Free radicals and other reactive oxygen and nitrogen species (ROS and RNS) are created as a result of normal cellular oxidative metabolic reactions in our bodies. Such molecules are also formed as a consequence of diseases (e.g., inflammations), and from tobacco smoke, environmental pollutants, natural food constituents, drugs, ethanol, and radiation (Preedy 2011). If not quenched by antioxidants, these highly reactive compounds will react non-enzymatically with, and potentially alter, the structure and function of several cellular or extracellular components (Halliwell, 1996; Gutteridge & Halliwell, 2000).
When the critical balance between generation of free radicals and the antioxidant defenses is unfavorable, oxidative damage can accumulate. Oxidative stress is defined as “a condition that is characterized by accumulation of non-enzymatic oxidative damage to molecules that threaten the normal function of the cell or the organism” (Blomhoff, 2005). Compelling evidence has emerged that oxidative stress makes a significant contribution to a lot of diseases.

An antioxidant enzyme is “a protein that limits oxidative stress“ (Blomhoff, 2005).

A plant-based diet protects against chronic oxidative stress-related diseases. Dietary plants contain variable chemical families and amounts of antioxidants. This is a comprehensive food database consisting of the total antioxidant content of typical foods as well as other dietary items such as traditional medicine plants, herbs and spices and dietary supplements.

The results demonstrate that there are several thousand-fold differences in antioxidant content of foods. Spices, herbs and supplements include the most antioxidant rich products, shown in a lot of studies, some exceptionally high.

It shows that plant-based foods introduce significantly more antioxidants into human diet than non-plant foods. 

It is widely accepted that a plant-based diet with high intake of fruits, vegetables, and other nutrient-rich plant foods may reduce the risk of oxidative stress-related diseases [1-6]. 

Antioxidants can eliminate free radicals and other reactive oxygen and nitrogen species, and these reactive species contribute to most chronic diseases. It is hypothesized that antioxidants originating from foods may work as antioxidants in their own right in vivo, as well as bring about beneficial health effects through other mechanisms, including acting as inducers of mechanisms related to antioxidant defense [7,8], longevity [9,10], cell maintenance and DNA repair [11].

Several assays have been used to assess the total antioxidant content of foods, 

A complex endogenous antioxidant defense system has been developed to counteract oxidative damage and oxidative stress. Such an antioxidant defense is essential for all aerobic cells. The endogenous antioxidant defense has both enzymatic and non-enzymatic components that prevent radical formation, remove radicals before damage can occur, repair oxidative damage, and eliminate damaged molecules (Halliwell, 1996; Gutteridge & Halliwell, 2000; Lindsay & Astley, 2002).

In addition to the endogenous antioxidant defense, it has been hypothesized that dietary components also may contribute to the antioxidant defense either by providing redox active compounds that can directly scavenge or neutralize free radicals or other ROS and RNS, or by providing compounds that can induce gene expression of the endogenous antioxidants (Blomhoff, 2005).

DIETARY COMPOUNDS WITH THE ABILITY TO INDUCE

PRODUCTION OF ENDOGENOUS ANTIOXIDANTS

An important antioxidant defense mechanism involves detoxification enzymes.

Dietary plants rich in compounds that induce detoxification enzymes include the vegetables broccoli, Brussels sprouts, cabbage, kale, cauliflower, carrots, onions, tomatoes, spinach and garlic. The evidence for phase 2 enzyme inductions at ordinary intake levels of plant foods in humans is, however, limited, and the importance of this defense mechanism in the overall protection against oxidative damage is still uncertain.

DIETARY COMPOUNDS WITH THE ABILITY TO DIRECTLY

SCAVENGE OR NEUTRALIZE REACTIVE OXIDANTS

In addition to the well-known antioxidants, vitamin C, vitamin E, and selenium, there are numerous other antioxidants in dietary plants. Carotenoids are ubiquitous in the plant kingdom, and as many as 1000 naturally occurring variants have been identified. At least 60 carotenoids occur in fruit and vegetables commonly consumed by humans (Lindsay & Astley, 2002).

Phenolic compounds are also ubiquitous in dietary plants (Lindsay & Astley, 2002).. Over 8000 plant phenols have been isolated. Plant phenols are antioxidants.

MEASUREMENTS OF TOTAL ANTIOXIDANT CONTENTS IN FOODS e

METHODOLOGICAL CONSIDERATIONS

Several assays have been used to assess the total antioxidant content of foods.

TOTAL AMOUNTS OF ANTIOXIDANTS IN FOODS

We have recently performed a systematic measurement of the total antioxidant content of more than 3100 foods. This novel antioxidant food table enables us to calculate the total antioxidant content of complex diets, to identify and rank potentially good sources of anti- oxidants, and to provide the research community with comparable data on the relative anti- oxidant capacity of a wide range of foods.

  

   

There is not necessarily a direct relationship between the antioxidant content of a food sample consumed and the subsequent antioxidant activity in the target cell. Factors influencing the bioavailability of phytochemical antioxidants include the food matrix, absorption, and metabolism. Also, the methods measuring total antioxidant capacity do not identify single antioxidant compounds, and they are therefore of limited use when investigating the mech- anisms involved. This is, however, not the scope of this initial study. With the present study, food samples with high antioxidant content are identified, but further investigation into each individual food and phytochemical antioxidant compound is needed to identify those which may have biological relevance, and the mechanisms involved.

