In the present study, we have shown that CGM enabled the visualization of postprandial glucose profiles in people without diabetes after consumption of different sequences of meals with standardized nutritional composition supporting its use in non-diabetic people. As is widely known, the application of CGM in people with diabetes to assess glucose data has been shown to be useful in optimizing glucose management and achieving glucose targets (13, 25, 26). Although CGM systems were initially developed for people with diabetes, this investigation implies that this technology could also be valuable when integrated in the everyday life of people without diabetes as it allowed distinguishing between meals of different nutritional content and illustrated the impact of a previous meal on the following eating behavior. Being aware of glucose responses of different foods might be helpful for example for obese people in weight loss programs to individually adjust their nutritional behavior according to the recorded glucose profiles.
The glycemic profiles reported in this study after consumption of standardized meals with identical carbohydrate amounts (50 g) showed that not only the CHO content but also other meal components, like fat and protein content, have to be considered when focusing on postprandial glucose (27, 28). In this study, highest postprandial glucose traces were seen after TM “fast”. This TM had fast absorbable character consisting mainly of simple CHO and a low content of fat and protein. The AUCs after consumption of TM “fast” were largest within the first hours followed by a rapid decline. TM “slow” was characterized by complex CHO, a high fat content and slightly higher protein content than TM “fast”. Compared to TM “fast”, TM “slow” caused a low and prolonged postprandial glucose increase; even 5 hours after meal intake, glucose values did not return to baseline. These results are in line with other study results, demonstrating that complexity of CHOs and meal composition itself influence the magnitude and extent of postprandial glucose changes, due to differing ability to be digested and absorbed in the intestine. Thus, meals with rapidly absorbed carbohydrates and low fiber content induce a rapid increase in postprandial glucose (16, 29, 30).
For TM “slow”, the AUC was larger and glucose traces were higher after lunch than after breakfast. This finding is in contradiction to a previous study we conducted in 2007 (18), where TM “slow” for breakfast led to higher glucose excursions and AUC in comparison to lunch. A possible explanation for this effect could be that the subjects of the present study arrived at the study site in the morning, which required at least a low level of physical activity. In the 2007 study, however, subjects stayed overnight so that they only had negligible levels of activity between getting up and having breakfast. As reported by Büsing et al. and Fencher et al., low-intensity physical activity can improve postprandial glycemic response of individuals without diabetes (31, 32).
The delayed glycemic response and low peak after TM “slow” with high fat content matches with our previous analysis in 2007 (18) and another study result focusing on the impact of dietary fat in people with type 1 diabetes mellitus (27). Moreover, compared to TM “fast”, TM “slow” contained more fiber, which is reported to increase gastrointestinal content, decrease the gastric emptying rate and reduce the glucose absorption rate (15, 33).
Additionally, protein preload was reported to blunt the glycemic response to a subsequent meal with high CHO amount (34–36). In the current analysis, this effect cannot be judged because proteins were not separately consumed beforehand.
Besides the effect of single meals, meal sequences seem to be relevant for the glycemic profile as well in our study. When comparing the glycemic data after TM “fast” for breakfast and TM “slow” for lunch and with opposite order, the glycemic courses differ although the same TM were consumed and pre-meal glucose levels were comparable. Previous studies indicate that the meal composition of breakfast can influence the glycemic response of the following meal (29, 37, 38). Our results show that when TM “slow” was consumed before TM “fast”, an elevated glycemic response (on average + 17 mg/dL) was seen compared to the glucose course after TM “fast” consumption without a pre-meal intake. Ando et al. also reported 20 mg/dL higher blood glucose concentrations when a previous high fat meal was consumed (29). Additionally, Fechner et al. presented significantly higher glucose responses of a rice meal challenge after a low glycemic load diet in overweight and slightly obese participants without diabetes which might be due to a physiological adaptation to a low glycemic eating pattern which only becomes visible when challenged with high glycemic foods (32).
Similarly, another study demonstrated that a high-fat breakfast can lead to an elevated glycemic response at lunchtime (29).
Moreover, the circadian rhythm of glucose metabolism can influence the postprandial glucose course as well. As nutrient metabolism exhibits a day-night variation, the postprandial response of metabolic functions may depend on the time of meal intake. Due to the time-dependent regulation of glucose utilization, a higher glucose tolerance is reported in the morning than in the evening in healthy individuals (39–41). As reported by Sutton et al. (42) eating in alignment with circadian rhythms in metabolism, e.g. by increased food intake at breakfast time and reduced intake at dinnertime, improves glycemic control. Their trial of early time-restricted feeding indicated that in men with prediabetes an early time-restricted feeding improves aspects of cardio metabolic health like insulin sensitivity and satiety. Thus, meal timing may have an impact on the development of metabolic disorders as well (42).
The specific and rather uncommon composition of the TM represents a limitation of the study. However, the TM were chosen based on previous analyses assessing glucose profiles in healthy (18) and people with type 1 and type 2 diabetes after consumption of the same TM. To be able to compare the results of each analysis in a next step, it was decided to use the identical TM for the present evaluation. Another limitation of the study is the use of a first generation intermittent-scanning CGM which may not be as accurate as current generation devices. Sufficient accuracy of CGMs in response to meals in healthy individuals is indispensable to reliably predict glycemic postprandial responses and needs to be taken into account when interpreting CGM values. One study compared postprandial CGM accuracy against plasma glucose measurements in healthy subjects showing that CGM significantly underestimates glucose values during 8 hours after a meal and CGM accuracy was dependent on different macronutrient compositions (43). However, only ten individuals were included in this study and the population consisted of older and overweight subjects so that further validation with a larger cohort is needed.
An energy intake exceeding the energy requirement contributes to the progression of obesity and associated complications, like type 2 diabetes mellitus (13). Thus, the identification of food and meals that suppress hunger and promote satiety is one of the main interests for therapy and prevention of obesity and type 2 diabetes mellitus (20, 44). The glucostatic theory describes a causal relation between the short time appetite regulation and the blood glucose level. According to this theory, high blood glucose levels signal satiety, whereas low glucose levels trigger food intake (45). In this study, the meal sequence or previous meal may have influenced the feeling of satiety and energy intake of the subsequent meal. Irrespective of the TM sequence, the same energy content was consumed for breakfast and lunch combined; however, the feeling of satiety before dinner was lower when TM “fast” was consumed for lunch. Consistently, the energy intake for dinner was higher as well. Likewise, satiety was higher before lunch when TM “slow” was consumed in advance. TM “slow” promoted the feeling of satiety for a longer time and decreased the risk of an increased energy intake at a subsequent meal. The results support previous studies, describing positive correlations between fat and fiber content and the feeling of fullness (20).
In conclusion, we showed that CGM can be a valuable tool for individuals without diabetes to monitor nutritional behaviors as it was able to reliably reflect postprandial glucose profiles according to the nutritional content of different meals in our study as well as after different sequences of predefined meals suggesting a potential application of this technology also in non-diabetic people as health and lifestyle application. The glycemic response data as well as the feeling of satiety in relation to specific meals and sequences in individuals without diabetes collected in this evaluation may further contribute to identify beneficial dietary patterns with the use of CGM that could be considered in the management of metabolic disorders.