# TOFI: visceral and ectopic fat at a normal body weight

- Canonical: https://nutrifit.health/knowledge/tofi-phenotype-ectopic-fat-visceral-adiposity
- Language: en
- Published: 2026-10-08T16:13:12.858Z
- Updated: 2026-10-08T16:13:12.858Z
- Author: NutriFit Editorial

> Why normal BMI does not rule out metabolic risk: adipose expandability, organ lipids, insulin signaling, correct TyG, HOMA-IR and FLI formulas, assessment methods and intervention limits.

## What a normal BMI can conceal

TOFI stands for Thin Outside, Fat Inside. It describes a mismatch between apparent body size and internal fat distribution. MONW, metabolically obese normal weight, primarily describes metabolic abnormalities, whereas TOFI emphasizes distribution. They overlap but are not interchangeable diagnoses. Research definitions differ, and there is no single universally accepted TOFI test.

Body mass index is calculated as

$$
\mathrm{BMI}=\frac{m}{h^2},
$$

with mass $m$ in kilograms and height $h$ in metres, yielding kg/m². A person weighing 72 kg at 1.78 m has a BMI of approximately 22.7 kg/m². The calculation cannot distinguish muscle, bone, fluid, and fat. The adult category 18.5–24.9 kg/m² does not guarantee minimal individual risk. BMI remains useful at a population level, complemented by waist measurement, history, and metabolic findings.

Ruderman and colleagues described normal-weight individuals with insulin resistance and related abnormalities in 1981 and 1998. The Thomas and Bell group's imaging work demonstrated substantial variation in subcutaneous, abdominal, and organ fat at similar BMI. This does not mean that most slim people have a disease. Widely repeated figures of “45% of women and 60% of men” cannot be generalized without defining the phenotype and sample. Fixed relative risks for diabetes or mortality likewise do not apply automatically to every person labeled TOFI.

Subcutaneous adipose tissue, SAT, lies beneath the skin. Visceral adipose tissue, VAT, includes abdominal depots such as the omentum and mesentery. Ectopic fat refers to lipid accumulation in other organs and tissues, including liver and skeletal muscle. These compartments are related but cannot reliably substitute for one another. A small waist reduces the likelihood of marked abdominal accumulation without ruling out liver steatosis. MRI and proton spectroscopy investigate distribution and particular lipid pools; they complement other risk measures.

### Muscle mass and sarcopenia

Normal weight can coexist with relatively high body fat and low muscle mass. Low mass alone does not establish sarcopenia: strength, muscle quantity or quality, and physical performance matter. Appropriate norms vary with age, sex, and population. Body-fat thresholds of 25% for men and 35% for women are not universal TOFI diagnostic boundaries.

Muscle is a major site of insulin-stimulated glucose disposal, glycogen storage, and fatty-acid oxidation. Frequently cited proportions of 75–80% refer to particular conditions of insulin-stimulated metabolism, rather than all glucose following every meal. Reduced muscle mass, myosteatosis, and inactivity may lower metabolic capacity. Muscle is not the sole “BCAA depot,” and muscle loss does not establish that all dietary carbohydrate must become liver fat.

## Adipose expandability: why storage location matters

The expandability hypothesis discussed by Virtue and Vidal-Puig considers SAT's ability to store excess energy safely. Its capacity is limited and varies between individuals. Expansion involves both cell enlargement and formation of new adipocytes, influenced by depot, genetics, age, and environment. “Infinite subcutaneous capacity” contradicts the hypothesis itself. Lipodystrophies clearly illustrate how little subcutaneous fat can coexist with severe metabolic dysfunction, but cannot be equated with every TOFI presentation.

During adipogenesis, early C/EBPβ and C/EBPδ help induce PPAR-γ and C/EBPα. PPAR-γ partners with RXR and regulates FABP4, GLUT4/SLC2A4, LPL, PLIN1, and ADIPOQ. Wnt/β-catenin signaling can inhibit differentiation. Precursor dysfunction and extracellular-matrix remodeling are possible contributors to limited storage. PPARG hypermethylation or precursor exhaustion is not, however, an established obligatory defect in every normal-weight person with insulin resistance.

