Diabetes and Fatty Liver

Nonalcoholic fatty liver disease (NAFLD) affects more than 25% of the adult population worldwide. According to analyses for 2016, Germany ranks third behind Greece (41%) and Italy (25.4%) in the prevalence of NAFLD (22.9% of the total population). An increase in the prevalence of NAFLD to 26.4% has been calculated for Germany (2.3) for the year 2030. At around 70%, the frequency of NAFLD is particularly high in people with obesity and/or type 2 diabetes [2]. However, NAFLD also occurs in about 7% of lean people and is then primarily of genetic origin [2]. In Europe and the USA, NAFLD is now regarded as the most frequent cause of chronic liver diseases although most people with NAFLD die from secondary diseases resulting from diabetes or cardiovascular diseases. Therefore, it is particularly important to test patients with type 2 diabetes for the presence, and especially the degree of severity, of NAFLD, and to plan therapy accordingly [3] [4].

Definition

A fatty liver can have many causes. First, a systematic evaluation is performed, and if suspected, laboratory tests to confirm specific illnesses or drug therapies are carried out ([Table 1]). If no evidence is found for these diseases, it is usually because NAFLD is present. There are two types of NAFLD: Nonalcoholic fatty liver (NAFL) is not associated with any relevant inflammation or liver fibrosis and affects about 70% of people with NAFLD. The second type includes nonalcoholic steatohepatitis (NASH), liver fibrosis and cirrhosis of no specific cause. These represent advanced stages of NAFLD, with NASH present in about 30% of people with NAFLD. In people with fatty liver and diabetes, the probability of having NASH is >40% (1,4).

Table 1 Causes of fatty liver.

Causes	Diagnostic
Non-alcoholic fatty liver	Steatosis with none of the causes listed below
Alcohol	>21 standard drinks* per week for men.>14 standard drinks* per week for women
Medication	E.g. glucocorticoids, estrogens, amiodarone, tamoxifen, tetracycline, methotrexate, valproic acid, antiviral drugs, perhexiline maleate, chloroquine
Viral hepatitis	Virus serology
Autoimmune hepatitis	Autoimmune serology
Hemochromatosis	Elevated ferritin levels and transferrin saturation in serum
Wilson's disease	Lower levels of caeruloplasmin in serum
Alpha-1-antitrypsin deficiency	Lower alpha-1 antitrypsin levels in serum
Celiac disease	Anti-gliadin antibodies, anti-tissue transglutaminase
Other	E.g. severe malnutrition, hypobetalipoproteinemia, lipodystrophy, pronounced chronic inflammatory bowel diseases

*1 standard drink contains 14g alcohol.

Diagnostic

NAFL is currently diagnosed by ultrasound examination, proton magnetic resonance spectroscopy 1H-MRS and MR imaging (MRI) [Fig. 1]. The two non-invasive MR methods allow a precise determination of the lipid content of the liver and are therefore preferred to quantification of the lipid content of the liver using liver biopsy. The liver biopsy is currently the most suitable method for diagnosing inflammatory changes, i. e. NASH, as well as for the diagnosis of liver fibrosis. Ultrasound or MR-based techniques such as FibroScan and MR elastography (MRE) are quite accurate, but also expensive, non-invasive methods for diagnosing fibrosis ([Table 2]). Tests and scores based on anthropometric and laboratory chemical parameters are also available and can be used for risk assessment of NASH and fibrosis. In addition to aminotransferase (ALT/AST), special tests are available which are primarily used for diagnosing fibrosis stages 3 and 4 [4] [5] [6] [7] although their accuracy seems to be lower, especially in diabetes mellitus [8].

Fig. 1 Diagnostic flow-chart to assess and monitor disease severity in the presence of suspected NAFLD and metabolic risk factors, according to [6], [rerif].

Table 2 Diagnosis of NAFLD.

