It was a randomized, controlled, four-arm, parallel exercise and/or dietary intervention study (ClinicalTrials.gov: NCT03934476), with the primary aim of examining the VLCHF and HIIT effect on body composition and cardiorespiratory fitness level (6). The method for random assignment is presented in Supplemental Material. There were 91 participants allocated to the four study groups and these completed a 12-week experimental period (Figure 1). Participants were randomly allocated to four study groups: 1) high-intensity interval training (HIIT) and habitual diet, 2) very low-carbohydrate, high-fat diet (VLCHF) and habitual physical activity (no regular exercise training), 3) VLCHF diet and HIIT, and 4) Control (habitual diet and physical activity, no regular exercise training). Dual-energy X-ray absorptiometry (DXA) and graded exercise test to volitional exhaustion were used for the body composition and cardiorespiratory fitness (CRF) assessments, respectively.

To obtain measures of no intervention, a control group was utilized. Participants in the control group were advised not to change their habitual diet and physical activity regime. Therefore, no diet advice was provided.

Results for the primary outcome were previously reported, that a VLCHF diet, either in isolation or in combination with HIIT, caused a significant reduction in visceral adipose tissue (VAT) mass and body composition variables. HIIT alone did not induce such effects on body composition, but improved exercise capacity (6). We utilized the infrastructure of this trial to conduct a preplanned ancillary study focused on clinically relevant risk factors of cardiometabolic health. We analyzed blood samples following a 3-h fast before the experimental period (T₀) and after 4, and 12 weeks (T₁ and T₃). Blood pressure was analyzed also after 8 weeks (T₂). The participant set used for the primary analysis was identical with the participant set presented in this study.

We enrolled adults aged 20–59 years with BMI 25.00–40.00 kg/m², who were not engaged in any regular exercise. Participants with known chronic diseases were excluded. Additional eligibility criteria are listed in Supplementary Table 1. The participants had no previous experience with the VLCHF diet or HIIT. The recruitment details and dropouts during the study are shown in Figure 1. Written informed consent was obtained from all study participants. The study design was approved by the Ostrava university Ethics Committee (nr. 1/2018).

Prior to the intervention, the participants were provided with detailed instructions on the HIIT program. This was for both the HIIT and VLCHF+HIIT groups and done both in verbally and written form. The participants were instructed to complete 3 HIIT sessions per week where one HIIT session was completed during weeks 4, 8, and 12 when the participants visited the laboratory and two were home-based and self-performed. Each HIIT session had a warm up and cool down period of 5-min of slow walking. HIIT consisted of a 3 min interval of high-intensity walking (Borg's scale RPE 18–19) followed by a 3 min interval of low-intensity walking (RPE 9–11). Participants performed 4, 6, and 8 high-intensity intervals in the first, second, and third 4-week period, respectively. Therefore, duration at high-intensity was 12, 18 and 24 min and total session time increased from 31 to 43 min and 55 min during each 4-weeks, respectively. Training intensity was measured with a heart rate monitor (Polar M430; Polar Electro, Oy, Finland). These data were subsequently uploaded to Polar Flow (Polar Electro, Finland) and analyzed regularly to track compliance. Participants were instructed to record all additional training sessions of any type in addition to the study protocol.

Both the HIIT and control groups were asked to maintain their habitual dietary intake without restriction. The VLCHF diet was defined as allowing no more than 50 g of carbohydrates (CHO) per day (20). Neither diet included a specific calorie or energy goal. However, participants in the VLCHF group were advised to compensate for the total energy decrease caused by CHO intake restriction by increasing their natural non-trans-fat intake (e.g., cream, butter, olive, and coconut oil). A target protein intake of 1.5 g/kg lean body mass was recommended. Contrary to the strict CHO restriction, participants were asked to keep to targets. The use of all sweetened and grain-based products had to be minimized. The recommended food included whole food sources, such as meats, vegetables, non-sweetened products, full-fat dairy items, nuts, and seeds. A dietitian provided detailed dietary advice before and during the study (on request or at least once a month). In addition, a handbook was provided to participants containing food lists, guidelines for estimating macronutrient amounts, and sample recipes. To record all foods and quantities consumed an app was used in all study groups (www.kaloricketabulky.cz). This commenced seven days before the start of the intervention. Alcoholic beverages were restricted during the intervention period, and dietary supplements were not permitted 1 month before and during the intervention period. Caffeinated beverages were restricted only before the laboratory sessions.

