acid. Although the index only accounts for the fall of the omega-6 metabolite ARA, it ultimately indirectly reflects deficiency in both omega-3 and omega-6 essential fatty acid families to then result in the increased oleic to Mead acid production to see a rise in the Mead acid: arachidonic acid ratio. The Holman Index remains reliable because the metabolic response it captures—accumulation of Mead acid with a loss of ARA—occurs whenever the essential fatty acids are deficient.

The ongoing appeal of the Holman Index lies in its simplicity, physiological relevance, and reproducibility. By understanding the shift toward Mead acid synthesis in the setting of EFA deprivation, the index remains a robust biomarker that is interpretable across diverse clinical settings and laboratory methods. Considering the inconsistent and occasionally contradictory interpretations introduced by alternative frameworks, newer technologies and methods should be carefully evaluated and potentially used alongside the Holman Index, rather than as isolated substitutes. The Mead acid: arachidonic acid ratio ≥ 0.2 remains the recommended diagnostic threshold because it balances sensitivity and specificity for EFAD. It is low enough to detect biochemical deficiency before physical signs develop, but high enough to avoid flagging many patients who may have marginal imbalances with no clinical effect. Lower thresholds proposed have not been validated against clinical outcomes and may markedly increase the number of ‘EFAD’ diagnoses without clear benefit. Maintaining the Holman Index as the diagnostic benchmark ensures continuity in clinical care between different practice sites, comparability across studies, and diagnostic clarity in vulnerable populations at risk for EFAD.

Pediatric Considerations and Monitoring in High-Risk Populations

Preterm infants as previously discussed are a subgroup of patients at higher risk for the development of EFAD. Pediatric guidelines emphasize that infants require higher percentages of calories from essential fatty acids (at least 3% compared to 1–2% in adults) due to limited fat stores and rapid growth requirements, suggesting standard thresholds may be inadequate for vulnerable populations [21]. As such, unique safety considerations exist for the use of ILEs in neonatal and pediatric populations, given their different physiologic requirements compared to adults. The Holman index with its definition of biochemical EFAD at threshold value of 0.2 may spark concern for potentially missing clinically relevant EFA deficiency requiring intervention, and thus the continued use of the Holman index in context of the clinical picture may raise appropriate alarm in trends suggestive of impending EFAD in vulnerable populations.

A case report by Riedy [22] highlights a severely malnourished premature infant with normal baseline EFA profiles receiving FOLE, who developed a rising Mead acid: arachidonic acid ratio peaking at 0.075. Evidence has shown that when FOLEs are dosed at 1 g/kg/day, EFAD typically does not occur. However, despite receiving appropriate FOLE dosing, the infant had an increasing Holman index and elevated concentrations of Mead acid in the setting of poor growth, raising early concern for the risk of developing EFAD. FOLE dosing was ultimately increased by 50% to 1.5 g/kg/d with successful clinical and biochemical outcomes with monitoring of the Holman index showing improvement down to 0.005 by week 20.

This report raised the importance of close monitoring of EFAD development particularly in severely malnourished patients, as prior studies have shown that reintroduction of fats in experimentally starved rats resulted in a temporary but significant decrease in baseline EFA status. It is of critical importance that in these high-risk patient populations, trends in the Holman index - even if not yet at standard threshold significance - are carefully monitored in their appropriate setting and intervened upon early to prevent EFAD development in these patients. Identification and intervention at lower thresholds may be necessary in these populations and clinical settings. In this situation, the Holman Index successfully detected developing EFAD in context of clinical concern and objectively tracked improvement when the dose was increased from 1.0 to 1.5 g/kg/d, highlighting its clinical utility and sensitivity in complex circumstances involving malnutrition and ILEs. While further research is needed to determine the clinical significance of mildly elevated Mead acid: arachidonic acid ratios in neonates, until such data are available clinicians should use the Holman Index both as a baseline guide and for monitoring while continuing to rely on clinical assessment and risk factors to confirm a diagnosis of EFAD.

Conclusion

Essential fatty acid deficiency remains a clinically relevant condition particularly in the setting of ongoing parenteral nutrition administration in both adult and pediatric populations. While intravenous lipid emulsions have been formulated to reduce the incidence of EFAD and prevent the clinical manifestations of the disease, the development of novel formulations alongside historic and modern debates over the diagnostic threshold for EFAD continue to necessitate a validated standard for diagnosis and monitoring in our patients. The Holman Index remains one of the most validated and widely accepted biochemical markers for diagnosing EFAD.