The aim of the present study was to screen foods to identify the total antioxidant capacity of fruits, vegetables, beverages, spices, and herbs, in addition to common everyday foods. In nutritional epidemiologic and intervention studies, the antioxidant food database may be utilized to identify and rank diets and subjects with regard to antioxidant intake, and as a tool in planning dietary antioxidant interventions.

The results show large variations both between as well as within each food category; all of the food categories contain products almost devoid of antioxidants (Table 6.1).  

We find that plant-based foods are generally higher in antioxidant content than animal-based and mixed food products

The antioxidant food table is a valuable research contribution, expanding the research evidence base for plant-based nutritional research, and may be utilized in epidemiological studies where reported food intakes can be assigned antioxidant values. It can also be used to test antioxidant effects and synergy in experimental animal and cell studies, or in human clinical trials. The ultimate goal of this research is to combine these strategies in order to understand the role of dietary phytochemical antioxidants in the prevention of cancer, cardiovascular diseases, diabetes, and other chronic diseases related to oxidative stress.

The total content of antioxidants has been assessed in more than 3100 foods.
l The results show large variations both between as well as within each food category.
l Nuts and seeds are among the food categories that include the most antioxidant-rich food

items.
l Walnuts contain massive amounts of antioxidants.
l Pecans, chestnuts, peanuts, pistachios, and sunflower seeds are very rich in total

antioxidants.
l Hazelnuts, almonds, Brazil nuts, macadamias, pine kernels, cashew nuts, flax seeds, poppy

seeds, and sesame seeds contain significant amounts of total antioxidants. l A significant portion of nut antioxidants is located in the pellicle.

Antioxidants may occur naturally in different source materials or may be synthe- sized by different means. Higher plants and their constituents provide a rich source of natural antioxidants, such as tocopherols and phenols/polyphenols, which are found abundantly in spices, herbs, fruits, vegetables, cereals, grains, seeds, teas, and oils. Antioxidants from marine origin such as algae, sh/shell sh, and marine bacteria have also been considered (Amarowicz, Karamac, & Shahidi, 1999; Athukorala et al., 2003; Shahidi & Amarowicz, 1996).

The effectiveness of antioxidants is generally influenced by their structural features, concentration, temperature, type of oxidation substrate, and physical state of the system, as well as presence of prooxidants and synergists (Yanishlieva-Malslarova, 2001). The chemical structure of an antioxidant determines its intrinsic reactivity toward free radicals and other ROS and, hence, the antioxidant activity. Efficiency of antioxidants also depends on their concentration and location in the system (e.g., distribution at interface) (Shahidi & Zhong, 2011; Zhong & Shahidi, 2012a). The reaction kinetics plays an important role in their short-term or long-term protection against oxidation, and it involves the rate at which an antioxidant reacts with a speci c oxidant, the thermodynamics of the reaction, and the extent to which the antioxidant reacts (Antolovich, Prenzler, Patsalides, McDonald, & Robards, 2002).

Our results show large variations both between as well as within each food category; all of the food categories contain products almost devoid of antioxidants. Please refer to Additional file 1, the Antioxidant Food Table, for the FRAP results on all 3139 products analyzed. The categories "Spices and herbs", "Herbal/traditional plant medicine" and "Vitamin and dietary supplements" include the most antioxidant rich products analyzed in the study. The categories "Berries and berry products", "Fruit and fruit juices", "Nuts and seeds", "Breakfast Cereals", "Chocolate and sweets", "Beverages" and "Vegetables and vegetable products" include most of the common foods and beverages which have medium to high antioxidant values. We find that plant-based foods are generally higher in antioxidant content than animal-based and mixed food products, with median antioxidant values of 0.88, 0.10 and 0.31 mmol/100 g, respectively. Furthermore, the 75^th percentile of plant-based foods is 4.11 mmol/100 g compared to 0.21 and 0.68 mmol/100 g for animal-based and mixed foods, respectively. The high mean value of plant-based foods is due to a minority of products with very high antioxidant values, found among the plant medicines, spices and herbs. In the following, summarized results from the 24 categories are presented.

Most of the spices and herbs analyzed have particularly high antioxidant contents. Although spices and herbs contribute little weight on the dinner plate, they may still be important contributors to our antioxidant intake, especially in dietary cultures where spices and herbs are used regularly. We interpret the elevated concentration of antioxidants observed in several dried herbs compared to fresh samples, as a normal consequence of the drying process leaving most of the antioxidants intact in the dried end product. This tendency is also seen in some fruits and their dried counterparts. Thus, dried herbs and fruit are potentially excellent sources of antioxidants.

Nuts are a rich source of many important nutrients and some are also antioxidant-rich. The observed increase in antioxidant content in nuts with pellicle compared to nuts without pellicle is in good agreement with earlier studies showing the flavonoids of many nuts are found in the nut pellicle.

Chocolate have for several years been studied for its possible beneficial health effects. Our results show a high correlation between the cocoa content and the antioxidant content, which is in agreement with earlier studies.

As demonstrated in the present study, the variation in the antioxidant values of otherwise comparable products is large. Like the content of any food component, antioxidant values will differ for a wide array of reasons, such as growing conditions, seasonal changes and genetically different cultivars [46,58], storage conditions [59-61] and differences in manufacturing procedures and processing [62-64]. Differences in unprocessed and processed plant food samples are also seen in our study where processed berry products like jam and syrup have approximately half the antioxidant capacity of fresh berries. On the other hand, processing may also enhance a foods potential as a good antioxidant source by increasing the amount of antioxidants released from the food matrix which otherwise would be less or not at all available for absorption [65]