The results of the anthropometric analysis have previously been reported (6). In summary, the total body mass and visceral adipose tissue (VAT) mass significantly decreased in the VLCHF (by median [IQR]: −6.9 ([−8.4; −5.6]) % and −23.2 [−26.5; −14.7] %, respectively) and VLCHF+HIIT (by −9.0 [–10.9; −7.9] % and −17.6 [−23.8; −10.8] %, respectively) groups after 12 weeks despite no significant changes in the HIIT and Control groups.

Fasting blood samples were collected from the antecubital vein. Whole blood samples with EDTA as an anticoagulant were used immediately for blood count and HbA1c determination. Serum collection tubes were allowed to clot for 30 min and subsequently centrifuged at 2 500 g for 10 min to separate the serum. Blood serum was divided into three 1-ml aliquots, which were frozen at −80°C until analysis. The S-Monovette® system (Sarstedt, Nümbrecht, Germany) was used for blood sample collection.

Blood count parameters were measured using a UniCel® DxH™ 800 hematology analyzer (Beckman Coulter, Inc., Brea, CA, USA). Glycated hemoglobin (HbA1c) was measured using a D-10™ Bio-Rad device (Bio-Rad Laboratories, Inc., Hercules, CA, USA). Glucose, triglyceride (TG), total cholesterol, and high- and low-density lipoprotein cholesterol (HDL-C and LDL-C, respectively) concentrations were measured using an AU 5820 device (Beckman Coulter, Inc., Brea, CA, USA). Serum levels of leptin, adiponectin, TNF-α, IL−1RA, IL−1β, and IL-10 were determined by multiplex technology using a Bio-Plex MAGPIX system (Bio-Rad Laboratories, Redmond, WA, USA). Hs-IL-6 concentrations were measured using a Human IL-6 Quantikine ELISA kit (R&D Systems, Minneapolis, MN, USA) on a DSX device (Dynex Technologies, Chantilly, VA, USA). Insulin concentration was measured using a UniCel DxI 800 analyzer (Beckman Coulter, Inc., Brea, CA, USA). Lipoprotein(a) [Lp(a)] concentration was measured using a BN ProSpec system (BN ProSpec, Siemens Healthcare Diagnostics Product GmbH, Germany).

The intra-assay coefficients of variation for biochemical and blood count parameters were < 5%. Leptin, adiponectin, TNF-α, IL−1RA, IL−1β, IL−10, and hs-IL−6 were determined with an inter-assay coefficient lower than 10%.

Triglyceride-glucose (TyG) index was calculated by applying the following equation (21):

Homeostatic model assessment of insulin resistance (HOMA-IR) was calculated using the formula (22):

A capillary blood sample was drawn from a finger to measure β-hydroxybutyrate (βHB) (FreeStyle Optium Neo, Oxon, United Kingdom). All participants self-analyzed it twice a week (every Monday and Thursday) in a fasting state in the morning to monitor responses to the VLCHF diet and to control for adherence.

Systolic and diastolic blood pressure (BP) was automatically measured three times with 1–2 min apart after the participant had been sitting for ≥10 min in a quiet room by applying a standard device (Nissei DM 3000, Nihon Seimitsu Sokki Co., Japan). This procedure is in line with the recommendations of the American College of Cardiology, American Heart Association, and European Society of Hypertension (23, 24). Participants were instructed to avoid caffeinated beverages for at least 60 min before the blood pressure measurements.

The categorical variable (sex) was described by frequency ratio, and numerical variables were described by median and interquartile range (IQR) at each time point. Subsequently, the absolute changes of the monitored variables at time T₁, T₂, and T₃ with respect to the baseline (T₀) were analyzed. The absolute changes were tested for normal distribution using the Shapiro-Wilk test. In some cases, significant deviations from normality were detected such that non-parametric methods of data description (median and interquartile range) and statistical inference were used. Significance of change was tested by 95% confidence interval (CI) of median and two-tailed Wilcoxon signed-rank test for each variable, each group, and each time. The effect size (ES) of the observed changes was specified by the Wilcoxon effect size (r), including its 95% confidence interval. Threshold values for ES were 0.10 to < 0.30 (small), 0.30 to < 0.50 (medium), ≥ 0.50 (large).