For a geometric spherical model,

$$
V=\frac{4}{3}\pi r^3.
$$

Doubling radius or diameter increases volume eightfold. A real cell is more complex; this illustrates geometry rather than a diagnostic hypertrophy threshold. Blood supply, oxygen consumption, and capillary organization contribute to oxygenation alongside size. Neither a universal critical diameter of 100–120 µm nor identical tissue oxygen pressure is established for all people.

With inadequate oxygen, prolyl hydroxylation of HIF-1α decreases, reducing recognition by pVHL and proteasomal degradation. HIF-1α associates with HIF-1β to regulate the hypoxic response. In experimental adipose tissue this program is connected to angiogenesis, collagen remodeling, and LOX-mediated cross-linking. Fibrosis can constrain depot adaptation. Human relationships between oxygenation, cell size, and insulin resistance are more complex than an inevitable linear chain.

### Endoplasmic-reticulum stress and immune cells

The endoplasmic reticulum responds to disturbed protein folding through IRE1α, PERK, and ATF6. PERK/eIF2α/ATF4 and CHOP connect adaptive responses with injury under sustained stress; IRE1α can engage TRAF2/JNK. Mitochondrial BAX/BAK signaling and caspases participate in particular death pathways. It is unjustified to assert that every TOFI adipocyte dies exclusively through necrosis or pyroptosis.

Macrophages surrounding dying adipocytes form crown-like structures, CLS. CCL2/MCP-1 and CCR2 contribute to monocyte recruitment; NF-κB and stress kinases regulate inflammatory programs, while NLRP3/caspase-1 participates in IL-1β and IL-18 maturation. TNF-α can impair insulin signaling, and chronic IL-6 signaling is associated with the hepatic acute-phase response. Adiponectin, AdipoR1/R2, and AMPK contribute to metabolic regulation.

The M1/M2 distinction is a useful model, but human tissue contains intermediate and specialized macrophage states. CD11c is not a universal TOFI marker. Palmitate should not be presented as an undisputed direct TLR4 ligand: TLR4 dependence of an inflammatory response does not demonstrate direct fatty-acid binding. These pathways explain possible adipose dysfunction without replacing clinical evidence.

### Insulin and the lipolytic cascade

Insulin is an important inhibitor of lipolysis. In a principal model, INSR/IRS/PI3K/Akt signaling supports PDE3B, lower cAMP, and reduced PKA activity. Perilipin phosphorylation changes ABHD5/CGI-58 access to ATGL, while HSL moves to lipid droplets. Particular phosphorylation sites depend on species and model and are not patient diagnostic markers.

Sequential hydrolysis can be represented as

$$
\begin{aligned}
\mathrm{TAG}+\mathrm{H_2O}&\xrightarrow{\mathrm{ATGL/ABHD5}}\mathrm{DAG}+\mathrm{FA},\\
\mathrm{DAG}+\mathrm{H_2O}&\xrightarrow{\mathrm{HSL}}\mathrm{MAG}+\mathrm{FA},\\
\mathrm{MAG}+\mathrm{H_2O}&\xrightarrow{\mathrm{MGL}}\mathrm{glycerol}+\mathrm{FA}.
\end{aligned}
$$

TAG, DAG, and MAG are tri-, di-, and monoacylglycerols; FA denotes a fatty acid. ATGL is encoded by PNPLA2 and HSL by LIPE. Reduced antilipolytic sensitivity may increase nonesterified fatty-acid, NEFA, delivery to other tissues. This need not mean total signal abolition or continuously maximal lipolysis. Adipocyte glycerol-kinase activity is relatively low, but claiming absolute absence is too categorical.

Omental and mesenteric depots drain into the portal vein and influence liver substrate delivery. Subcutaneous fat releases NEFA into systemic circulation and can also be a major source of liver fatty acids. The pancreas does not receive portal blood as its nutrient supply: its arterial circulation rules out the original manuscript's proposed direct “portal assault” on islets.

