Introduction:
Traditionally, the foundations of nutritional guidance have rested upon the principles of balance, moderation, and variety. Following these principles ensures that no food is excluded from one’s diet, and each item can provide a valuable contribution tailored to the individual’s health status and needs. In a time when concerns about positive energy balance prevail in Western societies and emerge as a global issue, high-fat and energy-dense foods are frequently labeled as problematic. However, nuts defy this conventional perspective, as scientific evidence challenges such assumptions. This review delves into the impact of nut consumption on aspects such as appetite, energy intake, energy metabolism, and body weight.
Nuts within the Context of a Comprehensive Diet:
Nuts play a dual role in shaping energy and nutrient intake through various mechanisms. Primarily, nuts serve as potent sources of energy, delivering an array of essential nutrients such as tocopherols, magnesium, and potassium, along with antioxidants. Each type of nut possesses a distinctive sensory profile, influencing individual consumption choices. Processing methods like roasting and frying introduce modifications, enhancing fragility, darkening color, and creating novel flavor combinations. These alterations in physical properties significantly impact the palatability of nuts.
Moreover, the introduction of flavoring compounds directly onto the nut surface—ranging from salt and sugar to cinnamon and capsaicin—augments sensory variety. Such enhancements, by promoting uniformity effects, potentially facilitate regular nut consumption and subsequent nutrient intake. Sensory attributes emerge as pivotal factors guiding eating decisions.
Beyond direct effects, nut consumption also exerts indirect influences on overall energy and nutrient intake. Changes in the sensory, nutritional, or physical characteristics of nuts prompt shifts in the secretion of intestinal hormones and appetitive responses among consumers. Furthermore, nuts frequently integrate into various food matrices, such as pastries or baked goods, reshaping the flavor profiles of both components. This fusion creates a novel, integrated sensory stimulus that may influence the consumption of the combined substance or food, subsequently impacting the acceptability and choice of other items within the broader diet. The extent to which this contributes to increased energy consumption remains an area requiring further exploration.
Appetite and Energy Intake Dynamics:
Human feeding trials consistently demonstrate that incorporating nuts into diets tends to regulate postprandial appetite. Notably, the consumption of almonds and peanuts has been associated with diminished hunger, reduced cravings, and heightened satiety ratings after ingestion. A four-day daily intake of peanuts has also exhibited increased fasting satiety and satiety levels, crucial attributes for effective weight management. This reduction in postprandial hunger holds potential to extend the delay between meals, discouraging unnecessary eating. Additionally, heightened fasting satiety levels may translate into smaller meal sizes, contributing to overall calorie moderation.
The satiating impact of nuts is contingent on two key factors. Firstly, the shape of nuts influences appetite differently, as observed when comparing the consumption of peanut butter to whole nuts, revealing varying levels of hunger suppression and post-consumption return of hunger. However, whole almonds have demonstrated satiety levels comparable to almond butter. Secondly, the timing of nut consumption plays a role in appetite control. Consuming almonds with a meal doesn’t significantly impact appetite control during that meal. In contrast, consuming almonds independently as a snack has been shown to reduce hunger and desire to eat compared to those not consuming nuts. Similarly, studies suggest that consuming peanuts or peanut-containing snacks as a snack, as opposed to part of a meal, results in more effective energy compensation. Therefore, the form and timing of nut consumption emerge as factors influencing appetite, with evidence supporting greater satiety effects for whole nuts consumed as a snack. Hence, the form and timing of nut consumption may moderate feelings of appetite, with evidence that satiety effects may be greater for whole nuts consumed as a snack.
Understanding the Mechanisms of Nut-Induced Appetite Effects:
The mechanisms underlying the appetitive effects of nut consumption remain elusive, primarily due to limited research in this area. Current evidence suggests that the satiating impacts of nuts are unlikely to be mediated by delayed gastric emptying or glucose-dependent release of specific appetite-regulating peptides, such as glucagon-like peptide-1 (GLP-1). Studies involving the ingestion of 3 grams of pine seed oil, in either fatty acid or triglyceride form, have reported reduced food intake and increased cholecystokinin (CCK) secretion. These findings suggest that the satiating effects of nuts may involve CCK or peptide YY (PYY) secretion, potentially linked to the dietary protein or fat content in nuts. The proposed role of high unsaturated fat content as a primary driver of satiety is based on the observation that unsaturated fats are oxidized more rapidly than saturated fats, generating a quicker and more robust satiety signal.