Finally, the absolute changes in the given variables for the HIIT, VLCHF, HIIT+VLCHF, and Control groups were compared using the Kruskal-Wallis test at each time point. Dunn's test was used to analyze specific sample pairs for stochastic dominance. Dunn's test multiple comparison p-values was adjusted with the Benjamini-Hochberg method. The effect size of the observed differences was assessed using the eta squared based on the H-statistic, including its 95% confidence interval. Threshold values for ES eta squared were 0.01 to < 0.08 (small), 0.08 to < 0.26 (medium), ≥ 0.26 (large) (25).

An a priori power analysis using GPOWER (26) with power set at 0.80 and significance level set at 0.05 was calculated retrospectively. The power analysis indicated that a total sample of 76 people would be needed to detect large effects (f = 0.40) for this study with 4 groups. A total sample of 180 people would be needed to detect medium effects (f = 0.25) (27). Thus, the sample size was sufficient to reveal that a large effect could not be interpreted as non-significant.

In all cases, statistical significance was set at p < 0.05. Statistical analyses were performed using R Core Team (28).

The flow chart of participants through the trial, as well as the reasons of dropouts, are depicted in Figure 1. Participant characteristics at baseline are listed in Table 1.

Total energy intake decreased (p < 0.05) in the HIIT (median [95% CI]: −6.1 [−0.2; −13.4] %), VLCHF (−19.7 [−12.5; −25.2] %), and VLCHF+HIIT (−25.8 [−20.5; −28.0] %) groups. Carbohydrate intake decreased (p < 0.05) by −81.8 [−79.1; −82.9] % and −82.8 [−80.4; −85.7] % in the VLCHF and VLCHF+HIIT groups, respectively. Fat intake increased by 44.6 [36.1; 61.7] % and 34.8 [24.6; 47.3] % in the VLCHF and VLCHF+HIIT groups, respectively. Protein intake did not significantly change in any of the study groups. Total energy, protein, and carbohydrate intake did not significantly change in the control group, whereas fat intake decreased (p = 0.023, −6.0 [−1.0; −17.5] %) (Supplementary Table 2).

There were substantial between-group differences in the training characteristics. Total training time in the HIIT and VLCHF+HIIT groups (median 1424 and 1452 min, respectively) was substantially higher than those without the HIIT intervention (VLCHF−124 min, Control−105 min). A detailed training session analysis has already been published in the parent study (6).

There were no significant between-group differences in all the biochemical variables at baseline.

Absolute changes in HOMA-IR after 12 weeks differed by study group (p = 0.013; ES 95% CI: small to large). However, no intervention group significantly differed from the Control group. The between-group significant differences were caused by the differences between HIIT and VLCHF groups. The most substantial HOMA-IR decrease was in the VLCHF group (median [IQR]: −35.7 [– 20.2; −44.0] %). This decrease in the VLCHF group was caused by changes of both HOMA-IR components insulin and glucose. Unlike glucose changes, absolute insulin changes differed by study group (p = 0.023, ES 95% CI: small to large) (Table 2 and Figure 2).

Absolute changes in the adiponectin/leptin (Adpn/Lep) ratio after 12 weeks differed by study group (p < 0.001; ES 95% CI: large) with the differences between the HIIT and Control groups vs. the VLCHF and VLCHF+HIIT groups. The Adpn/Lep ratio increased in the VLCHF group by 120.4 [88.7; 287.1] % and VLCHF+HIIT group by 158.9 [49.5; 540.2] %. These Adpn/Lep increases in the VLCHF and VLCHF+HIIT groups were caused by both the leptin decreases (p < 0.001; ES 95% CI: large) and adiponectin increases (p = 0.054; ES 95% CI: small to large) (Table 2 and Figure 2).

The TyG index and TG/HDL-C ratio significantly decreased in the VLCHF (-0.74 [-2.11; −0.27] and −0.13 [−0.40; −0.04], respectively) and VLCHF+HIIT (−0.68 [−1.89; 0.00] and −0.18 [−0.65; −0.03]) group (Table 2 and Figure 2).

Absolute changes in LDL-C after 12 weeks differed by study group (p = 0.003; ES 95% CI: small to large) A post hoc analysis revealed differences between the both diet groups (VLCHF, VLCHF+HIIT) and HIIT and Control groups. LDL-C non-significantly changed in the VLCHF group by 1.6 [−7.8; 20.0] % and in the VLCHF+HIIT group by 7.4 [−6.1; 20.3] %). However, LDL-C decreased in the HIIT (p = 0.012; −8.7 [−12.7; 0.3] %) and Control (p = 0.016; −10.1 [−18.3; −1.0] %) groups (Table 2). The complete dataset is reported in the Supplementary Material (Supplementary Tables 3–5).

