"Get fruity to get fit: Eat more berries to beat a big belly," The Sun reports. The advice is based on the findings of a major new study looking at the effects of foods rich in the compound flavonoid, such as berries and apples, on body weight.
The researchers tracked around quarter of a million people over 24 years.
Results suggest that eating more flavonoids – specifically anthocyanins (coming mainly from blueberries and strawberries), flavonoid polymers (from tea and apples), and flavonols (from tea and onions) – was linked to less weight gain.
Every extra 10 milligrams (mg) of anthocyanins, 138mg of flavonoid polymers, and 7mg of flavanols per day, was linked to 70-100g less weight gained over four-year intervals. While this may not seem a lot, it adds up over a number of years.
An inherent limitation of this type of study design is that it cannot prove cause and effect – it can only highlight associations.
It would be unwise to take this study as advice to only eat berries or apples, as a balanced diet containing a wide variety of fruits and vegetables remains important for overall health. Nonetheless, the study is broadly in line with more robust evidence suggesting you should consume at least five portions of fruit or vegetables a day to reduce your risk of a variety of diseases.
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The study was carried out by researchers from the Chan School of Public Health, Beth Israel Deaconess Medical Center, Brigham and Women’s Hospital, and Harvard Medical School, all based in Boston (US), and the Norwich Medical School, University of East Anglia (UK). It was funded by the US National Institutes of Health, and the Biotechnology and Biological Sciences Research Council (BBSRC).
Two of the authors reported that they had received grants from the US Highbush Blueberry Council. These grants were unrelated to this study.
The UK media generally reported the facts of the study accurately, but most didn’t highlight the limitations of the study design, which the researchers themselves went out of their way to do.
Importantly, the study made no guarantees that if you eat or drink more flavonoids, you’d lose weight or be more likely to be a healthy weight. People eating flavonoids might have diets that are healthier in many ways, such as being high in fibre, which could explain why they are better able to maintain a healthy weight over the long term than people reporting lower flavonoid intakes. All, some or none of the weight benefits may relate directly to flavonoids.
The Daily Mirror sensibly pointed out that although flavonoids are found in various types of chocolate and wine, the calories may counter any positive weight loss effect.
This research combined results from three cohort studies looking at whether dietary intake of specific flavonoid subclasses (including flavonols, flavones, flavanones, flavan-3-ols, anthocyanins, and flavonoid polymers) was associated with weight change over time.
The study team describe how previous studies show that those who eat more fruit and veg are more likely to maintain a healthy weight, which is unsurprising. What is less clear is the specific mechanism by which the fruit and vegetables lead to a healthy weight, or prevent weight gain.
This study looked specifically at the flavonoids, to see whether they were the crucial weight-maintaining ingredients.
Readers should be aware that these types of studies can find associations between consumption of certain food ingredients (like flavonoids) with weight gain or other health benefits. However, they cannot prove that by increasing your consumption of flavonoids you will put on less weight than if you didn’t. A study where you randomly assign people a diet high in flavonoids for a long period of time would be needed to test this, and may not be the most practical to implement. Most people don’t follow diets given to them for long periods.
The study analysed diet and weight change information on 124,086 adult health professionals over 20 to 24 years from three cohort studies in the US: the Health Professionals Follow-up Study (HPFS), Nurses’ Health Study (NHS), and Nurses’ Health Study II (NHS II).
Self-reported change in weight was collected every two years via a questionnaire and was converted into a weight change measure spanning four-year intervals.
All participants self-reported their diet every four years using a validated, food frequency questionnaire. Foods were looked up on a US Department of Agriculture database to estimate their flavonoids content.
Information on other lifestyle habits were collected every four years via the same questionnaire. This was then incorporated into the analysis to minimise their effects, in an effort to isolate the effect of flavonoids. This adjustment of the analysis for confounders included the following variables:
They also took into account changes in intake of the following foods/nutrients:
Citrus juices were a main source of both flavone and flavanone intake in the study, so the researchers used non-citrus juice rather than all-fruit juice as a covariate for analyses of these two flavonoid subclasses. In additional models, they further adjusted for change in total fibre intake (g/day).
