"Eat to Live, Not Live to Eat"
John H Weisburger, 2000
There is currently a worldwide obesity epidemic, initially observed in developed countries but increasingly now also seen in those that are still developing. It’s a complex phenomenon with many driving factors that individuals, health authorities and national governments have struggled to effectively combat. In most OECD countries about two thirds of adults are overweight or obese (body mass index, BMI, of 25 or more) with profound consequences on well-being and happiness, morbidity and mortality, and family and government finances. Effective public health strategies are desperately needed, and two obvious targets are diet and physical activity, both of which can address the fundamental calories in-calories out equation underlying weight gain. The following study from a Harvard/Karolinska Institute team (Journal of Nutrition (2015), 145, 960-8) considered the effects of fruit and vegetable intake combined and individually, and also fibre on BMI in a large epidemiological study on women over an extended period of time. The following is a summary of their work:
Fruit, vegetable, and dietary fibre intake have all been associated with lower risk of cardiovascular disease (CVD); however, little is known about their role in obesity prevention. Our goal was to investigate whether intake of fruits, vegetables, and dietary fibre are associated with weight change and the risk of becoming overweight and obese. We studied 18,146 US professional health workers aged 45 y or older from the Women’s Health Study who were free of CVD and cancer with an initial BMI of 18.5- 24.9 kg/m2. Fruit, vegetable, and dietary fibre intakes were assessed at baseline through a 131-item food-frequency questionnaire (FFQ), along with obesity-related risk factors. Women self-reported body weight on annual questionnaires. During a mean follow-up of 15.9 y, 8125 women became overweight or obese (BMI 25 or more kg/m2). Intakes of total fruits and vegetables, fruits, and dietary fibre were not associated with longitudinal changes in body weight, whereas higher vegetable intake was significantly associated with greater weight gain. In multivariable analyses, controlling for total energy intake and physical activity along with other lifestyle, clinical, and dietary factors, women in the highest vs. lowest quintile of fruit intake had a significant 13% lower risk of becoming overweight or obese. No association was observed for vegetable or dietary fibre intake. The association between fruit intake and risk of becoming overweight or obese was significantly modified by baseline BMI where the strongest inverse association was observed among women with a BMI <23 kg/m2. Our results suggest that greater baseline intake of fruit, but not vegetables or fibre, by middle-aged and older women with a normal BMI at baseline is associated with lower risk of becoming overweight or obese.
The average age of the women at entry was 53 y. In the FFQ they answered questions on how often they consumed a specific food on average during the past year. There were 9 levels of response ranging from never or less than once/mth to 6+/d. There were 16 fruit and 28 vegetable items, with for example the latter including spinach, kale, lettuce, broccoli, cabbage, cauliflower, Brussel sprouts, carrots, yellow squash, yams, sweet potato, beans, peas, string beans, tofu and tomatoes. When the women were divided into 5 groups based on consumption, the bottom and top 20% ate <1 and 3.1 or more servings of fruit/d resp, while the corresponding ranges for vegetables and fibre were <2 and 5.4 or more servings/d, and <12.4 and 24.7g/d or more resp. So there was a good range of foods and intakes considered. This is important as in some study populations particular foods are consumed in such low or high amounts that many participants can be either under or over what could have an effect, or the range was not great enough to show any difference in outcome between low and high consumers. At the start of the study, women in the top 20% of intake for fruit and vegetables were on average older with lower BMI, were more physically active and likely to use multivitamins, had higher fibre and energy intake and were less likely to have hypercholesterolemia. Note the high incidence of women becoming overweight or obese (45%), a feature now common in so many countries.
This was an epidemiological study and as such has several drawbacks. They can only ever provide evidence for associations between factors, not cause and effects. Nevertheless when many studies conducted under varying conditions and in different populations round the world come to similar conclusions, this evidence is strengthened. As a prospective study the present one provides stronger evidence for observed effects than case-control studies, but neither design can be as convincing as controlled clinical trials given that the possibility of additional unaccounted confounding variables and the accuracy of subject recall and collection of unbiased information is always present. And there is also the possibility that dietary habits in free-living people could have changed from those at baseline over such a long period; this can be more likely with disease diagnosis and consequent strong medical advice. Countering these possibilities is that entrenched cultural cuisines and many life-long habits are formed before middle age, and these can be very resistant to change. Despite these caveats, the study did involve a large number of women which tends to average out individual behaviour and reporting, and it covered a long study period that is often more relevant than short term studies. When progress of particular medical conditions occurs over many decades, epidemiological studies are often the only form of investigation possible, as controlled clinical trials over such lengthy periods are logistically and financially prohibitive.
In most countries the consumption of fruit and vegetables falls far short of national health recommendations. In addition to having demonstrated effects on well-being and chronic disease prevention, they also have the favourable attributes of being nutrient dense and light on calories. Substituting them for nutrient-poor, highly processed fast foods and sugared beverages has multiple benefits, including as shown in this study, a reduced risk of becoming overweight or obese as the years pass. The absence of the effects of vegetables when considered separately from fruits may be surprising to some people as they are usually recommended together without differentiating between them. Although vegetables have approximately equivalent fibre to fruit and both have low calories due to their high moisture content, vegetables are usually processed in some way to increase their palatability while fruit are more normally consumed raw and calorie content and nutrient properties are not compromised. This study did not collect information on food processing so the magnitude of any such effects could not be evaluated. A good example of the significance of cooking techniques on nutrition is provided by considering French fries or chips. Whatever nutritional properties raw potatoes may start with, these are considerably compromised by frying in oil, substantially increasing calories due to absorption. This nutritionally negative impact on calorie content is separate to other important adverse effects that peeling and frying have on potato nutrients. It wouldn’t be a problem if fried chips were only eaten occasionally, but for many people they can become a staple. Similar negative effects to varying degrees can be expected for most vegetables (eg leafy green, cruciferous and dark yellow vegetables) depending on cultural traditions and cooking practices.
It is interesting that the strongest significant effect of fruit on BMI at the end of the study period was for women who were initially in the lower part of the normal range (BMI <23). They were more likely to maintain normal weight throughout the lengthy study period, whereas those who began with a normal BMI >23 but still less than 25 showed no significant fruit effects. These initially higher BMI women may have already developed habits earlier in their lives which predisposed them to ultimate weight gain over the years. it is an example of a factor being preventively effective but not necessarily remedial if other factors dominate to produce the unwanted outcome. Comparable examples are found in many other areas of medicine and nutrition. The classical focus of medicine has been on remedies after diseases or problems occur, but increasingly governments and individuals are realising that if possible, prevention is a far better strategy. Refocussing budgets and effort in this direction has nevertheless been slow, and we live in times when people are conditioned to believe they can continue with their sometimes unbalanced lifestyles and if problems develop at some stage there’ll be powerful and perhaps simple methods like taking tablets to deal with it.
Despite the above qualifications, studies like these provide support for making fruit a regular and significant part of your diet, and what better way to do this than to grow your own healthy and delicious product. If adopted as a long-term lifestyle choice, you could well travel better than most of your peers over the years.