on July 01, 2026
Insulin is one of the main hormones regulating human energy metabolism. It is produced by the beta cells of the endocrine pancreas, located in the islets of Langerhans, while alpha cells secrete glucagon, a hormone with a functionally complementary action.
Insulin's main task is to coordinate the transfer of nutrients from the bloodstream to the tissues that use or store them. In particular, it promotes the entry of glucose into insulin-sensitive cells, especially muscle tissue, adipose tissue, and the liver, helping to maintain blood glucose within physiological ranges.
When insulin is secreted after a meal, it gradually reduces blood glucose concentration and activates a series of metabolic processes aimed at efficiently managing available energy.
From a biological perspective, insulin is considered an anabolic hormone: it promotes the synthesis and replenishment of energy reserves, stimulates glycogen formation, and reduces certain catabolic processes.
This function is neither pathological nor undesirable: it represents one of the fundamental mechanisms of human survival.
However, the insulin response changes significantly depending on the body's energy status.
After physical activity or a sufficiently long period between meals, muscle cells may show a reduction in glycogen stores. In this scenario, a larger portion of the nutrients consumed is used to replenish intracellular energy reserves and support recovery processes.
When, however, energy intake repeatedly exceeds requirements and reserves are already largely available, a greater portion of the energy consumed tends to be stored over time.
This process does not depend exclusively on insulin but on the interaction between energy availability, insulin sensitivity, physical activity level, dietary composition, circadian rhythms, and neuroendocrine signals.
In recent years, the concept of signaling medicine has brought greater attention to the fact that the body responds not only to the amount of calories consumed but also to the biological quality of metabolic signals.
The hypothalamus continuously integrates information from the gastrointestinal tract, adipose tissue, muscle, and blood to decide whether to direct metabolism toward energy use or conservation.
Among these signals, fluctuations in blood glucose and insulin are particularly important.
Foods with high energy density, highly refined and quickly digestible, can cause more rapid increases in blood glucose compared to foods naturally rich in fiber and intact plant structure.
When the insulin response is particularly intense, especially in predisposed or metabolically vulnerable individuals, a subsequent drop in blood glucose may occur, subjectively perceived as early hunger, reduced energy, or cravings for sweet foods.
This phenomenon is defined as reactive hypoglycemia when it meets specific clinical criteria and does not automatically coincide with the normal return of blood glucose to baseline levels.
The idea that every insulin increase inevitably leads to weight gain is an oversimplification.
Insulin is necessary for life, the maintenance of muscle mass, brain function, and the proper use of nutrients.
The problem arises when the system is exposed for a long time to a combination of chronic overfeeding, an excess of ultra-processed foods, physical inactivity, and reduced insulin sensitivity.
Under these conditions, inflammatory circuits involving metabolism and appetite control can be activated.
One of the studied mechanisms is the activation of the NF-κB signaling pathway.
Zhang and colleagues have shown that nutritional excesses can activate inflammatory pathways in the hypothalamus that affect energy control and metabolic balance.
Chronic low-grade hypothalamic inflammation is now considered one of the possible mechanisms contributing to altered satiety signals and energy dysregulation.
The problem, therefore, is not the occasional consumption of a single food but the dietary context repeated over time.
In this scenario, dietary fiber plays a central role.
Fiber modifies the rate of gastric emptying, the postprandial glycemic response, the production of intestinal metabolites, and the dialogue between the microbiota and metabolism.
Fruit, vegetables, legumes, and minimally processed plant foods are among the main sources of fiber and bioactive compounds.
For years, some nutritional models attributed an unfavorable role to fruit in weight control due to the presence of natural sugars.
The available epidemiological and clinical evidence, however, shows a different picture.
Regular consumption of whole fruit is generally associated with better dietary quality, greater satiety, better weight control, and a reduced risk of numerous chronic diseases.
It is important to distinguish between whole fruit and derived products.
The natural matrix of the fruit—water, fiber, cellular structure, and chewing time—profoundly influences the metabolic response.
Juices, industrial purees, and sweetened products do not necessarily produce the same effects as whole fruit.
The concept of “negative calories” also requires scientific clarification.
There are no foods that require more energy to be digested than they provide.
However, there are foods with very low energy density and a high satiety effect, such as water- and fiber-rich vegetables, that can spontaneously facilitate a lower overall calorie intake.
Celery, fennel, lettuce, cucumber, cabbage, asparagus, and many other vegetables are excellent examples of foods favorable for energy control without being biologically “negative calorie.”
Fruit also generally maintains a moderate glycemic load thanks to its high water and fiber content.
The glycemic load is, in fact, a more useful measure than the glycemic index alone for understanding the actual effect of a food portion.
Regarding the timing of fruit consumption, there is no solid evidence that consumption at the end of a meal significantly compromises digestion in the general population.
Consuming fruit away from meals may be useful for some people due to individual preferences or specific gastrointestinal conditions, but it should not become an obstacle to achieving adequate plant-based intake.
Metabolic health does not depend on rigid rules but on the interaction between dietary quality, movement, meal distribution, sleep, inflammatory status, and continuity of habits.
Understanding the language of metabolic signals means shifting attention from simple calorie counting toward a broader view of the relationship between food, physiology, and biological adaptation.
Bibliography
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Petersen MC, Shulman GI. Mechanisms of insulin action and insulin resistance. Physiol Rev. 2018.
Zhang X et al. Hypothalamic IKKβ/NF-κB and ER stress link overnutrition to energy imbalance and obesity. Cell. 2008.
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Hall KD et al. Ultra-processed diets cause excess calorie intake and weight gain. Cell Metabolism. 2019.
Reynolds A et al. Carbohydrate quality and human health. Lancet. 2019.
Aune D et al. Fruit and vegetable intake and risk of cardiovascular disease, cancer and mortality. Int J Epidemiol. 2017.
Slavin JL, Lloyd B. Health benefits of fruits and vegetables. Adv Nutr. 2012.
EFSA Scientific Opinion on Dietary Reference Values for carbohydrates and dietary fibre. EFSA Journal.