[![Fat distribution and systemic or portal fatty-acid delivery](https://api.nutrifit.health/api/v2/public/knowledge/files/images/nutrifit-scientific-20261002-tofi-phenotype-ectopic-fat-visceral-adiposity-fat-partition-en-v2.png)](https://api.nutrifit.health/api/v2/public/knowledge/files/images/nutrifit-scientific-20261002-tofi-phenotype-ectopic-fat-visceral-adiposity-fat-partition-en-v2.png)

## Organ mechanisms and their limits

### Liver: lipid intermediates and insulin signaling

Liver fat reflects fatty-acid uptake, de novo lipogenesis, oxidation, and VLDL export. CD36 and FATP2/5 participate in fatty-acid handling; FATP proteins are associated with acyl-CoA synthetase activity. A triglyceride droplet is a storage structure, not proof that every stored lipid is toxic. Intermediates, localization, and turnover matter.

Membrane sn-1,2-DAG can activate PKC-ε. Petersen and colleagues' 2016 work investigated human INSR Thr1160 phosphorylation and corresponding mouse Thr1150, using biochemical experiments and amino-acid-substitution mice. This provides strong experimental support for a mechanism, rather than proof of complete receptor shutdown in every patient. Ceramides, synthesized with serine palmitoyltransferase participation, form another signaling-lipid class whose effects depend on molecular species and compartment.

Reduced IRS/PI3K/Akt signaling can alter FoxO1, PCK1/PEPCK, and G6PC regulation. Lipogenesis controlled by SREBP-1c, ChREBP, substrate availability, and mTORC1 may remain increased. “Selective hepatic insulin resistance” describes inadequate suppression of glucose production alongside continuing lipid synthesis. It cannot be reduced to one linear branch point: indirect insulin action through adipose tissue and substrate delivery are also important.

PNPLA3 I148M and TM6SF2 E167K are associated with steatosis susceptibility. PNPLA3 variant accumulation on droplets and interaction with ABHD5 are studied as components of altered lipid remodeling. TM6SF2 affects VLDL lipidation and secretion; its variant may combine increased liver fat with lower circulating lipids. Thus steatosis and atherogenic dyslipidemia need not coincide. Genotype changes probability rather than predetermining severe fibrosis in every carrier.

MASLD denotes steatotic liver disease with cardiometabolic criteria. Older NAFLD studies used a different definition and cannot simply be renamed without considering their participants. Alcohol, medicines, viral disease, and other liver conditions remain relevant at normal weight. Normal ALT does not exclude substantial steatosis or fibrosis.

### Pancreas and the twin cycles

MRI-measured pancreatic fat includes different tissue components and is not a direct measurement of β-cell lipid. Experiments involving excess saturated fatty acids show ER stress, ceramide and mitochondrial signals, altered PDX-1/MafA, and changed insulin secretion. SNARE proteins participate in granule exocytosis. First-phase secretion may deteriorate in diabetes, but total loss, dedifferentiation, or β-to-α-like conversion does not follow from a single image of a fatty pancreas.

Taylor's twin-cycle hypothesis connects excess liver fat, glucose production, hyperinsulinemia, and triglyceride export with worsening β-cell function. The personal fat threshold offers an explanation for diabetes developing below conventional obesity cutoffs. It is a research model, not an available individual laboratory test for an exact threshold. Reduced organ fat and improved secretion in some study participants do not demonstrate that losing 0.5–1 g of pancreatic fat guarantees lasting remission for everyone. Diabetes duration and preserved β-cell function matter.

### Muscle, heart, and blood vessels

Muscle DAG/PKC-θ signaling can impair IRS-1/PI3K/Akt/TBC1D4, also called AS160, and Rab-regulated GLUT4 trafficking. IRS-1 Ser1101 is one investigated phosphorylation site; Ser307 generally refers to the rodent protein, with human Ser312 as its counterpart. Some serine modifications have context-dependent effects. A universal 70–85% decline in GLUT4 trafficking in TOFI is unestablished. High intramyocellular lipid, IMCL, also occurs in trained athletes with high insulin sensitivity: droplet turnover and oxidative capacity matter alongside quantity.

