Effects of intermittent fasting on metabolism in men ☆
Efeitos do jejum intermitente no metabolismo humano ☆
- Open Access funded by Brazilian Medical Association
- Under a Creative Commons license
Abstract
This
review analyzes the available literature on the impact of intermittent
fasting (IF), a nutritional intervention, on different aspects of
metabolism. The epidemic of metabolic disturbances, such as obesity,
metabolic syndrome (MS), and diabetes mellitus type 2 has led to an
increase in the prevalence of cardiovascular diseases, and affected
patients might significantly benefit from modifications in nutritional
habits. Recent experimental studies have elucidated some of the
metabolic mechanisms involved with IF. Animal models have shown positive
changes in glucose (lower plasma glucose and insulin levels) and in
lipid metabolism (reduced visceral fat tissue and increased plasma
adiponectin level), and an increased resistance to stress. Despite the
limited number of samples studied, positive results have been reported
on the impact of IF for human health. IF is reported to improve the
lipid profile; to decrease inflammatory responses, reflected by changes
in serum adipokine levels; and to change the expression of genes related
to inflammatory response and other factors. Studies on obese
individuals have shown that patient compliance was greater for IF than
other traditional nutritional approaches (calorie restriction), and IF
was found to be associated with low oxidative stress. Recent reports
suggest that IF exerts a positive impact on the metabolic derangements
commonly associated with cardiovascular diseases, and that it may be a
viable and accessible intervention for most individuals. Therefore,
further clinical studies are essential to test the effectiveness of IF
in preventing and controlling metabolic and cardiovascular diseases.
Resumo
Esta
revisão traz uma análise de dados disponíveis na literatura sobre o
impacto do jejum intermitente, uma modalidade de intervenção
nutricional, em diferentes aspectos do metabolismo. A epidemia de
anormalidades metabólicas, como obesidade, síndrome metabólica e
diabetes mellitus tipo 2, tem ocasionado um aumento na prevalência de
doenças cardiovasculares, condições em que os indivíduos afetados
apresentam importantes melhorias advindas de modificação nos hábitos
alimentares. Estudos experimentais recentes têm elucidado a modulação do
metabolismo por jejum intermitente. Testes com animais têm mostrado
alterações positivas no metabolismo glicídico (valores menores de
glicemia e insulinemia) e lipídico (redução no volume de gordura
visceral e aumento nos valores de adiponectina plasmática), além de uma
maior resistência ao estresse. Apesar dos estudos disponíveis
apresentarem populações muito reduzidas, observaram-se resultados
positivos com esta intervenção também na saúde humana. Os resultados
indicam melhorias no perfil lipídico, redução de respostas
inflamatórias, com redução na liberação de adipocinas inflamatórias e
alterações na expressão de genes relacionados com a resposta
inflamatória e de outros fatores. Em indivíduos obesos observou-se uma
melhor adesão ao jejum intermitente em relação a intervenções
tradicionais (restrição calórica), além da redução no estresse oxidativo
desta população. Dessa maneira, por se tratar de uma intervenção viável
e acessível para a maioria dos indivíduos, novos estudos clínicos são
necessários para testar a eficácia desta intervenção na prevenção e no
controle de doenças metabólicas e cardiovasculares.
Keywords
- Fasting;
- Cardiovascular diseases;
- Obesity;
- Metabolic syndrome X;
- Caloric restriction;
- Dyslipidemia
Palavras-chave
- Jejum;
- Doença cardiovascular;
- Obesidade;
- Síndrome metabólica;
- Restrição calórica;
- Dislipidemia
Introduction
The human genotype is believed to have evolved from 600,000 BC to 25,000 BC, when humans were hunter-gatherers.1
During this period, major energy oscillations appear to have selected
genes that regulate metabolism for efficient nutrient usage and
increased fat storage, which represents an evolutionary benefit
consistent with the thrifty genotype theory proposed by James V. Neel.2
While the environment changed drastically with urbanization and easy
availability of food, the genotype remains largely unaltered. This
imbalance has resulted in an epidemic of conditions characterized by
metabolic disturbances, such as obesity, metabolic syndrome (MS), and
diabetes mellitus type 2 (DM2).1, 3 and 4
While obesity is easily defined by body composition (body mass index [BMI] > 30 kg/m2),
and DM2 by elevated blood glucose levels, MS is recognized by a cluster
of metabolic markers whose importance and contribution have greatly
changed over time. Despite this difference, the three conditions have
common pathophysiological backgrounds. Firstly, all these conditions
involve insulin resistance, elevated levels of plasma lipids, and
increased levels of chronic inflammatory mediators. Secondly, the
consequent metabolic profile considerably increases cardiovascular risk.
