"Eat to Live, Not Live to Eat"
John H Weisburger, 2000
The following report is a familiar one in this series, namely to what extent does processing of whole fresh fruit influence their nutritional properties? Fresh strawberries have a very short storage life because of their high water content, metabolic activity and fungal load. As a result the enthusiastic home grower very often has to find ways to store them for longer periods to avoid wastage. Jams have filled this spot for ages now, not only because they store so well without any special needs such as refrigeration, but also because they're universally enjoyed and long ago became part of our national dietary habits, eg scones with cream and jam.
Strawberries, like all berries, are exceptionally rich in phytochemical antioxidants, the plant components that are thought to play such a prominent role in the health benefits of all fruits and vegetables. Following is a summary of a US study (International Journal of Food Sciences and Nutrition, (2010) 61: 759–769) showing a dramatic 96% decrease in total antioxidant activity when cooked up as a jam. Even frozen strawberries had only 59% activity remaining.
The present study was conducted to determine differences in antioxidant levels of fresh, frozen, and freeze-dried strawberries, and strawberry jam. Hydrophilic (water soluble, free) antioxidant activity (HAA) and lipophilic (lipid soluble, bound) antioxidant activity (LAA) were measured using the ABTS/H2O2/HRP de-coloration method. HAA and LAA were then summed to calculate the total antioxidant activity (TAA). Mean differences in TAA were expressed on an 'as consumed' and 'dry weight' basis and analysed using one-way analysis of variance and pairwise comparisons. The mean TAA based on dry weight for fresh strawberries (43.6 mcmol/g Trolox equivalents) was significantly higher than for freeze dried (30.1), frozen (25.6), and jam (1.6). Results agree with previous studies reporting that strawberries are a valuable source of antioxidants for consumers.
When TAA was expressed on an 'as consumed' basis, the TAA levels were fresh (3.4), freeze-dried (30.1), frozen (2.7), and jam (1.2). Fresh fruit are a low calorie food because they are mainly water, between 80 and 95%. When given amounts of phytonutrients are spread throughout this water the resulting concentrations will predictably be an order of magnitude lower than those more solid forms like dried fruit, where the moisture level may have been reduced to 5-10%. This simple dilution effect means only smaller amounts of dried fruit need be eaten to obtain a certain amount of vitamins, minerals and phytonutrients compared to the fresh form. But expressing activities on an 'as consumed' basis is not the best way to compare the nutrient density of different fruits or fruit products. This is best done using dry weight because moisture content of fresh fruit depends so much on horticultural factors like plant genetics, cultivar, climate, soil nutrition, harvest timing and precipitation. In late maturity and ripening stages, fruit weight and size can double in a couple of days with rainy conditions, but in this case it's almost all water, and phytonutrient amounts in the whole fruit will usually have changed very little. In addition to considering this simple dilution effect on concentrations, there can also be chemical changes in individual phytonutrients during processing steps as a result of oxidation or enzymatic action, and these changes must be taken into account in the final product. Cooking fruit for sufficient time to give the required degree of softening for a paste-like jam to be made commonly causes some nutrient decomposition.
Another important consideration when comparing the relative merits of a range of fruits is the chemical properties of the different antioxidants. The hydrophilic phytonutrients are easier to extract and assay than the lipophilic, and in many of the early studies these were the only ones reported. But most fruits have some in the lipophilic form and in many this type may contribute most of the activity. Consequently more thorough recent studies, as in the above, have reported both for a better evaluation.
From a separate nutritional perspective, a key negative of jams is the large quantity of added sugar, frequently 50%, to prevent microbial growth and deterioration. And this addition is to a fruit that's already sufficiently sweet to appeal to most palates. Pure sugar is calorie-dense and nutrient-empty, and it's the last thing we should be adding into our diets; current levels of obesity illustrate we're already overloaded. It produces a food that has a high glycemic index which predisposes to diabetes amongst other unwanted physiological and medical outcomes. Strawberries are best eaten fresh when possible. If just picked they may last for 1-2 weeks at 0-4°C in the refrigerator, less if you bought them and they've already been in the postharvest transport and marketing stages for some time. Enjoy them for their delicious flavour and aromas and be comforted in the knowledge that they're also good for you.