Method	Characteristics	Advantages	Disadvantages
Liver biopsy	Lipid droplets in >5% of hepatocytes	To date, the reference method for lipid determination The reference method for the determination of inflammation and fibrosis	Not suitable for screening Can result in sampling errors Invasive Prone to complications
Sonography	Liver and kidney echogenicity Border to the diaphragm and intrahepatic structures	Widely available Inexpensive	Low sensitivity and specificity at lipid content <25%.
Fatty liver index (FLI)	BMI Waist circumference Gamma GT Fasting triglycerides	Widely available Inexpensive	Low sensitivity and specificity at lipid content <25%.
Indices for fibrosis (non-commercial: NAFLD-FS, FIB-4 Commercial score: ELF, FibroTest, FibroMeter)	Formulas using the following parameters: Age, BMI, Fasting blood glucose, Diabetes diagnosis, (AST), (ALT), Gamma-GT (GGT), Platelets, Albumin and additionally specific blood markers	Widely available Inexpensive	Low sensitivity and specificity at lipid content <25%.
Transient Elastography	Propagation of the pulse of a low frequency transducer for estimating the lipid content and the degree of fibrosis	Non-invasive Can better assess lipid content than the fatty liver index or the fibrosis indices	Lower sensitivity and specificity for obesity Relatively expensive
Computer tomography	Houndsfield units	Can better assess lipid content than fatty liver index or transient elastography	Radiation exposure Inferior to MR imaging
MR imaging and spectroscopy	MR-based measurement of the proton density of triglyceride and water (MR-PDFF) 1H-MR spectroscopy	Very precise for diagnosis of lipid content Low sampling error	Extremely expensive
MR elastography	MR-based imaging of tissue excitation by low-frequency sound waves	Relatively well-suited for non-invasive diagnosis of fibrosis Low sampling error	Extremely expensive

Risk for Advanced Liver Diseases and Cardiometabolic Diseases in NAFLD

In a large meta-analysis of 11 studies it was shown that in people with NAFLD with fibrosis detected by liver biopsy, over a period of 2145.5 person years, progression was observed in 33% of people, stabilization in 43% and regression of fibrosis in 22% [9]. Interestingly, however, the same percentage of people with NAFL or NASH (about 18% each) without fibrosis in the first liver biopsy have progressed to advanced fibrosis in the subsequent biopsy [9]. In NAFLD, hepatocellular carcinoma can also develop directly from NAFL without having had NASH [1].

People with NAFLD have a 2–6 times higher risk of type 2 diabetes and/or cardiovascular disease [10]. This risk is particularly high if there is abdominal obesity and especially if there is insulin resistance. As more people with NAFLD die from complications of diabetes, including cardiovascular disease [1], is of utmost importance to above all diagnose and prevent diabetes-related and cardiometabolic diseases as well as advanced liver diseases.

Therapy for NAFLD

First and foremost, in the therapeutic approach and prevention of progression of NAFLD is a lifestyle modification including a balanced, calorie-reduced diet and an increase in physical activity ([Table 3]). The effectiveness of lifestyle intervention fundamentally depends on the achieved reduction in body weight. Weight loss of about 5% results in a 30% reduction of the liver lipid content. However, to positively influence hepatic inflammation and fibrosis, weight loss of more than 10% is likely necessary. For effective NAFLD therapy, revised nutritional mealplans should include a reduction in fast-digesting carbohydrates, especially of products containing fructose, and of saturated fatty acids. Endurance and strength training can also be effective in addition to diet modification [4].

Table 3 Effects of intervention on NAFLD and diabetes.