Finally, individuals with any of these conditions can benefit from
significant lifestyle changes.5, 6 and 7 Notably, modification of nutritional habits is now considered extremely important for reducing cardiovascular risk.8 and 9
Intermittent
fasting (IF) is an interventional strategy wherein individuals are
subjected to varying periods of fasting. IF has recently attracted
attention because experimental studies have highlighted its potential
for correcting metabolic abnormalities.10 This regimen has also shown better adherence than other methods.11
This review analyzes existing data on the impact of IF on different aspects of metabolism.
Methodology
Experimental
studies and clinical trials on IF available in the PubMed database at
the time of manuscript preparation were reviewed. Animal and human
studies were searched for by using the key words “intermittent fasting,”
“alternate day fasting,” and “starvation” either alone or combined with
“cardiovascular risk,” “obesity,” and “metabolic syndrome.”
The
first search retrieved over 22,000 results, mostly by using the keyword
“starvation” alone (21,735). A detailed search yielded 26 articles on
the impact of fasting or IF on metabolic parameters related to
cardiovascular risk.
Pathophysiological basis of obesity and MS
Obesity and MS were the main outcomes analyzed in the studies on IF.
Adipose
tissue is now known to function as an endocrine organ involved in
regulating metabolism, rather than a passive reservoir for energy
storage.12
Adipocytes, mesenchymal cells, and infiltrating macrophages together
produce cytokines and adipokines that have important regulatory effects
on inflammation, insulin sensitivity, coagulation, vascular homeostasis,
appetite, energy expenditure, etc. When this production is deregulated,
e.g., by excessive adipose tissue, the organism appears to develop
low-grade chronic inflammation, leading to insulin resistance and
cardiovascular disease.13
Adipocytes produce important proinflammatory adipokines, such as
leptin, tumor necrosis factor alpha (TNF-α), resistin, angiotensinogen,
interleukin-6 (IL-6), and plasminogen activator inhibitor-1 (PAI-1), as
well as nonesterified fatty acids and C-reactive protein (CRP), which
are atherogenic.14 and 15
Certain
adipokines have cardioprotective action, such as adiponectin, which is
abundant in human circulation. Adiponectin was primarily investigated
for its ability to promote insulin sensitivity by suppressing
gluconeogenesis and increasing fatty acid oxidation, which in turn
reduce triglyceride accumulation in the liver.13 and 16
Moreover, adiponectin regulates endothelial function by increasing the
production of endothelial nitric oxide; by inhibiting endothelial cell
activation and endothelium–leukocyte interaction; by enhancing
phagocytosis; and by suppressing macrophage activation,
macrophage-to-foam cell transformation, and platelet aggregation.17, 18 and 19
Adipokines
might represent the evolutionary pathway that led excessively nourished
humans to obesity, insulin resistance, MS, and cardiovascular disease.
Indeed, obese patients with excess visceral fat show increased adipokine
expression. However, weight loss is related to reduced plasma levels of
metabolically active substances.20
Effects of intermittent fasting on animal models
Metabolic mechanisms triggered by IF were well elucidated by recent experimental studies. Anson et al.10 investigated glucose metabolism and enhanced neuronal resistance to stress in C57BL/6 mice subjected to ad libitum diet, IF, or limited daily food intake for 22 weeks. Neuronal stress was induced at the end of the 22th
week through an injection of kainate, a seizure- and neuronal
damage-inducing exotoxin. The IF group showed significantly lesser
histopathological brain changes, and lower plasma glucose and insulin
levels than the others. These findings were reproduced in male Wistar
rats. 21
Similarly,
a study on the lipid metabolism of male C57BL/6J mice subjected to
alternate-day fasting (ADF) or alternate-day 50% calorie restriction
(ADCR 50%) showed a significant reduction in the size of adipocytes of
both visceral and subcutaneous fat. The authors also found that IF
causes oscillation of triglyceride metabolism between anabolism
(gluconeogenesis and de novo lipogenesis) and catabolism (lipolysis). 22
The same group later reported that female C57BL/6J mice subjected to
four-week IF showed significantly reduced visceral fat percentage,
increased subcutaneous fat percentage, increased plasma adiponectin
levels, and unchanged amount of fat tissue. 11
Mager
et al. investigated heart-rate variability in 12 Sprague-Dawley rats
during 16 weeks of nutritional intervention by using an implanted
telemetric transistor.23
After 16 weeks, rats in the IF group showed reduced sympathetic
activity associated with increased vagal tone. This reduction might have
immunomodulatory effects, which would attenuate the release of
proinflammatory substances, such as TNF-α, IL-6, and IL-18.