The anticipation is that nut-induced appetite modulation will lead to reduced energy intake through spontaneous adjustments in overall dietary balance, triggering a robust compensatory feeding response. Compensation data from trials involving various nuts, including almonds, hazelnuts, macadamia nuts, peanuts, pecans, pistachios, and walnuts, show compensation values ranging from 55% to 105%. Notably, dietary compensation may vary depending on the form of nuts consumed. For instance, peanuts in the form of peanut butter exhibited higher nutritional compensation than whole peanuts (105% vs. 150%), despite inducing a weaker satiety effect. In summary, nut consumption suppresses hunger, diminishes the desire to eat, and fosters a sense of satiety, contributing to compensatory adjustments in energy intake from the overall diet. However, robust compensation can occur independently of the reported appetitive effects, raising questions about potential imprecision in appetite measurement or the existence of an independent, yet unidentified, mechanism.
Chewing and Nutrient Absorption Efficiency in Nuts:
While the nutritional value of nuts is extensively documented, emerging evidence suggests that the physical properties of nuts can significantly modify published values. The unique structure and high fiber content of nuts play a pivotal role in altering the bioavailability and absorption of nutrients. To access the nutrients within nuts, their parenchymal cell wall must be disrupted, a process facilitated by enzymatic or microbial degradation, as well as mechanical processing during oral (chewing) and gastric digestion. The fiber in nuts can bind with compounds like fatty acids, potentially reducing their absorption efficiency. Additionally, fiber may influence gastric emptying, gastrointestinal transit time, and gut hormone secretion, thereby impacting appetite and energy intake. The cell walls of nuts can serve as a source of fermentable fiber in the large intestine, influencing both energy balance and gut health. These dynamic processes are central to understanding the nutritional impact of nut consumption, prompting focused research in this area.
The substantial effort required for oral processing of nuts may contribute to their lesser-than-expected impact on body weight. Chewing, as a mechanical act, generates satiety signals through cognitive, neural, endocrine, and physical mechanisms, including effects on gastric emptying. This heightened cephalic phase response related to appetite can influence digestion efficiency, increase average energy consumption, and trigger compensatory food intake. The intricate interplay between chewing, nutrient absorption, and satiety underscores the multifaceted nature of how nuts contribute to overall dietary dynamics.
Efficiency of Energy Absorption from Nuts:
Several studies assessing energy absorption from ground and tree nuts through feeding trials consistently demonstrate a significant increase in fecal fat reduction with nut consumption, though values range widely from approximately 5% to over 20%. Early trials, where peanut products constituted the majority of daily fat intake, highlighted a food shape effect, with whole peanuts resulting in an 18% dietary fat excretion, peanut butter at 8%, and peanut oil at 5%. Notably, a background dietary effect was observed, as fecal fat reduction values decreased when participants consumed lower daily crude fiber (from 30 grams to 5 grams), suggesting a link between fiber content and nutrient binding, particularly fatty acids.
Later studies extended this observation to various nuts, including almonds, walnuts, pistachios, and peanuts, with some showing a dose-response pattern in fecal fat increase. Recent research in similar conditions demonstrated comparable reductions in fecal fat for 45 grams of almonds and pistachios daily, despite differences in nut intake (85 g/day for almonds, ~10 g/day; and ~7 g/day for pistachios). This suggests relative consistency among nuts at recommended intake levels, possibly indicating a nonlinear dose-response relationship for certain nut types.
The inefficiency of fat absorption associated with nut consumption has both positive and negative implications. While it aids in energy intake modulation and potential contributions to positive energy balance and weight gain, it likely reduces the absorption efficiency of fat-soluble nutrients and other macronutrients. When nuts are part of the diet, fat contributes to approximately 55% of the increase in faecal energy loss, impacting other energy sources. Selective nut form choices, such as consuming chopped, sliced, or finely ground nuts, can be considered for optimizing nutrient intake, especially if whole nuts might pose concerns when modulating energy expenditure. Delayed fat absorption from whole nuts may also have potential benefits in moderating postprandial lipemia and blood glucose. Additionally, nut processing, such as roasting, influences the bioavailability of nutrients, affecting their decomposition characteristics when chewed.
Effects on Energy Cost:
Limited trials have explored the impact of nut consumption on postprandial thermogenesis, also known as diet-induced thermogenesis (DIT), or resting energy expenditure (REE). Many of these studies have focused on the composition of nut fatty acids. In an acute setting, a meal containing walnuts, compared to a dairy-based meal, significantly increased diet-induced thermogenesis, despite both sources being rich in unsaturated fats. However, this acute thermogenic effect of postprandial nuts has not been consistently confirmed.
Mixed findings on thermogenesis arise from short-term and long-term trials (ranging from 5 days to 12 months) of nut consumption, with no specific data recorded for walnuts. One study on almonds reported no effect on thermogenesis, while another observed an increase that accounted for approximately 15% of the energy derived from almonds. In a trial involving 8 weeks of peanut consumption (54 grams per day), a 10% increase in thermogenesis was noted. Additionally, an 8-week supplementation with peanut oil (constituting 30% of REE) increased REE by 5%, particularly in overweight subjects.