Entervention	Effects on the liver	Systemic effects
Lifestyle	Steatosis: ↓↓↓ Inflammation: ↓↓ Fibrosis: ↓or  =	Blood glucose: ↓↓ Insulin resistance: ↓↓ Dyslipidaemia: ↓ Weight: ↓
Bariatric surgery	Steatosis: ↓↓↓ Inflammation: ↓ ? Fibrosis: ?	Blood glucose: ↓↓↓ Insulin resistance: ↓↓↓ Dyslipidaemia: ↓ Weight: ↓↓↓
Pioglitazone	Steatosis: ↓↓↓ Inflammation: ↓↓ Fibrosis: ↓or =	Blood glucose: ↓↓ Insulin resistance: ↓↓↓ Dyslipidaemia: ↓↓ Weight: ↑
GLP-1 analogues	Steatosis: ↓↓ Inflammation: ↓ Fibrosis: =	Blood glucose: ↓↓ Insulin resistance: ↓↓ Dyslipidaemia: ↓ Weight: ↓
SGLT-2 inhibitors	Steatosis: ↓ Inflammation: ? Fibrosis: ?	Blood glucose: ↓↓ Insulin resistance: ↓ Dyslipidaemia: ↓Weight: ↓

Bariatric surgery for pronounced obesity or moderate obesity and type 2 diabetes causes a pronounced reduction in the liver lipid content as well as weight loss, although effects on inflammation and fibrosis of the liver have not yet been sufficiently investigated [4].

So far, no pharmacological therapy has been approved to treat NAFLD. If type 2 diabetes is present, however, drugs can be used to specifically treat diabetes in order to also treat NAFLD. The joint guidelines of the European Association for the Study of the Liver (EASL), the European Association for the Study of Diabetes (EASD) and the European Association for the Study of Obesity (EASO) as well as those of the American Association for the Study of Liver Diseases recommend the use of pioglitazone if there are no associated contraindications (heart failure, history of bladder carcinoma, increased risk of bone fractures) (4.5). Recent data from studies with relatively small case numbers indicate that GLP-1 receptor agonists (GLP-1: glucagon-like peptide 1) such as liraglutide and SGLT-2 inhibitors (SGLT-2: sodium-dependent glucose transporter 2) can reduce the liver lipid content in NAFLD and type 2 diabetes. All other pharmacological therapies for type 2 diabetes have so far shown no clinically-relevant effects on the course of NAFLD [4].

Outlook

The increasing prevalence of NAFLD in the most common metabolic diseases such as obesity and type 2 diabetes requires targeted screening and careful diagnosis of liver diseases in these patient groups. Early prevention or therapy of NAFLD will reduce both the liver-specific as well as the diabetic consequences and complications. In the future, this will require the full use of all existing diagnostic possibilities including fibrosis screening on the one hand, and, on the other hand, the further development of cost-effective and non-invasive or low-invasive tests. The aim is to reduce the use of liver biopsies for diagnosis and, above all, to assess the course of NAFLD and the effectiveness of therapies. At present, there are still no large studies that have convincingly demonstrated the effectiveness of new monotherapies or combination therapies of existing drugs. However, different innovative therapy concepts are already being tested experimentally and clinically so that specific therapy recommendations for the increasing number of patients with NAFLD and diabetes can be expected in the near future.

Conflict of interest

N. S. has participated in Scientific Advisory Boards of Gilead, Genkyotex, AstraZeneca, Boehringer Ingelheim, Sanofi, and clinical trials of AstraZeneca, Boehringer Ingelheim, Sanofi, DSM Nutritional Products and Roche Diagnostics. M. R. has participated on Scientific Advisory Boards of BMS, Boehringer Ingelheim Pharma, Eli Lilly, Fishawack Group, Gilead Sci., Novo Nordisk, Poxel S.A. Sociéte, Prosciento Inc., Sanofi, Servier Lab., Target Pharmasolutions, Terra Firma and in clinical trials of Astra Zeneca, Boehringer Ingelheim, Nutricia/Danone and Novartis.

German Diabetes Association: Clinical Practice Guidelines

This is a translation of the DDG clinical practice guideline published in:Diabetologie 2019; 14 (Suppl 2): S222–S225.

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