Increased resistance to stress after intermittent fasting
Resistance to stress might be a reasonable basis for increased resistance to cardiovascular and endocrine diseases.
Wan
et al. evaluated the response to cardiovascular stress in male Wistar
rats undergoing IF by using a telemetric transistor, and found that the
rats showed rapid return to basal values of blood pressure and heart
rate after induced cardiovascular stress, and presented no alterations
in the plasma levels of stress biomarkers, such as adrenocorticotropic
hormone and corticosterone, during stress.21
Further, analysis of heart-rate variability demonstrated a better
norepinephrine response to stress, thereby indicating an improved
autonomic response in the IF group versus the control group.
Wan
et al. later reported that a diet supplemented with 2-deoxyglucose
(2-DG) - a glycolytic inhibitor and metabolic stressor - on alternate
days could mimic the effects of IF in Sprague-Dawley rats.24
They observed that both IF and 2-DG supplementation were capable of
reducing heart rate, blood pressure, and glucose levels to similar
extents. The authors suggested that the mechanisms underlying the
positive metabolic response to IF may be attributed to the fact that
periodic metabolic stress can induce adaptive changes in cardiovascular
physiology and glucose metabolism that are associated with a “less
atherogenic” profile.
An
alternative cardioprotective action of IF was evaluated by Ahmet et al.;
they maintained 30 Sprague-Dawley rats under IF for three months and a
control group with normal feeding.25
Thereafter, all animals were submitted to coronary artery ligation to
induce myocardial infarction (MI) or to sham surgery. The IF group
showed lower left ventricular (LV) mass, lower LV wall thickness, and
significantly lesser ventricular remodeling than the control group.
Notably, 23 hours after surgery, a significantly reduced degree of
apoptosis and neutrophil infiltration was noted in the IF group with MI,
possibly contributing to a smaller ventricular size. The authors
proposed that IF induces an ischemic preconditioning in the cardiac
muscle that protects myocardial cells from ischemic damage. Similar
results were recently reported by Wan et al. on male Wistar rats.26
In this study, plasma adiponectin and IL-6 concentrations were
measured, which showed an increase in the former and a decrease in the
latter. The authors suggested that the observed benefits of IF might be
attributed to increased plasma adiponectin levels.
More recently, Katare et al., performed a study on the impact of IF started after MI induction and maintained for six weeks.27
The male Wistar rats subjected to IF showed a decrease in cardiomyocyte
hypertrophy and fibrosis area, reduced oxidative stress, better cardiac
performance, and better survival rates than the control group. The
authors also noted an increased expression of the BDNF gene,
responsible for the enhanced expression of vascular endothelial growth
factor (VEGF) in the cardiac muscle; this resulted in increased
angiogenesis and decreased apoptosis.
In
another experimental study, streptozotocin was used to induce diabetes
mellitus type 1 before diet intervention in Sprague-Dawley rats.28
After eight weeks of IF, the diabetic IF group showed blood pressure
levels similar to those of the non-diabetic control group, indicating
that glomerular damages promoted by diabetes were somehow prevented.
Other findings were normal blood levels of glucose, albumin, HDL-C, and
blood urea nitrogen; increased resistance to oxidative stress; and
reduced incidence and intensity of degenerative structures in the
kidneys. Changes in the expression of some genes involved with cellular
survival (p53, p38, and Sir 2) were also demonstrated.
Studies
evaluating the potential of IF in the recovery from spinal cord injury
have also shown intriguing results. Plunet et al. assessed the effect of
ADF in a group of male Sprague–Dawley rats after cervical spinal cord
injury.29
The intervention proved to be neuroprotective, with a 50% reduction in
lesion volume and increased sprouting of corticospinal axons. The
intervention also promoted plasticity; improved behavioral recovery,
evident by improved gait-pattern and forelimb function during
ladder-crossing; and enhanced vertical exploration. Jeong et al.
investigated the effect of the same dietetic intervention, started
before or after a different spinal cord lesion (thoracic contusion) in
Sprague–Dawley rats.30
Both groups subjected to the intervention (before or after the lesion),
showed positive results, with a better functional recovery, along with
improvement of several parameters of their walking pattern. The
prophylactic group (IF started before the lesion) performed slightly
better than the therapeutic group (IF started after the lesion). The
results were also superior in benefits when compared with a group of
rats consuming the same amount of calories as the ADF group (25% calorie
restriction) everyday. Davis et al. also found positive results
assessing not only spinal cord lesion but also traumatic brain lesion
(TBL) induced in Sprague–Dawley rats.31
They found that fasting for 24 hours after moderate TBL confers
neuroprotection, maintains cognitive function, and improves
mitochondrial function. The results confirm the beneficial role of this
kind of calorie restriction in other organisms.