Epicardial adipose tissue, EAT, lies adjacent to myocardium and coronary vessels, whereas PVAT surrounds blood vessels. Their normal metabolic and paracrine functions can be protective. Dysfunctional depots may influence vessel walls through inflammatory mediators, oxidative stress, and altered adiponectin in an outside-in direction. Experimental mechanisms include eNOS uncoupling, reduced NO availability, ICAM-1/VCAM-1, and impaired vasoregulation. EAT volume alone does not establish endothelial erosion, an unstable plaque, or an inevitable infarction; universal UCP1 loss is likewise unproven.

[![Liver and muscle lipids in experimental insulin-signaling mechanisms](https://api.nutrifit.health/api/v2/public/knowledge/files/images/nutrifit-scientific-20261002-tofi-phenotype-ectopic-fat-visceral-adiposity-organ-signaling-en-v2.png)](https://api.nutrifit.health/api/v2/public/knowledge/files/images/nutrifit-scientific-20261002-tofi-phenotype-ectopic-fat-visceral-adiposity-organ-signaling-en-v2.png)

## Risk assessment: different measures answer different questions

Waist-to-height ratio is $\mathrm{WHtR}=WC/h$, using identical length units. A value around 0.5 is a simple abdominal-risk guide, not a TOFI diagnosis or a rule-out test for organ fat below that level. WHR is waist divided by hip circumference. Waist and WHR cutoffs vary with sex, ethnicity, and recommendations; standardized measurement matters. No index has identical accuracy at every age and ancestry.

High triglycerides, low HDL-C, and small dense LDL may accompany insulin resistance. CETP-mediated lipid exchange and hepatic lipase contribute to particle remodeling. This combination is not pathognomonic for TOFI. ApoB reflects the number of atherogenic particles, including LDL, IDL, VLDL, and Lp(a), and can help when LDL-C and particle burden disagree. Clinical risk and treatment decisions require the wider context; neither ApoB 90–100 mg/dL nor a fixed ApoB/ApoA1 ratio establishes TOFI universally.

### TyG, HOMA-IR, and FLI: formulas without diagnostic substitution

A commonly used TyG expression is

$$
\mathrm{TyG}=\ln\!\left(\frac{TG\times G}{2}\right),
$$

where fasting triglycerides $TG$ and glucose $G$ are both numerically in mg/dL. Some literature instead uses $\ln(TG\times G)/2$. Division inside and outside a logarithm is not equivalent. With $TG=150$ and $G=100$, the first gives about 8.92 and the second 4.81. A cutoff near 4.5 cannot be transferred to the first scale, nor can the difference be explained simply as “no division by two.” Simental-Mendía's 2008 study included 748 people and reported 84% sensitivity and 45% specificity; the original claim of simultaneously high specificity is incorrect. Thresholds depend on definition and sample.

$$
\mathrm{HOMA\!\!\!-IR}=\frac{G\,(\mathrm{mmol/L})\times I\,(\mathrm{\mu U/mL})}{22.5}.
$$

$I$ denotes fasting insulin. With glucose in mg/dL, the denominator is 405. Assay method, population, and current state affect interpretation. A value above 2 does not prove ectopic lipotoxicity or supply a universal diagnosis. Glucose, HbA1c, or an indicated OGTT address glycemic status better than labeling TOFI from one surrogate.

Bedogni and colleagues' Fatty Liver Index uses

$$
\begin{aligned}
y&=0.953\ln(TG)+0.139\,BMI+0.718\ln(GGT)+0.053\,WC-15.745,\\
\mathrm{FLI}&=100\frac{e^y}{1+e^y}.
\end{aligned}
$$

Use $TG$ in mg/dL, $GGT$ in U/L, $WC$ in centimetres, and BMI in kg/m². FLI ranges from 0 to 100 and estimates ultrasound steatosis probability in its original model. It does not measure fat fraction or fibrosis. Cutoffs of 30 and 60 are used for ruling out and suggesting steatosis in appropriate settings; low BMI may affect sensitivity in lean people. GGT and ALT are nonspecific: normal results do not exclude liver disease and elevated results do not establish its cause.

