Paul A.M. Smeets

The first taste is always with the eyes: A meta-analysis on the neural correlates of processing visual food cues

Food selection is primarily guided by the visual system. Multiple functional neuro-imaging studies have examined the brain responses to visual food stimuli. However, the results of these studies are heterogeneous and there still is uncertainty about the core brain regions involved in the neural processing of viewing food pictures. The aims of the present study were to determine the concurrence in the brain regions activated in response to viewing pictures of food and to assess the modulating effects of hunger state and the foods energy content. We performed three Activation Likelihood Estimation (ALE) meta-analyses on data from healthy normal weight subjects in which we examined: 1) the contrast between viewing food and nonfood pictures (17 studies, 189 foci), 2) the modulation by hunger state (five studies, 48 foci) and 3) the modulation by energy content (seven studies, 86 foci). The most concurrent brain regions activated in response to viewing food pictures, both in terms of ALE values and the number of contributing experiments, were the bilateral posterior fusiform gyrus, the left lateral orbitofrontal cortex (OFC) and the left middle insula. Hunger modulated the response to food pictures in the right amygdala and left lateral OFC, and energy content modulated the response in the hypothalamus/ventral striatum. Overall, the concurrence between studies was moderate: at best 41% of the experiments contributed to the clusters for the contrast between food and nonfood. Therefore, future research should further elucidate the separate effects of methodological and physiological factors on between-study variations.

Cephalic phase responses and appetite

The current food supply in many parts of the world differs substantially from that which existed during most of human evolution. It is characterized by a high variety of palatable foods with high energy density and low fiber content. Many foods can be eaten very quickly, and there is not always congruency between the sensory properties of the food and the subsequent metabolic consequences of its ingestion, (e.g., as in the consumption of artificially sweetened foods). It is not presently known how the human body copes with this incongruent food environment in terms of short-term satiety responses and long(er)-term regulation of food intake. Cephalic phase responses (CPRs) are innate and learned physiological responses to sensory signals that prepare the gastrointestinal tract for the optimal processing of ingested foods. CPRs could be affected by inconsistencies in the associations between sensory signals and subsequent post-ingestive consequences. Reviewed here are the available data on how CPRs affect the control of food intake.

Functional MRI of human hypothalamic responses following glucose ingestion.

The hypothalamus is intimately involved in the regulation of food intake, integrating multiple neural and hormonal signals. Several hypothalamic nuclei contain glucose-sensitive neurons, which play a crucial role in energy homeostasis. Although a few functional magnetic resonance imaging (fMRI) studies have indicated that glucose consumption has some effect on the neuronal activity levels in the hypothalamus, this matter has not been investigated extensively yet. For instance, dose-dependency of the hypothalamic responses to glucose ingestion has not been addressed. We measured the effects of two different glucose loads on neuronal activity levels in the human hypothalamus using fMRI. After an overnight fast, the hypothalamus of 15 normal weight men was scanned continuously for 37 min. After 7 min, subjects ingested either water or a glucose solution containing 25 or 75 g of glucose. We observed a prolonged decrease of the fMRI signal in the hypothalamus, which started shortly after subjects began drinking the glucose solution and lasted for at least 30 min. Moreover, the observed response was dose-dependent: a larger glucose load resulted in a larger signal decrease. This effect was most pronounced in the upper anterior hypothalamus. In the upper posterior hypothalamus, the signal decrease was similar for both glucose loads. No effect was found in the lower hypothalamus. We suggest a possible relation between the observed hypothalamic response and changes in the blood insulin concentration.

Neuropsychological weaknesses in anorexia nervosa: Set-shifting, central coherence, and decision making in currently ill and recovered women

OBJECTIVE The purpose of this study is to examine set-shifting, central coherence, and decision making in women currently ill with anorexia nervosa (AN), women recovered from AN, and healthy control women. We aim to test whether these neuropsychological weaknesses persist after recovery, and explore relations between the impairments RESULTS Compared to control women, ill and recovered women showed poor set-shifting and decision making. There were strong correlations between set-shifting and central coherence in the ill and recovered women. Decision making did not correlate with the other measures. DISCUSSION The present findings suggest that impaired set-shifting and decision making are stable traits in women with AN. Because individual differences within these groups were large, a rigid thinking style is only present in a (sub)population of ill and recovered women. Decision-making performance is not related to a rigid thinking style, but further research in this area is warranted.