Metabolic changes during fasting in humans
Metabolic
changes due to fasting in humans were first investigated in the
beginning of the century for treating obesity and other conditions, such
as seizure disorders.32, 33 and 34
Kerndt
et al. investigated the metabolic effects of long-term fasting in human
subjects who underwent a 36-day complete fasting regimen for religious
reasons.35 They noted a significant decrease in blood pressure, reaching significance on the 33rd
day, accompanied by negative sodium balance. Changes in metabolic fuel
were observed soon after the fasting period started. Plasma glucose
levels dropped immediately at the beginning of the studies and remained
low throughout the fasting period. Lipolysis and ketogenesis increased,
gluconeogenesis remained higher than baseline levels, and glycogenolysis
was reduced to undetectable levels.
Carlson et al. subjected healthy human volunteers to 60 hours of fasting and collected blood samples at 12 and 60 hours.36
They observed a decrease in plasma glucose by 30% and in insulin by
50%; a significant increase in the extent of lipolysis and fat
oxidation; and moderate increase in the extent proteolysis and protein
oxidation. This increase of fat oxidation provides the substrate for
gluconeogenesis and compensates for the decline in carbohydrate
oxidation and glycogenolysis, thus confirming a switch of metabolic
fuels. ADF is considered a variation of complete fasting that is easier
to maintain for longer periods and has remarkable metabolic benefits.
Studies on healthy subjects demonstrated a similar adaptation of
energetic substrates. This action was also confirmed by changes in the
muscular expression of genes, such as GSK-3, which is responsible for the regulation of glycogen synthesis pathways, favoring glycogen reposition. 37 IF also down-regulates mTOR
expression in muscle tissue, another important gene responsible for the
modulation of nutrient signaling, and could reflect lesser protein
synthesis and increased expression of carnitine acyltransferase 1
(CPT-1) in muscle tissue, as well as an increase in fat oxidation. 38 and 39
Results
from these and other studies on similar populations have shown
improvements in the lipid profile, with higher HDL-cholesterol levels in
women and lower triglyceride levels in men, and increased muscular
expression of Sirt 1 (40) −a gene involved with the regulation
of food intake, fat metabolism, cell differentiation, apoptosis, and
prevention of aging. 35 In these studies, an increase in stress resistance was demonstrated after three weeks of intervention. 40 A summary of the main findings is presented in Fig. 1.
- Fig. 1.Potential targets for interventions using intermittent fasting (IF). IF impact in each different pathway of the organism. This is a summary of the different actions of this intervention (IF) within the metabolism demonstrated in all the studies that were used to compose this review. CHO, carbohydrate; HR, heart rate; BP, blood pressure; TG, triglycerides; HDL-c, high-density lipoprotein cholesterol; CPR, C-reactive protein; IL-6, interleukine 6.Grey boxes – Verified in human and animal studies--------------- - Verified in animal studies only__________ - Verified in human studies only⇑ - Increase⇓ - Decrease.
Clinical implications
The
general health benefits involved with intermittent fasting in humans
still have not been fully explored in the literature. Johnson et al.
described this phenomena after studying the article by Vallejo,
published in 1956 in a Spanish scientific magazine. In his study of
elderly subjects living in a nursing home, Vallejo did not change the
overall calorie consumption, only the pattern of eating.41 The subjects would alternate between ad libitum
diet for one day, followed by a very restrictive diet on the next day.
The results were quite interesting, revealing an important increase in
the lifespan, demonstrated by a shorter number of days spent in the
infirmary. It is important to emphasize that there was no calorie
restriction involved in this intervention, thus the results were a
consequence of the meal pattern alterations. Johnson concluded that this
was probably just the tip of the iceberg, and that the health
improvements derived from IF are probably seen in very different aspects
of the organism, as observed in other studies exploring different
conditions. 41
Some
latter studies were designed to evaluate the consequences of this
intervention in specific health conditions. One interesting model
analyzed consisted of individuals following Ramadan.