While evidence suggests that nut consumption may contribute to increased thermogenesis, the data are not robust, and the underlying mechanism remains unclear. One potential explanation is that nut lipids are absorbed gradually, providing a small but consistent substrate source for thermogenesis, which may manifest as an elevated REE. Further research is needed to elucidate these mechanisms and strengthen the understanding of the impact of nuts on energy cost.
Adipose Tissue and Fat Metabolism Effects of Nut Consumption:
Nut consumption has been proposed to enhance fat oxidation and selectively reduce body fat mass, particularly visceral fat, owing to their high unsaturated fat content. Animal studies indicate that higher polyunsaturated fatty acid (PUFA) intake suppresses the differentiation of fat cells and reduces specific adipocyte genes, targeting visceral fat reservoirs. In rodents, increased PUFA intake correlates with decreased mRNA concentrations of various genes associated with fat metabolism in retroperitoneal adipose tissues.
Evidence from acute feeding studies in humans suggests that a high PUFA diet enriched with walnuts can increase fat oxidation, although significance may vary. Notably, in overweight and obese adults, consuming 30 to 35 grams of walnuts led to a significant increase in fat oxidation, around 50% more compared to a control diet. This effect was particularly pronounced in overweight individuals, indicating potential BMI-related differences. Importantly, the fat-oxidizing properties of walnuts in this study were not solely attributed to PUFA content, as the dietary fat concentration and fat subtype composition were consistent.
While walnut-specific studies on fat oxidation exist, limited research on other nuts hampers generalization. Other nuts, richer in monounsaturated fatty acids (MUFA) compared to walnuts, have demonstrated comparable or even higher rates of fat oxidation. This suggests that similar or greater effects on fat metabolism can be anticipated with other nut varieties, extending the potential benefits of nut consumption for fat oxidation beyond walnuts and their PUFA content.
Clinical trials investigating the impact of increased fat oxidation from walnuts on long-term fat mass have shown promising results. In one study involving type 2 diabetic adults, incorporating walnuts into the weight maintenance diet led to a modest reduction in body fat over a 6-month period, with stable body weight. This trend persisted when the intervention was extended to 1 year, revealing a greater reduction in fat mass compared to the control group, despite comparable weights between the two groups. Preliminary evidence suggests that the loss of fat mass mainly targeted subcutaneous fat rather than visceral fat, although both fat stores decreased in size over time in the walnut group.
Studies on the effects of polyunsaturated fatty acids (PUFAs) on different body fat pools using animal models consistently show that higher PUFA intake reduces visceral fat. However, findings in humans thus far do not align with these results; instead, PUFAs seem to preferentially reduce subcutaneous fat. The reason for this discrepancy between human and rodent studies remains unclear. As both subcutaneous and visceral fat contribute to total body fat, evidence suggests that the reduction of visceral fat is primarily determined by the reduction of total fat mass.
To better understand the impact of nut consumption on body composition, human studies with real doses of different nuts and varied dietary compositions are needed. Such findings could provide valuable insights into managing body adiposity and mitigating the risk of developing metabolic syndrome.
Epidemiological studies consistently indicate that regular nut consumption is not associated with harm and may even contribute to weight maintenance. Controlled feeding studies with almonds, walnuts, and macadamia nuts reveal that nut consumption does not alter body weight when energy intake is continuously regulated. Importantly, under controlled or ad libitum conditions, studies demonstrate that incorporating nuts into regular diets does not lead to weight gain, despite some reports suggesting small but significant increases.
Moreover, in weight maintenance programs, there is evidence of a reduction in body weight compared to baseline when nuts are included, although the exact mechanism remains to be fully understood. It is unclear whether this effect is attributed to the greater thermic effect of nuts or their impact on resting energy expenditure (REE) compared to the foods they replace in the diet. Nonetheless, current data suggest that including nuts in weight maintenance programs does not result in weight gain and may contribute to weight loss.
In the context of energy-restricted diets, incorporating nuts does not impede weight loss. Some trials, even when nuts did not induce weight loss, revealed reduced cardiovascular disease risk indices in groups consuming nuts, suggesting additional health benefits beyond weight change. Nevertheless, there is a call for long-term randomized intervention studies with body weight as the primary outcome to further understand the effects of daily nut consumption in actual amounts on maximal and sustained weight loss.
In conclusion, the established understanding is that body weight and fat are influenced by energy balance rather than the macronutrient content of the diet. Despite being high-fat and energy-dense, nuts are shown to have minimal impact on weight management and may even support it. This can be attributed to their strong compensatory effects, inefficiency in energy absorption, and potential contributions to increased energy expenditure and fat oxidation. While energy plays a crucial role in body weight, the health impact of diets is largely determined by their macronutrient content and other constituents. Nuts, rich in unsaturated fats, minerals, vitamins, antioxidants, and fiber, contribute to a healthy diet with potential positive effects on overall health.