Consumption of caloric and non-caloric versions of a soft drink differentially affects brain activation during tasting

Sensory-specific satiety, which is defined as a relative decrease in pleasantness, is increased by greater oro-sensory stimulation. Both sensory-specific satiety and pleasantness affect taste activation in the orbitofrontal cortex. In contrast, metabolic satiety, which results from energy intake, is expected to modulate taste activation in reward areas. The aim of this study was to determine the effects of the amount of oro-sensory stimulation and energy content on consumption-induced changes in taste activation. Ten men participated in a 2×2 randomized crossover study. Subjects were scanned twice using functional magnetic resonance imaging: after fasting for at least 2h and after treatment, on four occasions. Treatment consisted of the ingestion of 450 mL of orangeade (sweetened with 10% sucrose or non-caloric sweeteners) at 150 mL/min, with either small (5 mL) or large (20 mL) sips. During scanning, subjects alternately tasted orangeade, milk and tomato juice and rated its pleasantness. Before and after the scans, subjects rated pleasantness, prospective consumption, desire to eat and sweetness for all tastants. Main findings were that, before treatment, the amygdala was activated more by non-caloric than by caloric orangeade. Caloric orangeade activated part of the striatum before, but not after treatment. We observed no main effects of sip size on taste activation and no interaction between sip size and caloric content. In conclusion, the brain responds differentially to caloric and non-caloric versions of a sweet drink and consumption of calories can modulate taste activation in the striatum. Further research is needed to confirm that the observed differences are due to caloric content and not to (subliminal) differences in the sensory profile. In addition, implications for the effectiveness of non-caloric sweeteners in decreasing energy intake need to be established.

Sip size of orangeade: effects on intake and sensory-specific satiation

Sensory-specific satiation (SSS) drives food selection and contributes to meal termination. We hypothesised that smaller sips would increase SSS due to increased oro-sensory exposure, irrespective of energy content. The objective was to determine the effects of sip size and energy content on ad libitum intake of orangeade and subjective SSS for orangeade. Orangeade intake and ratings of wanting and liking were measured before and after ad libitum orangeade consumption in a 2 x 2 cross-over design (n 53). Conditions differed in energy content (no-energy v. regular-energy orangeade) and in sip size (large, 20 g/sip v. small, 5 g/sip). The mean intake of both orangeades was lower when consumed with small sips than when consumed with large sips (regular-energy, 352 v. 493 g; no-energy, 338 v. 405 g; both P < 0.001). When consumed with large sips, the mean intake of no-energy orangeade was lower than that of regular-energy orangeade (P = 0.02). When consumed with small sips, subjective SSS (based on the desire to drink) was higher for no-energy orangeade than for regular-energy orangeade (P = 0.01), while mean intake was comparable. We concluded that smaller sip size, i.e. increased oro-sensory exposure per unit of consumption, can lower intake of sweet drinks. Only with low oro-sensory exposure (large sip size) was intake higher for an energy-containing sweet drink than for a no-energy sweet drink. This suggests that intake of sweet drinks is stimulated by (metabolic) reward value and inhibited by sensory satiation. This underpins the importance of SSS for meal termination.

Variations with elevation in the surface energy balance on the Pasterze (Austria)

