Diet
Although enzyme deficiencies in acute hepatic porphyrias are usually inherited, the enzyme deficiencies alone do not cause the disease. Additional factors that lead to a critical deficiency in the regulatory pool of heme within hepatocytes determine the disease manifestations [
1]. Moreover, it is widely accepted that increased excretion of ALA and PBG is not necessarily correlated with symptoms even though excretion of these metabolites is further increased during an acute attack. Also, a marked fluctuation in the excretion of ALA and PBG from day to day is found in symptomatic and asymptomatic AHP patients.
A stabilization in urinary levels of these metabolites was reported in AIP patients following a diet regimen that included a balanced content of protein, fat, and carbohydrate to maintain weight [
2]. Moreover, the same author reported that the reduced calorie intake to 60–80 percent and the isocaloric substitution of fat for protein alone or for protein together with carbohydrate were associated with increased excretion of both porphyrin-precursors. Considering the marked decrease in PBG excretion due to the addition of carbohydrate in the diet, they concluded that the fluctuation in the excretion of porphyrin-precursors is related to protein and carbohydrate intake rather than total caloric content per se.
Interestingly, increased calorie intake has been reported to be negatively correlated with urinary PBG levels in AIP patients suggesting that also the total energy intake affects the biochemical disease activity [
3].
Recently, nutritional assessments of VP and AIP patients have been carried out using validated questionnaires that included a 24 h recall during 7-day dietary record [
4]. No significant differences were observed in the nutrient intake and food pattern between symptomatic patients compared to asymptomatic carriers of VP [
5]. Moreover, no difference in sugar, candies, or slow-release carbohydrate foods intake was found between the asymptomatic and symptomatic AIP cases [
3]. On the contrary, significantly lower intakes of lipids, saturated fatty acids (SFA) and monounsaturated fatty acids (MUFA) were detected in AIP patients than in matched controls [
6]. However, energy, protein, carbohydrate, polyunsaturated fatty acids (PUFA), cholesterol, and daily fiber intakes were reported to be similar in both groups [
6].
Despite this, it has been reported that hypercholesterolemia may be present in AHP patients although without atherogenic potential [7. In order to improve the results of a diet study, it would be useful to use the gold standard method such as DEXA (dual-energy X-ray absorptiometry) for the calculation of basic metabolism and a direct approach through expert nutritional assessment [
8]. In summary, the changes in diet can alter the excretion of porphyrin-precursors in AHP patients. However, especially considering the lack of toxicity of ALA and PBG [
9], it should not be concluded that changes in diet are capable of affecting the clinical disease activity.
Glucose
The therapeutic effect of glucose in patients with AHP is well documented [
10,
11]. Carbohydrate loading is widely used not only for the treatment but also in the prophylaxis of the acute attack. Moreover, fasting is one of the precipitating factors in the AHP crises [
12]. The addition of glucose to cultured chick embryo hepatocytes and the carbohydrate feeding in rats have been first reported to cause a concentration-dependent impairment of drug-mediated induction of ALAS-1 enzyme [
13,14].
Successively, Handschin et al. revealed that PGC-1α (peroxisomal proliferator-activated cofactor 1α), a transcriptional co-activator of nuclear receptors and other transcription factors, is a key player in both porphyria induction via fasting and amelioration of symptoms by glucose treatment [
15]. Using mice with a liver-specific deficiency in PGC-1α and isolating primary hepatocytes, they demonstrated the ability of this factor to increase markedly the production of
ALAS-1 mRNA. Furthermore, the authors also established the effect of glucose loading in reducing the
ALAS-1 transcript levels 30 min after injection. The combination of glucose and insulin was more potent in inhibiting fasting-mediated induction of PGC-1α and ALAS-1, supporting the hypothesis that at least part of the beneficial effect of glucose in AHP attacks is mediated by the glucose-triggered increase of plasma insulin.