| Measure | What it can assess | Main limitation |
|---|---|---|
| Waist, WHtR, WHR | Abdominal distribution and risk | Do not measure liver or β-cell fat |
| DXA | Fat and lean tissue, bone mineral; VAT estimates on some systems | VAT is algorithmically estimated; lean tissue is not pure muscle |
| Grip strength and ASMI | Components of sarcopenia assessment | Appropriate norms and clinical context are required |
| Elastography CAP | Ultrasound attenuation associated with steatosis | Cutoffs depend on conditions; CAP is not fibrosis measurement |
| MRI-PDFF and ¹H-MRS | Quantitative organ-lipid assessment | Method and compartment matter; no universal pancreatic cutoff |

ASMI is appendicular muscle mass in kilograms divided by squared height in metres. EWGSOP2 uses low strength for probable sarcopenia, low quantity/quality for confirmation, and poor performance for severe disease. Common European ASMI guides of 7.0 and 5.5 kg/m² and grip-strength values of 27 and 16 kg for men and women are not TOFI criteria. “VAT volume 100 cm²” is dimensionally wrong: cm² measures area, cm³ volume. Neither 100 cm² nor 500 g supplies a universal phenotype threshold.

Historical CAP values around 248/268/280–295 dB/m arose in different samples and do not create immutable S1–S3 bands for every instrument and patient. Liver stiffness is assessed separately. Liver MRI-PDFF around 5% is often a research steatosis guide, with protocol-dependent definitions. Pancreatic fat of 6.2% is not a universally accepted clinical normal limit.

## Exercise and nutrition: what mechanisms justify

The objective is metabolic health, strength, and body composition, not obligatory weight reduction. Unnecessary severe restriction may impair nutrition and muscle preservation. Conversely, claiming that every energy deficit at normal BMI causes catastrophe or 40–50% muscle loss is incorrect. With established diabetes or steatosis, the appropriateness of reducing fat mass is individualized while preserving muscle; someone with low body mass may need a different goal.

Resistance exercise stimulates adaptation through mechanical signaling, mTORC1, p70S6K, and 4E-BP1. Contraction enhances glucose uptake through partly insulin-independent pathways involving AMPK, calcium signaling, and TBC1D4. Aerobic activity supports oxidative capacity, PGC-1α/NRF/TFAM-related adaptation, and mitochondrial metabolism. Long-chain fatty-acid transport is explained further in the article on [carnitine palmitoyltransferases and fat oxidation](/knowledge/carnitine-palmitoyltransferase-cpt1-fatty-acid-oxidation).

Irisin/FNDC5, IL-15, and muscle-derived IL-6 are investigated as myokines. Experimental effects do not guarantee clinical browning or selective VAT removal in every person. Acute muscle IL-6 is not equivalent to chronic inflammatory signaling. “Zone 2” depends on the zone system, while FATmax and lactate thresholds vary individually. A fixed lactate concentration of 1.5–2 mmol/L does not define optimal training for everyone. Training need not eliminate all IMCL; athletes may maintain high stores.

Protein supplies amino acids for muscle adaptation. Intakes of 1.6–2.2 g/kg were studied mainly in training contexts and generally refer to body weight, not automatically to a target lean mass. They are not mandatory TOFI doses; age, overall diet, and kidney disease affect decisions. Leucine/Sestrin2/Rag/mTORC1 explains amino-acid sensing, but a universal 2.7–3 g threshold at every meal does not establish individual requirements.

Fructose metabolism involves KHK, fructose-1-phosphate, and triose intermediates; large exposure can promote lipogenesis and alter cellular energetics. Considerable metabolism also occurs in the intestine. Ordinary portions of whole fruit are not equivalent to excessive sweetened drinks. Food quality, added sugars, and energy excess are reasonable topics without declaring fructose a universal hepatotoxin. Fibre supports dietary structure and microbial metabolites; SCFA/FFAR2/3 and GLP-1/PYY pathways do not guarantee therapeutic benefit from a fixed 35–45 g intake for all people.

