Influence of cell structure on nutrient delivery

With the advent of present-day analytical techniques and instrumentation, it is possible to describe the complex chemical nature of our foods with ever more accuracy and sensitivity. However, the types and quantities of either the nutrient or non-nutrient components of fruits and vegetables may have very little bearing on their potential contribution to our nutrient or 'health' status. The reason for this is that only a proportion of these food components can be absorbed and utilised. This proportion may be highly variable depending upon the physiological state of the consumer, the food matrix, dietary mix, process history and storage. Determination of the extent of the release of bioactive compounds from different types and forms of fruits and vegetables during human digestion (recently defined as the bioaccessibility of the compound) and the extent to which that nutrient is absorbed and targeted to sites of action within body tissues (defined as the bioavailability of the compound) is essential knowledge for those involved in food production and nutritional assessment.30

The influence of plant food structure on the bioaccessibility and subsequent bioavailability of many of the potentially bioactive components of foods is an area that has been poorly researched, particularly with respect to the lipid soluble compounds, so that there is only a small diffuse literature. However, the bio-accessibility of lipophilic microconstituents of fruits and vegetables (especially carotenoids) was an area of focus in a European collaboration and the key issues examined in this project are outlined below.30 The carotenoids have been chosen for special focus because they serve as an excellent example of where too little understanding of the complexity of their behaviour in foods and human tissues has confounded interpretation of their role in the putative health benefits of specific food plants.

There are two main mechanisms by which nutrients are released from the cell matrix of food plant tissue. First, if the plant cells are broken open, the digestive enzymes have free access to the contents and it would be predicted that this would allow rapid and efficient digestion. Second, if the cells are not broken open the rate of digestion will be modulated by the permeability of the cell wall (pore size) that regulates the rate of penetration of the cell by digestive enzymes and the rate of diffusion of the products from the cell. Small mobile hydrophilic molecules, for example sugars, fatty acids, amino acids and mineral ions, will diffuse easily but the diffusion of larger hydrophilic molecules, for example complex phenolic compounds, may be severely impaired. For large hydrophobic molecules that need to be dissolved in a lipid structure for transport, for example the carotenoids, the situation is more complex, since the cell wall is unlikely to be permeable to lipid emulsions or micelles, and the presence of lipases will strip away the solvating lipid.

Plant cells are compartmentalised membrane-bound structures contained within a semi-rigid cell wall composed mainly of cellulose and pectic substances. The main features of the cell are the vacuole, cytoplasm, nucleus and a range of sub-cellular organelles. This compartmentalisation is an essential mechanism for separating the various biochemical and physical functions of the cellular processes. Disruption of this physical separation, as in bruising, leads to metabolic chaos, resulting in cell death and the production of undesirable colours (enzymic or non-enzymic browning) and flavours (lipid oxidation), and destruction (vitamin C) or production (isothiocyanates, cyanide) of bioactive compounds. Cellular compounds are not free to move about within the cell and are bound to specific structures (for example, lipoproteins, glycoproteins) or associated with particular domains (for example, carotenoids associated with lipid membranes). The carotenoids are very hydrophobic and are normally associated with the lipid structures of the sub-cellular organelles. In green leafy vegetables, the main carotenoids, lutein and b-carotene, are bound to lipoproteins in the light-harvesting complex of the chloroplasts (organelles responsible for photosynthesis). In the carrot and tomato, the carotenoids may be present as membrane bounded semi-crystalline structures or present in lipid droplets. In fruits, the carotenoids are more frequently present in oil droplets, although the solubility of carotenoids in oil is low. The different types of plant tissue (leaf, root, fruit, seed) and the environment and physical nature of the cellular carotenoids have implications for the ease with which they are made available for absorption through processing (thermal or physical), mastication and digestion.

To be absorbed the carotenoids need to be released from the constraints of the gross physical structure of the plant tissue and from the plant cells and transferred to the free lipid phase of the processed product or digesta. In general, carotenoids in plant structures are stable and they will survive quite aggressive processing and intense light exposure with a minimum of loss or isomerisation. However, once released from the structure, they are more prone to degradation by heat, light and atmospheric oxygen. There is, therefore, a trade-off between maximising release and retention during storage. It should be noted that aggressive processing may result in conversion of the native all-irans carotenoids to their

Cell contents o

03 O

Cell separation

Cell separation

Cell contents

Cell rupture

Fig. 2.1 Food processing promotes cell separation but cell rupture, which is associated with the greatest release of plant cell constituents, does not always occur.

cw-isomers and the production of highly reactive species that can continue to degrade the carotenoids after processing is complete.

As a general rule, cooking and processing sterilises and softens the plant tissue leading to cell separation, which is the primary mechanism of tissue disintegration. In contrast, mastication of raw fruit and vegetables causes crushing and shearing of the tissue and tears the cells open (Fig. 2.1). Both mechanisms of particle size reduction will contribute to increased release, so it is not a foregone conclusion whether the raw or cooked tissues will provide more bioaccessible carotenoid. This is clearly demonstrated by examination of grated carrot strips fed to ileostomy patients. Carrot recovered from the terminal ileum (having passed through the gastrointestinal tract) shows loss of the carotenoid from only the fractured surface cells. There is no evidence for loss of carotenoid from deeper plant tissue.

It will be appreciated that the delivery of nutrients from foods is attenuated by the structure of the food and the way in which it is digested. Thus, delivery from the food structure occurs over the same timescale as gastric emptying. Carotenoids and other compounds isolated from the food structure are generally emptied from the stomach and absorbed more rapidly. These different rates of delivery may have profound effects on subsequent metabolism.

There are proven health benefits from 'slow release' carbohydrate foods; they do not stimulate the oversecretion of insulin, undesirable large excursions in blood glucose or unnecessary glycosylation of proteins. By analogy, the slower delivery of other food components may maximise health benefits by not overloading transport systems or causing undesirable excursions in plasma concentration. The fact that some portion of nutrients escape absorption in the ileum and are 'lost' to the colon should not automatically be interpreted negatively, since they may contribute positively to colon health and the production of beneficial products of colonic fermentation.

The complex nature of the mass transfer of carotenoids to absorbable lipid species, the diversity of raw and processed foods consumed and individual variations in the degree of mastication will lead to differences in the amount of carotenoid that becomes bioaccessible and potentially available for absorption. By understanding the underlying mechanisms of these processes, for a wider range of fruit and vegetable constituents, it will become possible not only to recommend 'five portions' a day but also to suggest domestic and commercial processing practice to maximise the potential health benefits.

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