Sylvie L. Turgeon* | Laurie-Eve Rioux
The older reader will probably remember the popular “I am Joe” article series from the Reader’s Digest dedicated to overviewing the human body’s function. One article was specifically dedicated to human stomach function (I am Joe’s stomach). More recently, the “dummies” series has devoted articles to the digestive system with topics highlighting food digestion. From back to the future, the understanding of food digestion will always be an important matter to consumers.
For a long time, the food we eat was considered as a sum of nutrients independently of the nature of the food. The nutritional facts table found on each packaged food product is a good example of this phenomena. Nutrients are presented as percentage daily value to consumers independently of the nature/structure of the product. The subsequent question arising is: Are foods containing the same nutrients all equal? Intuitively, we know some foods are more filling than others although their nutrition facts tables are similar. Therefore, the structure of the food, referred to as the food matrix, may be important. Recently, this topic has gained attraction in the scientific community and it is now emphasized that the composition of food, as well as its final structure will influence the way its nutrients are digested, released and absorbed.1,2 To exert a nutritional effect in humans, nutrients need to be released from the food matrix (bioaccessibility), transported through the epithelial membrane of the intestine (bioavailability) and metabolized by the targeted organs.3 This review aims to present through simple examples how the food matrix structure may impact their nutritional properties.
ABC’S OF THE FOOD DIGESTION PROCESS
Let’s first focus on what happens when food enters the mouth (Table 1). The structure (liquid, semi-solid or solid) impacts how the food is processed within the mouth. Liquid food is directed towards the mouth cavity and swallowed, while semi-solid food (e.g., yogurt) forms a bolus through the action of the tongue and is then swallowed. For solid foods the bolus is formed while chewing (hydrating and softening the food) and when the particles are small enough, the bolus is swallowed. The bolus formation also involves the secretion of saliva which acts as a lubricant to allow a safe deglutition and contains amylase which will begin to hydrolyse starch. After swallowing, the bolus is submitted to the peristaltic movements of the oesophagus and then delivered to the stomach. The food bolus is dispersed in the gastric juice containing a mixture of enzymes and acid to initiate protein and lipid hydrolysis to obtain the chyme. Mixing is realized through powerful stomach contraction to disintegrate food particles. When the chyme’s particles are smaller than 1-2 mm4, they can flow through the pylorus into the duodenum to carry on the hydrolysis of proteins and lipids, while the large ones are retained in the stomach. Free nutrients are absorbed by the intestinal epithelia while the undigested ones continue to the gut where the microflora may complete their breakdown (readers are referred to recent reviews for more details on the influence of food on the gut microbiota5,6).
THE FOOD MATRIX INFLUENCES THE DIGESTION RATE
As seen previously, gastric emptying controls when nutrients and small food particles can enter the duodenum and therefore may be a limiting step delaying nutrients bioavailability. Every food, natural or processed, have their constituents organized into a unique structure. Therefore, the food matrix structure may modulate the kinetics of nutrients release and absorption. Many factors may influence the food matrix digestion such as food microstructure and texture (liquid vs. solid), food constituents (protein sources, fibres, etc.), and processing treatment applied during food manufacture.
WHY IS THE KINETICS OF NUTRIENTS RELEASE IMPORTANT?
Food consumption regulates several metabolic functions. The well-known glycemic index introduced by Jenkins in 198110 is a relevant example. White and whole grain breads have respectively a glycemic index of 71 and 51. After ingestion, those two breads induce a different rise in blood glucose. High glycemic food results in a rapid rise in blood sugar which is beneficial in sports nutrition. However, low glycemic food with a slow appearance rate of blood sugar is associated with many positive health benefits11. The same concept of modulated amino acids appearance in the blood stream and some specific physiological effects was recently proposed with proteins and two examples will be presented next.
Eat less but feel full – a satiety concept
According to a 2011 report, one-quarter of Canadian adults are obese12 leading to important health consequences such as diabetes, hypertension, stroke, cancers, etc. This is mostly attributed to food overconsumption altering the energy balance. One approach proposed to improve weight management is through satiety, which refers to the processes that inhibit further eating during a meal (postprandial period). It is the state when you feel full. Several nutrients found in food have the potential to affect satiety. Proteins and fibres are good examples.13 Some dietary fibres are not metabolized in the gastrointestinal (GI) tract but absorb large amounts of water that may result in an increase in the chyme (digesta) viscosity affecting stomach distension and triggering the fullness signal.14 Also, when they reach the small intestine they can interfere in the nutrients-intestinal wall interaction delaying the release of satiating hormones. Some authors have also suggested that high viscous fibres included in a beverage may alter glucose absorption in the intestine resulting in decreased glycemic, insulinemic and appetite responses.15 Proteins may also affect satiety by slowing gastric emptying or stimulating the synthesis of GI hormones.16-19 For example, whey proteins elicit an higher satiating effect than casein17 attributed to the different digestion kinetics exerted by these two proteins20. Whey proteins (beta-lactoglobulin) transition through the stomach quickly and were shown to be resistant to pepsin activity18,21; their hydrolysis begins in the upper part of the intestine. In contrast, micellar caseins coagulate in the stomach due to an acidic pH allowing pepsin to hydrolyse these proteins before their arrival in the intestine. In the literature, these two proteins are often referred to as fast (whey proteins) and slow (caseins) proteins since the rate of appearance of plasma leucine differs20.