Nasopharyngeal cancer (NPC) shows a very skewed racial and geographic distribution. The incidence rate among Caucasian males in Westernised societies is <1/100,000 but it can be 20-40 in southern Chinese. Intermediate rates are found in several indigenous populations in Southeast Asia, and in natives of the Arctic region, North Africa, and the Middle East. A study on Chinese immigrants in California showed that the risk of NPC for the third generation was less than half that for the second generation but was still 10-fold greater than Caucasians. Alternatively, Caucasians born and raised in southern China have increased rates compared to those remaining in western countries. These findings indicate both genetic and environmental factors contribute to the disease, and several candidate genes have now been identified. About 90% of the world population is infected with the Epstein-Barr virus at a young age and this also has been shown to be a necessary but not sufficient factor in the onset of NPC. Incidence in males is 2-3 times that in females, and the effect of smoking on NPC is an order of magnitude less than with lung cancer or other respiratory tract malignancies.
If NPC is relatively rare in Caucasians, perhaps we in Australia should focus our health-directed efforts on other more pressing problem areas? But we are a multicultural society with an ever-increasing ethnic population, and to our benefit and enjoyment these people have very successfully introduced their national cuisines to the wider community here. Who doesn't enjoy going to a Chinese restaurant or cooking up one of their traditional meals? However a number of these foods and cooking practices were shown more than 40 years ago to be strong risk factors for NPC, and to the extent that we consume these meals we may all be subject to them, with resulting increased risk of NPC. The following summary of a study in the southern province of Guangdong, China (BMC Cancer 2010, 10:446) describes some of the dietary factors that can exacerbate or minimise the disease.
Nasopharyngeal carcinoma (NPC) is rare in most parts of the world but is a common malignancy in southern China, especially in Guangdong. Dietary habit is regarded as an important modifier of NPC risk in several endemic areas and may partially explain the geographic distribution of NPC incidence. In China, rapid economic development during the past few decades has changed the predominant lifestyle and dietary habits of the Chinese considerably, requiring a reassessment of diet and its potential influence on NPC risk in this NPC-endemic area. To evaluate the association between dietary factors and NPC risk in Guangdong, a large-scale, hospital-based case-control study was conducted. 1387 eligible cases and 1459 frequency matched controls were recruited. Odds ratios (ORs) were estimated using a logistic regression model adjusting for age, sex, education, dialect, and habitation household type. Observations made included the following.
In multivariate analyses, these associations remained significant. It can be inferred that despite rapid urbanisation and changing dietary habits, previously established dietary risk factors in the Cantonese population are still contributing to a high incidence of NPC.
This was a case control study where people with confirmed NPC were compared with subjects of similar age, gender, education etc but who did not have NPC. The reliability of this comparison is always dependent on how well the healthy controls match the case cohort or whether it is unknowingly influenced by other factors not considered. Prospective studies where healthy people are entered into a study, then followed for a certain time during which cases develop, and then identifying factors that were different between cases and those who remained healthy, provide stronger evidence. Nevertheless, when case-control studies with similar findings in many different studies are considered together they can provide valuable information.
To facilitate statistical analysis in the above work, intake levels for particular foods were grouped into three levels - less than monthly (serving as the reference), monthly, and weekly or more. Children defined as <12 years old were generally more susceptible to both positive and negative dietary effects than adults. Interviewing the mothers and family of people who developed NPC revealed that salted fish was one of the most common solid foods given to children when weaning, and these NPC cases were more likely to have other members in the family with NPC. The most extreme OR of 9.69 was for adults consuming preserved/cured meats in the highest strata of 'weekly or more often', highlighting the danger of eating these foods. It represents almost a 10 times greater risk of developing NPC, and in earlier studies this has been >40-fold. The effect of preserved vegetables and fermented pastes was not as pronounced, but still cautions against too frequent consumption.