Lp(a), TNF-α, hs-IL−6, IL−1RA, IL−1β, and IL-10 remained mostly under a detection level of the assay. Therefore, no further statistical analyses were conducted (Supplementary Table 6).

There were substantial increases in β-hydroxybutyrate concentration (βHB) in the VLCHF and VLCHF+HIIT groups. The highest βHB concentrations were achieved after 2 weeks of VLCHF diet intervention. βHB concentrations in the HIIT and control groups remained within the range between 0.0 to 0.3 mmol/l for the whole 12-week intervention. A detailed βHB analysis has already been presented in the parent study (6).

Systolic BP decreased (p < 0.05) in all three intervention groups after 8 weeks, when compared to the baseline level, and remained significantly decreased after 12 weeks in the VLCHF+HIIT group. However, systolic BP did not significantly differ by study group at any time point. The between-group differences were shown in diastolic BP after 12 weeks (p = 0.049, ES 95% CI: small to medium), with the most pronounced decreases in the VLCHF (ΔM [95% CI]: −4.0 [−6.8; −0.3] mmHg; ES 95% CI: small to large) and VLCHF+HIIT (−5.3 [−8.0 −3.3] mmHg; ES 95% CI: large) groups (Table 3).

Less than 50 g/day of CHO was required for the VLCHF groups in this study (29). The aim of the VLCHF diet intervention was not to reduce total energy intake. As such the VLCHF and VLCHF+HIIT groups were encouraged to compensate for the CHO intake restriction by increasing fat intake while maintaining protein consumption. Nevertheless, fat intake was insufficient to keep the total energy intake unchanged (Supplementary Table S2). This is indeed a common situation in real-life conditions. ßHB measures confirmed adherence to the diet in both VLCHF and VLCHF+HIIT groups as has been shown already (13). Notably, total energy intake significantly decreased in the HIIT group despite no diet modification. This effect was likely due to an increased interest in a healthy lifestyle when participating in such a research study.

We found that the HOMA-IR, a frequently used index to evaluate insulin resistance, was substantially reduced after 12 weeks of the VLCHF diet, when compared to HIIT. We showed this improved insulin sensitivity even if the participants with overfat constitution were without diabetes and within the normal range of HbA1c. Low carbohydrate diets proved to be more effective than higher carbohydrate (low fat) diets in improving fasting glucose and insulin and insulin sensitivity as measured by HOMA-IR in individuals with obesity and insulin resistance (9) and patients with obesity and non-alcoholic fatty liver disease (10). Not surprisingly, low carbohydrate diets are associated with a large (32 %) increase in remission of diabetes (7). Insulin resistance and excessive body fat are considered among the most important causes of several chronic metabolic and cardiovascular diseases. The cellular and physiological mechanisms are complex and involve adiposity-induced alterations in β cell function, adipose tissue biology, and multi-organ insulin resistance. All these perspectives can be improved with adequate body mass loss (30), which we also showed in this study (6) which may have contributed to the HOMA-IR reduction.

Another surrogate measure for the diagnosis of insulin resistance is the TyG index, which is independently and more strongly associated with arterial stiffness in patients with type 2 diabetes than HOMA-IR (31). The TyG index significantly decreased in both the VLCHF group by median 25.9 [IQR: −38.0; 1.2] % and VLCHF+HIIT group by median 23.1 [−54.7; 27.0] %, when the 12-week outcomes were compared to the baseline. However, these favorable changes were not sufficient to prove a significant between-group differences (p = 0.207). Nevertheless, we can suggest that a carbohydrate intake restriction might be beneficial not only for the visceral adipose tissue reduction as we showed in the parent study (6), but also for the treatment of the impaired insulin resistance, as already shown elsewhere (29, 32, 33).