The complexity of the meal modulated by the macronutrients present and food form (liquid vs. solid), will also influence its resulting satiety. Organized food structure like solid foods, elicits higher satiety than beverages in most studies.22-24 Also, a yogurt formulation containing a higher proportion of whey proteins led to a lower food intake at the next meal when consumed as a snack compared to the control yogurt.25 Even if both yogurts had equivalent proteins and calories the higher proportion of whey proteins did change their aggregation pattern which may affect digestion kinetics.
Muscle mass synthesis – the bodybuilder paradigm
In humans, there is a balance between muscle protein synthesis and muscle protein breakdown responsible for the net muscle growth. This is dependent on the food protein consumption and the resulting rise in blood amino acid (aminoacidemia). As we age our muscle mass and strength decline impairing our physical performance. This is a geriatric condition named sarcopenia. Physical activity and the dietary protein consumption may attenuate this condition (recently reviewed by Churchward-Venne, Breen26). However, a large body of evidence suggests that dietary proteins are not all equal in terms of muscle protein synthesis after exercise. For example, the whole body protein gain of elderly men fed a whey protein enriched meal was higher than when fed a casein enriched meal27. Different digestion kinetics may be relevant to support muscle, whey proteins being easily digested (fast protein) and consequently easily utilized.
Natural food structure vs processed foods
The natural and complex organization of plant based foods is a good example of how nutrients organization modulates nutrients release. The benefits of whole grain in cereal based products is of common knowledge.28 During food manufacture, the ingredients are cooked, fried, roasted, homogenized, extruded, mixed, frozen, milled, baked, etc. Some of these processes are also common culinary steps. Those processes may influence food structure. The main purpose is to improve food palatability and shelf life but it may impact their nutritional properties. Almond’s lipids (50-55%) are located within the seeds cell-wall. Mastication only frees part of the lipids resulting in a low lipid release (8-11%)29 and blood lipid appearance.30 Therefore, cells must be fractured by mechanical processing to increase lipid bioaccessibility.31 Also, raw almonds were emptied rapidly from the stomach32 and significantly lower amounts of lipids were released29 compared to roasted almonds. Increasing nuts lipid bioaccessibility may increase the absorption of vitamin E, a fat-soluble antioxidant. Almond’s vitamin E has reduced blood oxidized LDL33, a marker involved in cardiovascular diseases34. The better bioavailability of lycopene from cooked tomatoes is another example of the benefit of some processing on nutritional attributes35. Similarly, cooking carrots improves carotenoids bioaccessibility and bioavailability. For example, carotenoids are often embedded within the cell wall or attached at the cell surface in fruits and vegetables altering their bioaccessibility and bioavailability.36,37
Dairy products: food with many attributes
Dairy products represent interesting examples of digestion behaviour varying with food structure. The microstructure (liquid vs. solid) and the texture of food may influence the gastric transit time and consequently the nutrients bioaccessibility (amino and fatty acids). A recent animal study showed that minipigs fed milk or yogurt having the same composition led to different absorption kinetics of dietary proteins38 and rate of appearance of plasma leucine39. Liquid food was promptly emptied from the stomach after ingestion while viscous food was retained longer. Furthermore, yogurt-like gels and rennet like-gel (mimicking soft cheese) had different40 rate of plasma leucine appearance, the former showing a delay. It was hypothesized that gastric emptying was delayed due to contraction of the gel in the stomach acidic environment. Thus, food that looks the same may have different kinetics of protein digestion.
THE TAKE HOME MESSAGE
We have given examples of the crucial role of the stomach/duodenum and its complex relationship with food during its digestion. Each step of the digestion path has an equally important role in breaking down the food structure to free and process the nutrients for our bodies benefit. Designing food structure to deliver nutrients in an optimized way is an important topic for functional food development. But we still have a lot to learn on how the food structure impacts several physiological responses because we eat food and not nutrients. Food is a complicated combination of macronutrients having distinctive structures associated with a specific bioaccessibility and bioavailability. Food items are rarely consumed individually but are mostly included in a meal. Their interaction has been poorly investigated which can bring us “to infinity… and beyond*”…
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*From Toy Story movie, 1995.
STELA Dairy Research Centre, Institute of Nutrition and Functional foods, Université Laval, Québec city, QC, Canada, G1V 0A6
(*Corresponding author email: Sylvie.Turgeon@fsaa.ulaval.ca)