Alternative diagnostic criteria, most notably those proposed by Siguel, Teitelbaum, and Cober, utilize arbitrary lower thresholds based on self-reported data or population differences, or emphasize the value of absolute fatty acid concentrations. However, these approaches suffer from methodological limitations, lack of clinical correlation, and poor generalizability across populations. Siguel’s diagnostic thresholds have been shown to fall within normal biological variation and not contradict Holman’s data, and his concept of “EFA insufficiency” has not been validated in clinical practice. Meanwhile the population-specific thresholds suggested by Teitelbaum and Cober risk overdiagnosis without clinical correlation, as their use of stricter EFAD diagnoses using Mead acid: arachidonic acid ratios > 0.05 classified multiple additional patients with EFAD who never demonstrated associated symptoms. In contrast, the Holman Index directly reflects the compensatory shift to Mead acid synthesis during EFA deprivation, making it a sensical and clinically interpretable biomarker. To reconcile these perspectives, it is recommended continue to use the historical Holman Index threshold of 0.2 for diagnosing EFAD, but to closely monitor trends in the Mead acid: arachidonic acid ratio in high-risk patients. For example, if an infant’s ratio rises above normal-for-age using other existing reference ranges (e.g. into the 0.05–0.1 range), clinicians should increase essential fatty acid provision proactively rather than waiting for the ratio to exceed 0.2.

Although newer techniques offer more comprehensive fatty acid profile data, they should be viewed as complementary to the Holman index rather than as replacements, particularly considering the existing variability across the literature. Unlike in peroxisomal or fatty acid oxidation disorders, where absolute fatty acid levels have defined diagnostic value, EFAD is best assessed through the Holman Index as it reflects a physiologic process rather than population-based or laboratory analysis fluctuations. Importantly, animal and human studies with ILEs such as FOLEs utilized the Holman Index to successfully demonstrate that alternative fatty acid compositions do not result in biochemically or clinically significant EFAD when downstream metabolites such as DHA and ARA are present—further supporting the index as the appropriate diagnostic anchor. And although the Holman Index does not directly incorporate the omega-3 pathway, it indirectly accounts for deficiency in both omega-3 and omega-6 fatty acids by evaluating the physiologic rise in Mead acid synthesis. The continued use of the Holman Index ensures universal interpretability and clinical utility, making it a foundation in the diagnosis and monitoring of EFAD.

In pediatric populations, especially premature and malnourished infants, careful monitoring of the Holman Index is essential. Case reports highlight its sensitivity in detecting biochemical EFAD before overt symptoms arise, allowing clinicians to intervene early and prevent long-term consequences. Even when the diagnostic threshold of 0.2 is not reached, upward trends in the ratio may carry clinical significance in high-risk contexts, reinforcing its value as both a diagnostic and monitoring tool.

Despite the advent of newer ILE formulations and the continued advancement in laboratory methods, the Holman Index remains as the diagnostic gold standard for EFAD. Its physiologic basis, clinical validation, and cross-population reproducibility ensure consistency throughout clinical care and research. Existing knowledge of fatty acid metabolic pathways explains the rationale behind the Mead acid: arachidonic acid ratio and its established diagnostic threshold value make it ideal for easily assessing treatment efficacy and guiding necessary EFA supplementation - with modern-day emphasis not just on the resultant index value but also its trends. The simplicity and reliability of the Holman Index offer strong advantages in clinical decision-making, allowing for early detection and timely intervention before symptoms arise. Moving forward, the Holman Index should remain the benchmark for diagnosis, ensuring diagnostic reliability and protecting at-risk patients from the serious consequences of EFAD.

Highlights.

• The Holman Index (Mead acid: arachidonic acid ratio ≥0.20) in plasma remains the gold standard for EFAD diagnosis.

• Fatty acid profiling is variable and not reliable for EFAD diagnosis in isolation

• Holman Index reliably detects EFAD early and guides timely clinical intervention.

• Monitoring trends in high-risk infants works to prevent consequences of EFAD