### Supplements: plausibility is not established treatment

Omega-3 fatty acids can lower triglycerides and influence lipid regulation, including PPAR-α and SREBP. REDUCE-IT studied prescription icosapent ethyl 4 g/day in higher-risk statin-treated patients, not every EPA/DHA mixture in all slim people. Its results do not show that supplements break the pancreatic cycle. Product and indication must be considered in relation to the actual trial, including adverse events.

Berberine and dihydroberberine are investigated in relation to complex I, AMPK/ACC, and lipid metabolism. Many detailed mechanisms originate from cells or rodents; they do not establish universal clinical doses or prevention of TOFI complications. The article on [berberine, AMPK, and PCSK9](/knowledge/berberine-vs-dihydroberberine-ampk-pcsk9) further distinguishes evidence levels.

Phosphatidylcholine is needed for membranes and VLDL assembly. It is synthesized through the Kennedy CDP-choline pathway and PEMT, in which phosphatidylethanolamine receives three methyl groups from SAM, forming phosphatidylcholine and three SAH molecules. Choline deficiency can cause liver abnormalities. Physiological necessity does not demonstrate that alpha-GPC, lecithin, or 500–1000 mg additional choline treats every steatosis case; increased VLDL export is not equivalent to improved overall cardiovascular risk.

CoQ10 participates in electron transport and redox metabolism. Lipoate is a bound cofactor of PDH and OGDH; ingesting free R-ALA is not a direct replacement for intracellular lipoylation. Nrf2, glutathione, and lipid oxidation are research topics, not confirmation of islet restoration. SYDNEY 2 studied diabetic polyneuropathy symptoms, rather than TOFI remission or pancreatic-fat clearance. A universal omega-3, berberine, choline, CoQ10, and R-ALA combination is unestablished.

## Integrating assessment and recognizing review limits

| Area | Possible abnormality | Informative assessment | What cannot automatically be inferred |
|---|---|---|---|
| SAT/VAT | Limited storage and altered NEFA delivery | Distribution and metabolic context | Waist does not diagnose cellular fibrosis |
| Liver | Steatosis, signaling lipids, fibrosis risk | Causes of steatosis and separate fibrosis assessment | FLI or ALT does not grade fibrosis |
| Pancreas | Relationship between organ fat and β-cell function | Glycemic status and clinical presentation | MRI fat does not measure dedifferentiation |
| Muscle | Low strength/mass, myosteatosis | Strength, function, body composition | Low ASMI does not make every weight dangerous |
| Heart and vessels | Atherogenic particles and paracrine relationships | Blood pressure, lipids, context-specific ApoB | EAT does not establish an unstable plaque |

A reasonable sequence clarifies history and measurements, assesses standard risk factors and glycemic status, and selects additional methods for a particular question. This review does not justify universal DXA, MRI, pancreatic spectroscopy, or insulin testing for every slim person. Follow-up depends on disease and the decision it will inform, rather than a fixed schedule of laboratory testing at 12 weeks and DXA at 24 weeks.

This is a narrative scientific review of the source manuscript, with targeted verification of disputed formulas, Thr1160 signaling, and selected clinical conclusions on 8 October 2026. It is not a systematic search, independent GRADE assessment, external peer review, or clinically validated protocol. Some primary pages returned access restrictions or incomplete responses; a full audit of every bibliographic identifier was not completed. Original bibliography links are retained as the literature base with that limitation. Normal weight does not rule out metabolic risk, but its magnitude and appropriate actions depend on individual findings rather than the TOFI label alone.

## FAQ

### Can metabolic risk be high at normal weight?

Yes. BMI does not show fat distribution, muscle function or glycemic status. Assessment integrates several findings.

### Does a waist below half of height rule out TOFI?

No. WHtR is an abdominal-risk guide rather than a direct organ-fat test.

### Which TyG formula is correct?

Different definitions exist. State formula and units; cutoffs around 4–5 cannot be transferred to ln(TG×G/2), usually yielding around 8–9.

### Does every slim person need MRI?

No. Additional imaging should answer a particular clinical question and inform a decision.

### Do supplements treat hidden organ fat?

A molecular effect does not establish clinical efficacy. A universal TOFI supplement combination is unconfirmed.

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