The induction of PGC-1α during fasting is due to glucagon action on the cAMP response element-binding transcriptional factor (CREB), which binds directly to the
PGC-1α promoter [
16]. Also, the direct activation of the
ALAS-1 promoter by CREB was described [
17]. Moreover, it has been established that the insulin pathway involving the protein kinase B (Akt) in the liver inhibits
ALAS-1 transcription [
18]. It is also known that the activated Akt, in turn, phosphorylates the transcriptional factor FOXO1 (Forkhead box protein O1) disrupting its binding to PGC-1α [
19,
20]. Then, it was believed that the activation of
ALAS-1 expression by PGC-1α is due to the co-activation of FOXO1, which binds to the insulin-response element in the promoter of
ALAS-1 and this interaction can be disrupted by insulin signaling [
21] ().
Figure 2. Regulation of ALAS-1 transcription by glucagon and insulin.
However, discordant data have been reported on this aspect. An in vivo study indicated that the serum glucose level was unchanged, but fasting insulin levels were higher and the glucagon was lower in mice with AIP than in the wild-type mice [
22]. Moreover, the drug-mediated induction of ALAS-1 significantly disturbs the hormonal status that regulates carbohydrate metabolism by increasing insulin levels while decreasing glucocorticoids synthesis, metabolization, and plasmatic levels in animal models
23]. Also, the finding that AIP patients with acquired type 2 diabetes mellitus did not longer have symptoms of AIP supports the protective role of elevated blood glucose levels [
24]. On the contrary, symptomatic AIP patients showed decreased insulin release and C-peptide levels in plasma associated with increased disease activity, indicating that a decreased glucose uptake by cells may explain accelerated heme synthesis [
25]. Fasting insulin is lower, and the glucose/insulin ratio is higher in AIP patients with high urinary PBG levels than in patients with low urinary PBG levels [
3].
At the same time, the screening of serum hepatic proteins revealed that a significant number of AIP patients presented a decrease in insulin-like growth factor 1 IGF-1, transthyretin (prealbumin) or both [
26]. Due to its structural similarity with insulin, IGF-1 interacts with insulin receptors and has insulin-mimicking effects. Besides, transthyretin and IGF-1 are useful markers for predicting nutritional status and are known to decrease during inflammation or liver disease [
27,28,29]. The decrease in transthyretin and IGF-1 levels in AIP patients therefore could reflect a metabolic disturbance restricted to the liver and/or the existence of chronic liver inflammation. A higher resistin level in the symptomatic than in the asymptomatic AIP patients and its positive correlation with leptin levels may indicate that inflammation, adipokines, and hormones affecting insulin resistance may be involved in higher disease activity [
3]. However, most inflammatory biomarkers and cytokines were not correlated with ALA, PBG or porphyrin levels [
25].
Conversely, accelerated protein degradation, decreased rate of synthesis of liver proteins and increased amino acid catabolism and nitrogen loss may be secondary to inflammatory disease because of increased metabolic demands [
30]. Thus, a change in liver energy metabolism in AIP patients could support the induction of ALAS-1, thereafter, worsening the symptoms of the disease and contributing to the persistence of the clinical manifestations. It was also found that liver graft recipients from an AIP patient developed AIP symptoms and increased PBG levels [
31] and that liver transplantation in severe AIP and VP patients normalizes the excretion of ALA and PBG [
32,
33] confirming the importance of the liver in the pathophysiology of this disease.
The activation of PGC-1α takes place through fasting and glucagon release that links its receptor (GLR) with consequent ATP reduction by adenylate cyclase in cAMP in the cytosol of hepatocytes. In turn, it leads to the activation of protein kinase A (PKA). In the nucleus of hepatocyte, the active form of PKA leads to the activation of transcription factor CREB that links the active site of CRE on PGC-1α gene promoter. The PGC-1α and FOXO-I bind together with the insulin responsive element (IRE) site on the ALAS-1 gene promoter, inducing transcription. The introduction of glucose and subsequent production of insulin, detected from membrane Insulin Receptor (IRs), lead to the phosphorylation of FOXO-I by the intervention of PI3K and AKTP. Phosphorylated transcriptional factor FOXO is carried out of the nucleus inhibiting the synergistic activation with PGC-1α on the ALAS-1 promoter.