The uniqueness of this study, however, lay in including HIIT, alone or in combination with VLCHF diet, into consideration. An exercise intervention program proved to be effective in the treatment of insulin resistance in individuals with overweight/obesity (34), metabolic syndrome (35) or type 2 diabetes, i.e., reduces fasting insulin, HOMA-IR, fasting blood sugar, HbA1c, and body mass index (36). However, we did not show any or additional effect of HIIT on these variables after 12 weeks. This inconsistency can be related to participants characteristics, study duration or other design issues. A diet adjustment seems to be, therefore, crucial within any lifestyle modification for body mass management and reducing health risk variables, despite we still consider physical activity and regular exercise important, e.g., for the maintenance or improvement of the CRF level (6). There is a solid evidence that the CRF level is inversely associated with all-cause, CVD and cancer mortality. A dose-response analyses even showed that a per one-MET increase of the CRF level was associated with 12 %, 13 %, and 7 % reduced risk of all-cause, CVD and cancer mortality (37).

We showed significant beneficial changes in the leptin and adiponectin concentrations in the VLCHF group after 12 weeks. The significant decrease in leptin levels also occurred in the VLCHF+HIIT group, whereas the adiponectin increase was not significant. No such beneficial changes were detected in the HIIT and Control groups. A low adiponectin-leptin ratio has been proposed as a promising marker of adipose tissue dysfunction and may lead to chronic systemic inflammation. Leptin is involved in inflammatory responses, and its increased levels are induced by adiposity. In contrast, a decrease in adiposity leads to increased adiponectin levels, which is considered an anti-inflammatory marker (38, 39).

We have already demonstrated favorable changes in serum adiponectin and leptin concentrations in healthy young individuals following a comparable 12-week VLCHF diet (16). The present study shows that a VLCHF diet can induce similar changes in individuals with overfat constitution. Unlike our previous study with healthy young individuals, in which body mass reduction was only small (16), the decrease in leptin and increase in adiponectin levels in this study might be associated with the large body mass changes in both VLCHF and VLCHF+HIIT groups, which were not present in the HIIT and Control groups. Since the diet intervention in the present study induced a substantial increase in βHB (6), the beneficial changes in leptin and adiponectin could also be related to the anti-inflammatory effect of βHB. βHB inhibits histone deacetylase enzymes, free fatty acid receptors, and the NLRP3 inflammasome, inhibiting inflammation, oxidative stress, and the development of chronic diseases (40, 41).

We also assessed other biomarkers associated with low-grade chronic inflammation, since an excess dietary intake of both CHO and lipids is considered harmful (42, 43). Neither anti-inflammatory (IL-1RA, IL-10) nor pro-inflammatory (TNF-α, IL-6, IL−1β) cytokines were analyzed because they mostly remained under the clinically relevant levels at baseline and in response to all interventions. Therefore, from these specific perspectives, a 12-week CHO restricted high-fat diet cannot be considered harmful.

A common concern about the VLCHF diet is its potential atherogenicity due to the high dietary fat intake and subsequent elevated LDL-C levels (44, 45). An elevated LDL-C is traditionally considered a risk factor for atherosclerotic cardiovascular disease (ASCVD) (46). However, a harmful effect of the LDL-C, as well as an elevated LDL-C level induced by the increased saturated fat intake (a.k.a. diet heart hypothesis), have been challenged (47, 48). Paradoxically, it has been shown that those individuals with the high levels of LDL-C live just long or longer than those with low LDL-C (49, 50). A low LDL-C level can even be interpreted as a biomarker of illness severity (51). Therefore, not surprisingly, the LDL-C level is not directly considered among the most important factors for the latest QRISK®3 calculator of a cardiovascular disease risk estimation (52). Unlike our previous study in healthy young individuals (16), we did not show significant LDL-C changes in both the VLCHF and VLCHF+HIIT groups in the individuals with overfat composition despite a large increase in SFA intake (Supplementary Table 2). The significant between-group differences were caused by LDL-C decrease in the HIIT and Control groups. However, substantial inter-individual differences in LDL-C levels were observed and needs to be considered.