Ramadan
is a holy month in the Islamic calendar, in which Muslims must refrain
from eating or drinking during daylight hours, thereby resulting in
approximately 12 hours of fasting. These features make Ramadan a natural
model to study IF in humans. Aksungar assessed cardiovascular health in
Muslim individuals during Ramadan, with special emphasis on
coagulation.42
The results showed improvements in the lipid profile, with increased
HDL-cholesterol levels and decreased values of HDL risk factor (CT/HDL),
during the fast and 20 days after it; decreased levels of D-dimmer and
reduced homocysteine, which translates in an improved coagulation
profile, were also observed. Three years later, in a similar study
design, the same author measured inflammatory markers levels, such as
IL-6 and CRP, during Ramadan. The results demonstrated a decrease in the
inflammatory response, since plasma levels of IL-6 and CRP were
consistently reduced by fasting.43
Allard et al. assessed cellular stress in healthy individuals subjected to IF for three weeks.44
Serum samples obtained before and after the intervention were added to
cultures of human hepatoma cell line (HepG2). The effects on growth,
stress resistance, and gene expression were observed. HepG2 cells were
then subjected to acute stress by treatment with freshly prepared
hydrogen peroxide. The results demonstrated cell proliferation and
increased stress resistance. This study also confirmed previous reports
of an increased expression of the Sirt 1 gene after IF. The results
obtained showed that IF had a significantly greater cell-protective
effect than calorie restriction.
Another
study on healthy subjects appraised changes in adipokine levels after
the energetic oscillations caused by IF. After two weeks of
intervention, subjects showed an increase in plasma adiponectin levels
and increased insulin sensitivity in muscle and fat tissue, which was
confirmed by increased lipolysis inhibition.3
Finally,
Varady et al. presented a revision of the impact of IF on the
prevention of chronic diseases in healthy human and animal models.45
Although most human results were found to be inconclusive, they showed a
tendency to improve metabolic conditions (glycemia and blood lipids),
which would probably reflect in cardiovascular risk reduction. The lack
of significance found in human studies is probably a consequence of the
short length of the interventions and of the absence of a control group
in most studies, reflecting the need for more studies to precisely
determine the impacts of this intervention in humans.45
Although
there are important differences in the strength of the results found,
these findings in humans reflect the benefits observed in animal models,
and also show similar underlying mechanisms, such as genetic
modulation. Indeed, the described results motivated the investigation of
IF and its possible therapeutic effects in diseased individuals.
Protective actions of intermittent fasting in obese individuals
IF
for a period longer than a few weeks is difficult for most individuals,
since they tend to develop headaches, dizziness, and irritability.40
Therefore, an alternative approach - alternate-day calorie restriction
(ADCR) - has been developed to promote patient compliance. In this
approach, patients have ad libitum and very-low-calorie diet on
alternate days (ranging from 25% to 85% restriction). When tested on
obese individuals, this regimen showed higher adherence scores than
others. 22
Johnson et al. investigated the therapeutic properties of ADCR for two months in ten obese individuals with asthma.46
The study revealed that IF affected improvement in the lipid profile,
systemic inflammatory status (reduced serum TNF-α levels), and oxidative
stress (decreased levels of reactive oxygen species). The authors
concluded that ADCR induces mild stress, thereby causing cells to adapt
by activating antioxidant mechanisms; this would imply that IF may be
beneficial in other disorders involving inflammation and oxidative
stress, such as atherosclerotic heart disease.
The cardio-protective action of IF was examined by Varady et al.11 16 obese individuals underwent eight weeks of ADCR 25%, i.e., they consumed 25% of their baseline energy needs, and ad libitum
food on alternate days. The study results confirmed the significant
cardio protective action of IF, such as weight loss; reduction of fat
tissue mass, blood pressure, and heart rate; and improvements in lipid
profile, with decrease in total cholesterol and LDL-cholesterol levels,
and an increase in HDL-cholesterol levels.
Conclusions
The
development of cardiovascular disease, the leading cause of death
worldwide, is directly connected to lifestyle factors causing metabolic
disorders. Traditional approaches to counter these risk factors have
been proven ineffective in most individuals. However, IF has recently
been shown to have a positive impact on cardiovascular health.
Few
studies conducted hitherto have explored the effectiveness of this
intervention in metabolism regulation. The studies discussed in this
paper have mostly been conducted in very small populations, on healthy
individuals, and for short periods, which limits the strength of the
results achieved. Further investigations are required to determine the
frequency and/or duration of IF required to exert a positive effect on
metabolism and cardiovascular outcomes.
Conflict of interest
All authors declare to have no conflict of interest.
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