Major insights have been gained into the response of our gut to meal-related stimuli. However, the investigatory techniques available are scarce and cannot be applied for the whole range of foods. For instance, widely used methods such as intubation and aspiration can only be used in liquid meals and are usually annoying for the patient. In contrast, imaging the human gastrointestinal tract using magnetic resonance imaging (MRI) offers a non-invasive approach for studying the behavior of food materials and fluids in the context of gastrointestinal function and transit.
A new narrative review, written by Robin Spiller and Luca Marciani from the University of Nottingham (United Kingdom), updates how MRI has helped in evaluating the impact of nutrients and drugs on gastrointestinal physiology and pathology.
MRI is a non-invasive medical imaging technique, based on the use of radiofrequency, which allows users to explore how solid and liquid meals alter stomach and small and large intestine function in response to feeding.
Through MRI images, the role of physical factors—involving viscosity, fat and fiber content—in controlling gastrointestinal functions, such as gastric processing and distribution of foods and gastric emptying depending on food particle size, has been elucidated. Likewise, MRI allows researchers to quantify total gastric secretion in response to meal viscosity, which in turn affects how the meal is rapidly diluted and later becomes accessible to digestive enzymes.
That gastric volume is associated with the sense of fullness, which appears to be higher in high viscosity foods, is also an interesting point. In this regard, MRI may pave the way for weight-loss strategies based on modifying food factors, including meal viscosity and homogeneity. For example, MRI has shown that preparing homogeneous mixtures of solid and liquid foods, making oil-water emulsions more stable using emulsifiers (MRI can selectively image fat components), and consuming whole fruits may result in delayed gastric emptying, prolonged fullness and reduced hunger, which could be exploited for tackling obesity.
Furthermore, MRI has also allowed for the study of changes induced by foods (e.g., poorly absorbed carbohydrates such as fermentable oligo-, di-, mono-saccharides and polyols) in intestinal secretions and colonic water content.
Not all carbohydrates are absorbed as soon as they leave the stomach. Studying small bowel water secretion through MRI may allow for a better understanding of the impact of osmotically active and poorly absorbable small molecules (e.g., fructose, lactulose and polyols) on gastrointestinal physiology. This is relevant in the clinical setting, as not all patients with functional gut conditions show alterations in sugar absorption, and scientists still struggle to identify the major culprit behind carbohydrate-related gastrointestinal symptoms. On the other hand, an increase in small bowel water content may also contribute to explaining the impact of specific foods on drugs’ bioavailability. This is the case with grapefruit juice consumption, which is thought to promote drug absorption by increasing the small bowel’s water content.
Beyond foods and nutrients, MRI has also been used to explore the underlying mechanisms of action of commonly used osmotic laxatives such as lactulose, bran and psyllium. Lactulose and PEG solutions has been shown to lead to a greater increase in small bowel secretion, whereas mannitol led to a decreased response as short-chain fatty acids, caused by the laxative’s fermentation by the gut microbiota, are absorbed. Cross-section MRI image studies have also shown that the soluble fiber psyllium works by trapping water, whereas bran increases small bowel water content by stimulating secretion.
On the whole, MRI has provided researchers and clinicians with a better understanding of the intraluminal impact of food and intestinal fluid secretion and absorption in response to food and drugs. Due to its non-invasive nature and safety, MRI is a good technique for exploring the impact of nutrients and drugs on gastrointestinal physiology and disease. There are also, however, a number of limitations to MRI that are worth highlighting. Firstly, it is an expensive technique. Secondly, MRI cannot provide insights regarding secretion and absorption separately; rather, it measures the net overall effect. In the meantime, a novel area of research concerning MRI involves determining which patients can respond better to a diet and treatment, which may help with guiding treatment in the future.
This review article belongs to the special issue “Food and Diet for Gut Function and Dysfunction” in the peer reviewed open access journal Nutrients. This issue was instigated by the European Society of Neurogastroenterology and Motility, guest edited by Profs Fernando Azpiroz and Paul Enck, and made possible through an unrestricted educational grant from Danone.