Thesis Abstract

Abstract
The use of salad dressings in enhancing the taste and palatability of salads is a common practice. However, the amount of oil in salad dressings can significantly impact the bioavailability of carotenoids and fat-soluble vitamins present in salad vegetables. This study aimed to investigate the effects of varying amounts of oil in salad dressings on the absorption of carotenoids and fat-soluble vitamins from salad vegetables in the human body. The research employed a randomized, crossover design where participants consumed salads with dressings containing different oil levels (low, medium, and high) over a period of time. Blood samples were collected at specific intervals to analyze the concentrations of carotenoids (such as beta-carotene, lutein, and zeaxanthin) and fat-soluble vitamins (including vitamins A, D, E, and K). Additionally, fecal samples were collected to assess the excretion of these nutrients. The results indicated that the bioavailability of carotenoids and fat-soluble vitamins was influenced by the amount of oil in the salad dressings. Participants who consumed salads with dressings containing higher oil levels showed increased absorption of carotenoids and fat-soluble vitamins compared to those with lower oil levels. This could be attributed to the role of dietary fats in enhancing the solubilization and absorption of these hydrophobic compounds. Furthermore, the study found that the type of oil used in the salad dressings also played a role in nutrient absorption. Dressings made with healthy oils rich in unsaturated fats, such as olive oil or avocado oil, exhibited higher bioavailability of carotenoids and fat-soluble vitamins compared to dressings made with saturated fats. This highlights the importance of choosing the right type of oil in salad dressings to maximize the nutritional benefits of salads. In conclusion, the findings of this study emphasize the significance of the amount and type of oil in salad dressings in influencing the bioavailability of carotenoids and fat-soluble vitamins in salad vegetables. Incorporating dressings with higher oil content, particularly those rich in unsaturated fats, can enhance the absorption of these essential nutrients, thereby maximizing the health benefits of consuming salads.

Thesis Overview

INTRODUCTION 

1.1 Overview 

High intakes of fruits and vegetables have been recommended to prevent the development of chronic diseases. The beneficial effects of fruit and vegetable intake on human health have been closely linked to substances in plant foods, such as fiber, micronutrients and phytonutrients. Numerous studies have shown an association between vegetable intake and plasma level of fat-soluble nutrients (Campbell et al., 1994; Chopra et al., 2000; Drewnowski & Popkin, 1997; Olson, 1984; Su & Arab, 2006). Among the most frequently consumed plant foods, salad vegetables significantly contribute to vitamin and other micronutrient intakes in the U.S. population. In the Third National Health and Nutrition Examination Survey (NHANES) salad consumption was reported by one third of US adults and was associated with higher serum concentrations of micronutrients, including vitamins A and E, as well as provitamin A carotenoids (α- and β-carotene) (Su & Arab, 2006). Salad vegetables are among the major sources of vitamin E in the U.S. diet (Maras et al., 2004; Murphy et al., 1990). However, the amounts of micronutrients absorbed from raw salad vegetables may not be adequate to meet requirements, especially for the fat-soluble nutrients, due to the low bioavailability of fat-soluble nutrients in raw vegetables. The bioavailability of carotenoids and phylloquinone was reported to be significantly lower from raw vegetables compared with processed vegetables (Gartner et al., 1997; Rock et al., 1998). Many factors influence the bioavailability of fat-soluble nutrients from vegetables, including dietary factors, physicochemistry of fat-soluble nutrients and host-related factors. Dietary factors such as the plant matrix, nutrient species, amount of co-ingested fat and other compounds can be modified in order to enhance the absorption of fat-soluble nutrients from vegetables. Improving the absorption of fat-soluble nutrients in salad vegetables may help in meeting the recommended dietary intake of these nutrients. One way to improve the bioavailability of the 2 fat-soluble nutrients is to incorporate fat during processing and/or ingestion of vegetables. Absorption of fat-soluble nutrients from vegetables was improved by increasing the fat content of a meal (Gijsbers et al., 1996). An in vitro digestion model showed a high bioaccessibility of α-tocopherol, but not γ-tocopherol, in romaine lettuce homogenized with fat (Reboul et al., 2006b). In our previous study, the absorption of carotenoids from salad vegetables was higher when ingested with full fat compared with reduced and fat-free salad dressings (Brown et al., 2004). The necessity to increase the amount of co-ingested fat raises a concern considering the association between high fat intakes and the obesity epidemic (Corella et al., 2007; Snell-Bergeon et al., 2009). It is necessary to identify the minimal amount of fat needed to improve the bioavailability of carotenoids or other fat-soluble micronutrients in unprocessed vegetables. Although oil is known to be essential for the absorption of fat-soluble micronutrients, little or no information is available on the effect of low levels of oil on the relative bioavailability of a wide range of fat-soluble micronutrients from salad vagetables. In addition, no detailed information is available on the relationship between the overall amount of oil and the effect on the relative bioaccessibility of salad vegetables.

 1.3 Study objectives 

The overall objective of this study was to improve the bioavailability of dietary fatsoluble nutrients and bioactives to lower the risk of chronic disease and enhance overall health. The specific goals of this study were therefore to: 

1) obtain the dose-response relation in terms of the influence of the amount of added fat (oil) on the intestinal absorption of carotenoids, phylloquinone and tocopherols in salad vegetables;

 2) investigate whether low levels of oils, i.e. 2 g and 4 g, can result in a statistically significant increase in the absorption of the salad bioactives compared with 0 g of oil.