Teas, herbs and spices have some of the highest antioxidant phytochemical levels of all plant-based foods, and since they're thought to play a central role in minimising cancers, it might be expected that they would be of benefit in NPC. Their impact is muted because the amounts typically consumed are much smaller than for other staple foods, but they can still represent the major source of antioxidants when diets are insufficiently balanced and adequate. Levels in a given plant species vary greatly depending on geography, variety, climatic and soil conditions, fertilisation and management, harvest, storage conditions etc, plus when consumed with other foods in a meal, absorption and effects may be variably compromised. This scenario is further complicated when several herbs are involved and the effectiveness and reproducibility of products becomes increasingly unpredictable. As a result it's not surprising that the effects of Chinese teas and soups made from 30 different species as in this study may have shown beneficial effects whereas in others the reverse has been seen.
As with all cultures worldwide, there has been a pressing need in the past to be able to preserve foods in times of plenty to cover periods when less was available. Historically the principal means of achieving this were dehydration (sun-dried), smoking, fermentation and salting. We now know that all these have their drawbacks nutritionally and newer less-destructive or harmful techniques such as autoclaving and pasteurisation, spray drying, micro-wave treatment, high pressure sterilisation, gamma radiation, freeze drying etc have been introduced. But the foods produced by the traditional techniques have been so strongly integrated into national cuisines over centuries that it's difficult to achieve meaningful changes – old habits and practices that should be abandoned or minimised prove very resistant. There has been some reduction in incidence of NPC in southern China following gradual adoption of foods subject to more modern processing techniques and the advice of public health authorities, but NPC incidence still remains much higher than in the West.
With still incomplete understanding of the disease, appropriate dietary habits remain the principal means of prevention. The damage from traditionally processed foods in NPC is development of carcinogens such as nitrosamines which may then be eaten and/or inhaled. Salt preserved foods are a dietary staple in all endemic NPC populations, with increased incidence in lower socioeconomic groups less aware or receptive to public health advice. The message from the present study and others suggests that consumption of preserved/cured meats and other salted products, including eggs, fruit and vegetables should be limited, particularly in childhood, however much loved or enjoyed. Eating fresh fruit regularly can do wonders for prevention, reducing the risk of NPC by about 90%.
The current obesity epidemic is due to changes in society - modernisation results in over-nutrition and a sedentary lifestyle. The phenomenon largely began in the developed world only a few decades ago and has rapidly become global. Australia is no exception, with almost two thirds of adults being either overweight or obese (body mass index, BMI>25). The situation is serious as high BMI is at the core of so many degenerative and chronic diseases, and people are generally aware that something needs changing to minimise dramatic loss in quality and longevity of life, economic productivity and over-whelming individual and societal costs. Long term dietary changes are one arm of a preventative strategy that can address the problem. But losing weight and keeping it off once gained is very hard for almost everyone, and many resort repeatedly to the latest fad diet. only to find it doesn't amount to real gains. Despite endless discussion about the impact of genes, family and traditional habits, psychology etc, putting on weight comes down to an imbalance between energy intake and expenditure. All these driving factors may be involved. but if the imbalance has developed, what can be done about it?
Amongst all the knowing advice out there about how to minimise energy intake, how many times have you been told in TV documentaries, health magazines, newspapers, web articles etc that drinking a glass of water or other low-calorie drink before a meal will fill part of the stomach. so that when food is subsequently eaten, the full feeling will be experienced earlier, resulting in fewer total calories consumed? Well, the following recent US study (Appetite (2009), 52, 416–422) investigated this perceived 'wisdom' with different fruit products and found it to be false. It's common practice for many people to consume a glass of (say) orange juice either before or during their breakfast or other meals. The healthy nutritional benefit from this practice is a separate issue to the weight control focus here, and as these considerations have been discussed elsewhere in this series (see More Fruit, Less Juice and others) they won't be discussed further.
