Cardiometabolic Effects of Intermittent Fasting

In January 2025, a panel of experts commissioned by the scientific journal The Lancet Diabetes & Endocrinology proposed a new definition of obesity to better reflect the varied effects of excess weight on individual health. Firstly, the committee recommended moving away from defining obesity solely based on an elevated Body Mass Index (BMI ≥ 30 kg/m²). Instead, it suggested incorporating other anthropometric measures, such as waist circumference, to highlight the dangers of abdominal fat accumulation. Several studies have shown that a waist circumference greater than 100 cm in men and 88 cm in women is strongly associated with an increased risk of chronic diseases—even when BMI is below 30. Conversely, some individuals (notably athletes) may have a BMI over 30 due to high muscle mass and are at lower health risk.

The committee also recommended considering the pathological manifestations resulting from excess weight, leading to identifying two major categories of obesity:

1. Preclinical obesity: when excess weight does not (yet) have measurable short-term effects on physiological functions or overall health.
2. Clinical obesity: when excess fat is associated with the emergence of health issues (such as abnormal blood glucose levels, hypertension, fatty liver disease, etc.).

The aim of this new classification is to tailor therapeutic strategies based on the health risks associated with being overweight. Preclinical obesity, for instance, represents a future risk factor for chronic diseases, as studies show that this condition is often temporary and tends to worsen over time. Prevention should thus be emphasized—primarily through lifestyle changes, especially dietary adjustments. Clinical obesity, on the other hand, can be considered as a disease requiring treatment using current medical therapies (e.g. blood sugar-lowering drugs, GLP-1 agonists, antihypertensives), due to the damage caused by fat accumulation to metabolic function and/or key organs (such as the heart, kidneys, or liver).

While this distinction between preclinical and clinical obesity is interesting, the reality is that for the vast majority of people, excess weight is a significant short- to medium-term risk factor for chronic diseases. Based on my clinical experience, glucose metabolism disorders are commonly observed in overweight patients—even if their blood sugar is normal at the first consultation, it typically worsens over the following years. In such patients, insulin levels are often abnormally high due to the chronic inflammation caused by fat accumulation, which decreases the cells’ sensitivity to insulin (a condition known as insulin resistance). To maintain blood sugar at a near-normal level, the pancreas compensates by producing more insulin. Over time, however, the pancreas cannot sustain this output, leading to chronic hyperglycemia, type 2 diabetes, and a range of complications—especially cardiovascular ones. In other words, whether preclinical or clinical, obesity should be addressed as early as possible with lifestyle changes aimed at reducing caloric intake and normalizing body weight.

Eat Less, Less Often

Time-restricted eating has emerged in recent years as a simple and effective strategy to improve the metabolic health of overweight individuals (see our article on this topic). This form of intermittent fasting involves limiting calorie intake to a specific daily window—for example, from 11:00 a.m. to 7:00 p.m.—alternating with 16-hour fasting periods. In the vast majority of approximately 500 clinical trials on time-restricted eating, significant improvements were observed in body weight and various aspects of metabolism (see here and here, for example). These effects are likely due to an involuntary reduction in calorie intake (by around 300 kcal per day) and better synchronization with circadian rhythms that regulate metabolic processes.

Another advantage of time-restricted eating is its practicality—it doesn’t exclude food categories, unlike some highly restrictive diets, and is thus generally well tolerated.

Metabolic Syndrome

The positive impact of time-restricted eating is well illustrated by a recent study involving obese individuals with metabolic syndrome, a common complication of overweight affecting about 20% of Canadians. Metabolic syndrome is diagnosed when a person meets at least 3 of the following 5 criteria:

• Abdominal obesity (waist circumference > 88 cm for women, > 100 cm for men)
• Elevated blood pressure (≥ 130/80 mm Hg)
• High fasting blood sugar (≥ 5.6 mmol/L)
• High triglyceride levels (≥ 1.7 mmol/L)
• Low HDL cholesterol (< 1 mmol/L for men, < 1.3 mmol/L for women)

Metabolic syndrome typically involves insulin resistance and greatly increases the risk of developing type 2 diabetes (by 5-fold) and cardiovascular disease (by 2–3-fold). According to the new classification proposed by a Lancet Commission, this is clearly a case of clinical obesity, with measurable negative health effects caused by excess fat.

In this study, researchers recruited 108 obese individuals (average BMI of 31) with metabolic syndrome, all of whom had significant disruptions in glucose metabolism (all participants had fasting blood glucose > 5.55 mmol/L, HbA1c ≥ 5.7%, or were already on glucose-lowering medications). Participants were randomly assigned to one of two groups:

1. Control group: Participants received standard lifestyle recommendations, including regular physical activity and healthier eating habits (e.g. the Mediterranean diet).
2. Intervention group: In addition to the same lifestyle advice, participants were asked to restrict their food intake to an 8–10-hour daily window, reducing their normal eating period by at least 4 hours (for example, from 13 hours down to 9).

All participants were followed for 3 months. They logged their food intake times daily using a smartphone app (myCircadianClock). The effects of the intervention were measured by assessing various anthropometric and cardiometabolic parameters at the beginning and end of the study.

Overall, the results show that restricting caloric intake to an 8–10-hour window leads to significant improvements in several risk factors associated with metabolic syndrome (see Figure 1).

In addition to the weight and fat loss—about 2–3%, which is consistent with findings from several other studies—the research clearly highlights that the most significant improvements from the intervention were seen in glucose metabolism markers. Specifically, there was a marked reduction in fasting insulin, insulin resistance (HOMA-IR), and continuously monitored blood glucose levels. These reductions were correlated with a decrease in glycated hemoglobin (HbA1c), a key marker of chronic hyperglycemia.

Although the HbA1c reduction may seem modest at first glance (1.7%), prior studies have shown that changes of this magnitude are associated with a 58% reduction in the risk of developing type 2 diabetes. Simply limiting caloric intake to an 8–10-hour window can therefore produce measurable and meaningful improvements in glucose metabolism.

The intervention also appeared to have positive effects on other cardiometabolic risk factors, such as LDL cholesterol levels, CRP (a marker of chronic inflammation), and, to a lesser extent, blood pressure.

These results are particularly noteworthy given how mildly restrictive the eating window was: participants only reduced their eating period by 4 hours, and only for 12 weeks. It’s likely that a slightly more intensive approach—such as a 16:8 or 18:6 time-restricted eating schedule sustained over a longer period—could yield even more pronounced benefits.

This aligns with my clinical observations: obese patients with hyperglycemia who commit to long-term time-restricted eating often improve their glucose metabolism to such an extent that it becomes possible to reduce or even eliminate their medications.