Iron
Iron is classified as a nutritional and fundamental microelement required for oxygen transport, electron transfer, oxidase activities, and energy metabolism. The major intake of iron is through the diet. Foods that contain a relatively high concentration of iron include meat, fish, cereals, beans, nuts, egg yolks, dark green vegetables, potatoes, and fortified foods. The median dietary intake of iron in adults ranges between 9.4 and 17.9 mg/day (higher in males compared to females) with the average basal iron loss ranges 0.95–5.9 mg/day in men and 1.34–7.4 mg/day in women. Nutrient composition data for iron are derived from the European Food Safety Authority (EFSA) Nutrient Composition Database [
34].
Iron has a central role in heme biosynthesis and also in erythropoietic cutaneous porphyria due to the positive feedback between iron and ALAS2, the first enzyme of erythroid heme biosynthesis. Moreover, it serves as a substrate of the ferrochelatase enzyme in the last step of the heme pathway. Iron deficiency can occur in EPP and XLP patients [
35]. While iron overload is reported in PCT (in 90% of patients) [
36,37] and CEP (all patients) [
38]. The presence of hepatic iron in PCT patients has been associated with hemochromatosis (HFE) gene mutations, hepatitis C (HCV), and human immunodeficiency (HIV) viral infections. The presence of hepatic iron generates a UROD inhibitor, uroporphomethene, identified in the human liver biopsies of patients with PCT [
39].
The presence of intracellular iron also has a strong impact on the cellular redox status leading to an increase in oxidative status in these patients. Interestingly, a study on the dietary intervention was performed on a group consisting of 13 male PCT patients to decrease iron overload. Patients were evaluated for different parameters including serum iron level, before and after three weeks of the vegetable–fruit diet, and its daily caloric content was ca. 500 kcal/day. The results showed a significant decrease in iron and ferritin levels after dietary caloric restriction [
40]. Given that, dietary caloric restriction could be used in support of phlebotomy (specific therapy for the reduction of porphyrins in the blood) in order to reduce iron overload as suggested in the latest review on PCT [
41]. Future studies on the reduced intake of iron-rich foods should be carried out.
Iron overload in CEP patients is due to ineffective erythropoiesis that characterizes this disease and blood transfusion therapy. Dietary iron restriction in these patients has not been described because the large amount of this mineral is not ameliorable with a diet but only with iron-chelation and phlebotomy [
42,
43].
It should be emphasized that there is no well-designed clinical trial that can establish the role of iron supplementation in EPP and XLP patients. Moreover, while the iron is a substrate of ferrochelatase (FECH), the defective enzyme in the EPP, it is the limiting substrate in XLP caused by a gain of function of ALAS2. Therefore, the response of oral supplementation could be different in the two forms of protoporphyria. Only one case of XLP was reported in which iron substitution increased hemoglobin concentration and decreased concentrations of both protoporphyrin IX (PPIX) and zinc protoporphyrin (ZnPP). Hence, iron supplementation could be useful for XLP patients with mild microcytic anemia [
44].
Regarding EPP, the mechanism of iron deficiency is not completely understood, and the benefit of iron supplementation is controversial [
45] despite the strong evidence that confirms the benefit of mild anemia on photosensitivity symptoms in EPP patients [
46,
47]. In the literature search, six studies have shown that oral iron produced biochemical (an increase of PPIX) and clinical (increasing of photosensitivity) worsening of symptoms in EPP patients [
48,
49,
50]. By contrast, bibliographic research found two cases that described a reduction in photosensitivity during iron supplementation [
51].
In summary, in the presence of in vitro tests on the improvement of the clinical symptoms in anemia condition, but in the absence of clinical trials that also exclude the placebo effect, it is necessary to go deep on this controversial aspect in patients affected by protoporphyria, to realize a personalized therapy. It is important to underline that in six clinical cases reporting a negative effect of iron, patients also had a slightly compromised liver condition. Only the study of these patients will lead to the formulation of an adequate oral supplementation followed by a control regarding the dietary intake of this important vital microelement.