The rationale for population-wide restriction of dietary SFA to prevent cardiovascular diseases and reduced mortality is not supported by the results of randomized clinical trials and observational cohort studies (53). SFAs appear to be less harmful than the proposed polyunsaturated alternatives because of their resistance to lipid peroxidation associated with oxidative stress and inflammation (54). In addition, a standard lipid panel analysis may not be sufficient to consider cardiovascular risk (55). An assessment of the LDL-C and HDL-C particle size attracted a research attention because the cardiovascular risk was shown to be associated only with the small, dense LDL-C and HDL-C particle sizes (53, 56). Unfortunately, this analysis is highly demanding and was not performed in this study. However, the triglyceride/HDL-C ratio can predict particle size and is closely related to cardiometabolic diseases (57). In the current study, triglyceride levels and HDL-C tended to change in a favorable manner in the VLCHF and VLCHF+HIIT groups. The triglyceride/HDL-C ratio was therefore lower in both the VLCHF (p = 0.011; median [IQR]: −11.9 [−34.1; 7.2] %) and VLCHF+HIIT (p = 0.037; −24.3 [−37.6; 6.9] %) groups when comparing the 12-week outcomes to the baseline. However, these changes were not sufficient to cause significant between group differences (p = 0.06).

We revealed significant between-group differences in diastolic BP after 12 weeks. Post-hoc analysis showed that diastolic BP decreased in the VLCHF and VLCHF+HIIT groups by median 4.6 (IQR: [−8.6; 2.4]) % and 7.0 [−9.8; −3.5] %, respectively. However, a recent systematic review and meta-analysis found no significant differences in BP changes between low carbohydrate high fat diets and low-fat high carbohydrate diets. The studies included in this review lasted from several weeks to 1 year (58). In contrary, significant favorable changes in both systolic and diastolic BP were shown in patients with type 2 diabetes or impaired glucose tolerance after an average of 2 years observation (IQR 10–32 months) of a low carbohydrate diet (59). Similarly to our findings, a 4-week carbohydrate restriction diet lowered systolic BP in women with obesity/overweight, although the combination with exercise (HIIT or MICT, i.e., moderate-intensity continuous training) had no additional benefits (60). The presented findings cannot, however, be unconditionally ascribed to the VLCHF diet. Again, the body mass loss, induced by the VLCHF diet in this study (as well as in all the cited studies above), could substantially contribute to the presented diastolic BP changes.

The VLCHF diet and HIIT are popular diet and exercise approaches. The main strength is their independent and combined investigation within one randomized clinical trial with sufficient statistical power, high participant adherence, and focus on cardiometabolic risk factors in the overfat population. We wanted to show a complete picture of these effects caused by the VLCHF diet and HIIT. Therefore, another strength of this trial is considering several perspectives associated with chronic non-communicable diseases, as also presented in the parent study (6). Finally, the real-life design of this study shows expectable participants responses to these diets and exercise lifestyle modifications.

There are some limitations of this study. The real-life application did not allow to collect some of the data in person (e.g., daily records of nutrition and exercise sessions). As such, the participant's adherence to all study requirements cannot be fully controlled. However, compliance as well as controlling intensity and duration of the HIIT sessions were done by regular checks of the HR records. Also, despite the very detailed diet instructions, diets could differ in many ways, such as the ratio and type of fatty acids, fiber type and amount, glycemic index, food processing, micronutrient content, as well as food timing and frequency. Furthermore, the study groups were not balanced for sex and, despite the randomization, the median age of the HIIT group was slightly higher than in the other study groups. Furthermore, the menstrual cycle and menopause status were not considered within the data collection and analysis. Another limitation is that there are between individuals' differences in the daytime of the blood withdraw (early morning to afternoon) and possible circadian rhythms have not been considered. However, each participant performed the laboratory sessions (blood withdraw included) during similar day hours (± 60 min). The adiponectin analysis was based on the total adiponectin only. Other forms of adiponectin were not considered. The sample size allowed us to be confident about revealing significant large effect. However, potential significant small or medium effects could be interpreted as non-significant. The statistical significance outcomes for HOMA-IR, Adpn/Lep ratio, and LDL-C were sufficiently robust, but between-group changes in diastolic BP (p = 0.049) are borderline and need to be interpreted cautiously, although some prior studies are supportive (59, 60). The presented results might be related to the body composition changes, which we showed in the parent study (6). However, this analysis is beyond the scope and aim of this study. We have to highlight also the interindividual differences in all the variables, despite the group statistical analysis revealed significant between group differences and changes from the baseline (Figure 2). The “one size does not fit all” approach needs to be considered and requires further investigation. Finally, even if we have no evidence of a detrimental effect of the VLCHF diet, long-term studies and individually adjusted carbohydrate intake restriction is needed for safety and life-long maintenance in various populations. Additionally, although the HIIT approach did not show additional effects, we need to consider other types and amounts of exercise with respect to the evidence regarding positive effects of exercise on health in the literature.