Consuming whole fruit reduces ratings of satiety more than fruit juice, but little is known about the effects of different forms of fruit on subsequent energy intake. This study tested how consuming pre-loads of apples in different forms prior to a meal (fresh apple, cooked applesauce, and apple juice with and without added fibre) influences satiety and energy intake in the subsequent meal. Pre-loads were matched for weight, energy content, energy density, and ingestion rate. Once a week for 5 weeks, 58 adults consumed one of four pre-loads (266g each, 125kcal) or no pre-load taken as the control, followed by a test meal consumed ad libitum 15 min later. Results showed that eating apple significantly reduced total energy intake (pre-load + test) compared to control (837 cf 1024kcal resp), with a reduction of 18%. Applesauce also had a significant but reduced effect (928kcal) but neither of the juices had any significant effect. Fullness ratings differed significantly after pre-load consumption with apple > applesauce > both juices > control. Overall, whole apple increased satiety more than applesauce or apple juice. Adding naturally occurring levels of apple fibre to juice did not enhance satiety. These results suggest that solid fruit affects satiety more than pureed fruit (applesauce) or juice, and that eating fruit at the start of a meal can reduce energy intake.
The interval between pre-load and main meal intake has significant effects on this phenomenon – the longer the interval the less effect. The present authors chose an interval of 15 min as being representative of the time between a typical entrée and the subsequent meal, for example between courses served in restaurants. Subjects were instructed to consume the pre-load in 10min and then after a further 5min to have the ad libitum test meal. To avoid social interactions and their effects on eating behaviour all participants ate the test meal isolated from one another. Fruit juices are very popular products, both commercially-supplied and home-made. Unless a puree is made with all fruit components being present, juices are invariably strained to remove off-colours, tastes and solid matter associated with skin, fibre and seeds. In the present study the authors tailored their pre-loads to reflect these background habits. For most meals, no pre-load is eaten so they took this as the control. The fruit juices were strained (skin and fibrous material removed). Because fibre has been shown to influence satiety, they then added apple pectin (soluble fibre to give the same fibre concentration as in whole apple) to a second juice pre-load. The applesauce was skinned apple cooked then pureed, and the whole apple with skin removed was served in pieces. As stated above, these four pre-loads were then matched for weight, energy content, energy density and ingestion rate (10min). There were a number of exclusion criteria from the trial such as people who had abnormal energy balance reflected in unhealthy BMI (<18 and >40), standard questionnaires were used to exclude any with symptoms of depression, those who were currently engaged in weight loss diets. taking significant medications etc. Each subject in a cross-over clinical trial acts as their own control, so minimising variability between people.
Other studies with apples, oranges and grapes on the greater effect of solid foods on satiety suggest that it's due to the longer times that food stays in the mouth while chewing and salivating that generates stronger hormonal/sensory feedback to the brain compared to more liquid forms which are quickly swallowed. A more general finding in this direction is that slower eating gives time for these hormonal messages to build up resulting in a feeling of fullness. Eating quickly often means you consume more than necessary before the internal 'finished/full' signal gets thru; the outcome if habitually maintained predisposes to weight gain. Of course an extra benefit with slower eating is that you have an increased chance to savour and enjoy what's being consumed.
Another facet of this story is the irregular and increased frequency of food intake associated with Westernised lifestyles, supported by the widespread availability of fast food outlets selling cheap, calorie-dense foods, highly-sugared drinks and treats, ready-prepared supermarket offerings, habitual snacking, skipping meals and generally being time-poor. For many, three regular and filling meals a day is no longer the norm and hunger builds up before the next regular meal. If additional snacks throughout the day are desired or necessary, fruit always represents a better choice than biscuits, cakes or confections. So too is a preference for fruit rather than ice-cream or cheese cake if you have dessert after meals. But the study quoted above suggests if you want to lose weight, then it would help to have fresh fruit or fruit salad as an entrée just before the meal instead of afterwards. If you have three adequate and filling low glycemic index meals a day, clinical trials have shown that fewer daily calories are consumed compared to 6 or 9 smaller meals at irregular intervals. Other studies in Japanese children have shown those who skip meals and eat irregularly are more likely to be obese. In addition to lower daily calorie intake with regular meals, there is an increased thermogenic effect of foods, meaning more carbohydrate and fat is burned (oxidised) than stored as fat in the body.