This paper presents the surface energy balance at five locations on the Pasterze, a glacier in Austria, during a period of 46 days in the summer of 1994. The computations are based on local measurements of radiative fluxes, wind speed, atmospheric temperature and humidity, and precipitation. These measurements served as input for a numerical mass-balance model that computes the surface temperature by simulation of subsurface processes. The surface temperature was then used for the computation of the outgoing long-wave radiative flux and for the computation of the turbulent fluxes by means of the “bulk method.” Values of the roughness length for momentum were computed by profile analysis. They ranged from 1.2 to 5.8 mm. At all locations, net short-wave radiation constituted the dominant energy flux, the latent heat flux was small, and the net long-wave radiative flux and the sensible heat flux were of intermediate magnitude. The energy balances at the three stations on the glacier tongue were similar to each other but differed from those at the two stations in the accumulation basin, which were also similar to each other. These similarities within the two areas are due mainly to the fact that during the experiment there was hardly any variation in temperature and albedo within the two areas. The ablation rate at U3 (2420 m above sea level (asl)) was even slightly higher than the ablation rate at Al (2205 m asl). At all stations the calculated mass balance agrees within 5% with the measured mass balance, which is within the margins determined by the uncertainties in the input variables and the model parameters and equations. This gives some confidence in the method used to compute the turbulent fluxes, but on the other hand, the agreement may also be caused by cancellation of errors. Because of the high temperatures during PASTEX the calculated mass balance would have been almost identical if the “zero-degree assumption” had been used instead of the subsurface module.

Glucose ingestion fails to inhibit hypothalamic neuronal activity in patients with type 2 diabetes.

OBJECTIVE—The hypothalamus plays a critical role in the regulation of energy balance and fuel flux. Glucose ingestion inhibits hypothalamic neuronal activity in healthy humans. We hypothesized that hypothalamic neuronal activity in response to an oral glucose load would be altered in patients with type 2 diabetes. RESEARCH DESIGN AND METHODS—In this randomized, single blind, case-control study, 7 type 2 diabetic men (BMI 27.9 ± 2.0 kg/m2) and 10 age-matched healthy men (BMI 26.1 ± 3.2 kg/m2) were scanned twice for 38 min on separate days using functional magnetic resonance imaging. After 8 min, they ingested either a glucose solution (75 g in 300 ml water) or water (300 ml). RESULTS—Glucose ingestion resulted in a prolonged significant blood oxygen level–dependent signal decrease in the upper and lower hypothalamus in healthy subjects but not in diabetic patients. CONCLUSIONS—Glucose ingestion fails to inhibit hypothalamic neuronal activity in patients with type 2 diabetes. Failure of neural circuits to properly adapt to nutrient ingestion may contribute to metabolic imbalance in type 2 diabetic patients.

Appearance matters: neural correlates of food choice and packaging aesthetics.

Neuro-imaging holds great potential for predicting choice behavior from brain responses. In this study we used both traditional mass-univariate and state-of-the-art multivariate pattern analysis to establish which brain regions respond to preferred packages and to what extent neural activation patterns can predict realistic low-involvement consumer choices. More specifically, this was assessed in the context of package-induced binary food choices. Mass-univariate analyses showed that several regions, among which the bilateral striatum, were more strongly activated in response to preferred food packages. Food choices could be predicted with an accuracy of up to 61.2% by activation patterns in brain regions previously found to be involved in healthy food choices (superior frontal gyrus) and visual processing (middle occipital gyrus). In conclusion, this study shows that mass-univariate analysis can detect small package-induced differences in product preference and that MVPA can successfully predict realistic low-involvement consumer choices from functional MRI data.

Representation of Sweet and Salty Taste Intensity in the Brain

The intensity of the taste of a food is affected mostly by the amount of sugars (mono- and disaccharides) or salt it contains. To season savory-tasting foods mainly table salt (NaCl) is used and to sweeten foods, sugars like sucrose are used. Foods with highly intense tastes are consumed in smaller amounts. The optimal taste intensity of a food is the intensity at which it is perceived as most pleasant. When taste intensity decreases or increases from optimal, the pleasantness of a food decreases. Here, we investigated the brain representation of sweet and salty taste intensity using functional magnetic resonance imaging. Fifteen subjects visited twice and tasted a range of 4 watery solutions (0-1 M) of either sucrose or NaCl in water. Middle insula activation increased with increasing concentration for both NaCl and sucrose. Despite similar subjective intensity ratings, anterior insula activation by NaCl increased more with concentration than that by sucrose. Amygdala activation increased with increasing NaCl concentration but not sucrose concentration. In conclusion, sweet and salty taste intensity are represented in the middle insula. Amygdala activation is only modulated by saltiness. Further research will need to extrapolate these results from simple solutions to real foods.