People with chronic disease at study start were excluded, due to the risk of their disease causing weight loss – messing up any link with flavonoids. This included some very common conditions like being obese (with a BMI of 30 or over) or having diabetes. This means the study group are a particularly healthy sub-group of the US general population, which is something we should bear in mind when interpreting the results.
The main analysis looked for links between four-year change in weight and change in intake of flavonoid subclasses over the same period, having adjusted for the long list of likely confounders.
Average age over the three cohorts was 36 to 49 (range 27 to 65) and over each four-year period, people gained an average of 1-2kg.
Consumption of most flavonoid subclasses (with the exception of flavones and flavanones) was associated with less weight gain among men and women aged 27-65 followed for up to 24 years.
The greatest magnitude of association was observed for anthocyanins (coming mainly from blueberries and strawberries), flavonoid polymers (from tea and apples), and flavonols (from tea and onions). Each set increase in anthocyanins (10mg), flavonoid polymers (138mg), and flavonols (7mg) was linked to 70-100g less weight gained over four-year intervals.
These associations remained statistically significant for anthocyanins and flavonoid polymers (including proanthocyanidins alone) after further adjustment for fibre intake, which was inversely associated with weight change in all three cohorts. This suggested that high anthocyanin and flavonoid polymer food sources may be associated with less weight gain through mechanisms other than fibre content.
The study repeated the analysis in those who were obese in the study and found the same links. These people had become obese while taking part in the study; obese people were excluded at the study outset.
The researchers concluded that: "Higher intake of foods rich in flavonols, flavan-3-ols, anthocyanins, and flavonoid polymers may contribute to weight maintenance in adulthood and may help to refine dietary recommendations for the prevention of obesity and its potential consequences."
The research team describe the magnitude of the effect as "small" but pointed out then even small decreases in weight gain may have an important public health impact. For example, they cite two studies they say suggest that "losing just 11-22lbs (5.0-10kg) is associated with a decrease in blood pressure". Bearing in mind that the maximum difference was 100g less weight gained per four years, it would take around 200 years to achieve this.
This study found that people who ate more flavonoids, specifically anthocyanins (coming mainly from blueberries and strawberries), flavonoid polymers (from tea and apples), and flavonols (from tea and onions), gained less weight than those consuming less over a 24-year period. Every extra 10mg of anthocyanins, 138mg of flavonoid polymers, and 7mg of flavonols was linked to 70-100g less weight gained over four-year intervals. This isn’t a lot, but adds up over the years.
Readers should be aware that cohort studies like this can find associations between consumption of certain food ingredients (like flavonoids) with weight gain or other health benefits, but they cannot prove that by increasing your consumption of flavonoids you will put on less weight. A study where you randomly assign people a diet high in flavonoids for a long period of time would be needed to test this, and may not be the most practical to implement.
However, the results are consistent with general public health advice to eat lots of fruit and vegetables. Most people in the UK don’t eat the recommended minimum of five portions of fruit or vegetables per day, so you may benefit from eating more and a larger variety.
The study was large and long term – both big strengths, thereby increasing the reliability of the findings.
However, no study is perfect and the study authors pointed out the major limitations of their work.
Firstly, the accuracy of estimates of flavonoid content can be affected by ripeness, storage conditions, food processing, and season. This measurement error would make it less likely to find the association the study did and may underestimate the effect of flavonoids on weight gain.
The results could again underestimate true associations if people who gained weight at the beginning of a four-year period modified their diet in response, eating more fruits and vegetables (flavonoids) to lose weight (reverse causality). On the other hand, the results could overestimate true associations if people who gained weight stopped eating fruits and vegetables.
Individuals who eat more flavonoid-rich foods like fruit and vegetables may have other lifestyle or dietary habits that lead them to put on less weight. The study made many practical efforts to limit this effect, such as controlling for fibre, smoking and many other foods, but may not have completely eliminated the meddling influence of other compounds on any potential flavonoid weight link.
Hence, we must be very careful about recommending high-flavonoid foods as being beneficial to health, although consuming at least five portions of fruit or vegetables a day has significant health benefits.
A healthy diet, while beneficial, may not be enough to prevent weight gain or help you lose previously gained weight. Regular exercise is also needed.
Read more about diet and exercise and how the NHS Choices weight loss guide uses both to help you lose weight.