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Michael and Saul will be happy about this one, since I'm going to criticize Dr. Greger's latest video (embedded below), about happiness titled Which Foods Increase Happiness? I'm also curious what Sthira might have to say :-), if he's still out there...?!

 

In the video, Dr. Greger reviews studies of the link between fruit/vegetable (FV) intake and happiness. In the first part of the video, he reviews a number of studies that have (not surprisingly) found a positive association between FV consumption and positive mood / happiness. Then, to his credit, he asks the perennial question for these kinds of observational studies, namely the direction of causality. Does FV intake increase happiness, or do happy people eat more FVs? He points to evidence for the latter. For example, study [1] found that people in a good mood are more likely than unhappy people to prefer healthy food alternatives (e.g. grapes) over unhealthy ones (e.g. M&Ms) when given the choice. 

 

But then he goes on to argue for causality in the other direction (FV intake  -> happiness) as well, based in large part on study [2]. Here is where my criticism comes in.

 

In [2], researchers tracked (via an internet questionnaire) the FV intake and mood of nearly 300 young adults (avg. age of 20), for three weeks. Affect was gauged each day by having them rate how closely their current mood matched each of 9 positive and 9 negative adjectives (e.g. relaxed, excited, happy, ... vs. depressed, anxious, sad, ...) on a 1-5 scale. The self-rated scores for the positive (and negative) adjectives were averaged together to form a positive (and negative) affect score for each subject each day. These details will be important below, in case you are wondering why I'm going into so much detail...

 

What they found was that higher reported FV intake on a given day was associated with a better mood on the same day. No surprise there, but also no insight as to causality. To try to get at causality, they then looked at FV intake on one day vs. mood on the next day, and visa versa - mood on one day vs. FV intake on the next day. What the found was that FV intake on one day predicted better mood on the next day, but better mood on one day didn't predict more FV intake on the next.

 

So case closed right? Doesn't this tells us that it is FV intake that causes (future) happiness?

 

That's (more or less) Dr. Greger's conclusion. But the key comes in the last few seconds of the video, when Dr. Greger says:

 

How many fruits and vegetables [to increase happiness]? Seems we need to consume approximately 7.2 daily servings of fruit or 8.2 servings of vegetables to notice a meaningful change.

 

So what does that somewhat ambiguous last statement actually mean, and how does it relate to the findings in the rest of the paper? That last statement is based on this sentence from [2]:

 

Because the typical daily consumption in our sample was 1.7 servings of fruit and 2.5 servings of vegetables, our data suggest that young adults would need to consume approximately 7.2 daily servings of fruit or 8.2 servings of vegetables to notice a meaningful change in positive affect.

 

In other words, because of the small size of the effect, the researchers found that a person would need to eat about 5.5 extra servings of FVs per day relative to their normal intake to experience a "meaningful change" in happiness level. What do they consider a "meaningful change"? Here is the footnote from the paper on their definition:

 

A meaningful change in positive affect was defined as an increase in 0.16 points (i.e., increasing from the mean of 2.59 –> 2.75 points on the positive affect scale). A change of 0.16 points reflects a Cohen’s d of 0.20, a small effect, which was computed by the formula 0.20 = (2.75–2.59)/0.80, where 0.80 equals the average within-person standard deviation in positive affect.

 

So they're defining a "meaningful change" as going from 2.59 to 2.75 on a 1-5 scale. Now if you ask me, that is a pretty tiny improvement from eating an extra 5.5 helpings of FVs. I looked up the Cohen's d measure they used a metric for effects size. Interesting statistical stuff. It seems like in the same way statisticians have agreed that P < 0.05 in a student t-test is 'significant', they've also (sorta supposedly) agreed on a definition for 'small', 'medium' and 'large' effects based on this "Cohen's d" metric. The authors of [2] equated a "meaningful change" with a "small effect" as measured by Cohen's d, which is basically defined as a change in a variable by 25% of one standard deviation in its data. Below is a helpful graphic to give you a feel for what that kind of "meaningful change" would really look like:

 

fDcfCTH.png

Imagine the dark grey gaussian represents the distribution of a single individual's happiness over time on a 1-5 scale - most days they felt a middling amount of happiness, but on a smaller number of days they were happier, and on a smaller number of days they were sadder. The extreme tails would represent the (very rare) best and worst days of their life.  Hence the gaussian distribution. The "meaningful change" the authors are postulating that would result from eating an additional 5.5 servings of FVs per day equates to shifting the happiness gaussian from the dark grey rightward to the light blue gaussian. 

 

I may be a curmudgeon, and you can judge for yourself, but to me this seems like a pretty small shift in happiness as a result of eating a whole lot more fruits and vegetables. So while Dr. Greger's portrayal of the relationship between FV intake and happiness isn't incorrect per se, he may be tilting the interpretation of the data in favor of fruits and vegetables, sort of like the authors of the apples and mortality study may have done in favor of apples.

 

This perspective appears to be shared by the authors of [2], as reflected in the title they gave to their paper (my emphasis): "Many apples a day keep the blues away - ...

 

And to their credit (the authors of [2] that is, not Dr. Greger), they do say in the discussion that they agree with the authors of the review article [3], which observed that double-blind, randomized control trials are needed to definitively determine if eating more FVs really does indeed lead to improvements in mood:

 

Of course, inferences about causality should be considered tentative until replicated with an experiment. Although our design allowed us to conduct lagged analyses, and these analyses suggested that fruit and vegetable consumption might be influencing positive affect, we agree that future research needs to include randomized controlled trials (RCTs) evaluating the influence of high fruit and vegetable intake on affect and well-being [3].

 

Sadly, Dr. Greger doesn't mention the randomized trials of FV intake and mood that have been done. Although small and not double (or single) blinded, this one [4] was encouraging. It found that, among 100 students randomly assigned to eat either a piece of fruit or a serving of "chocolate/crisps" each afternoon for 10 days, the fruit-eaters reported a better mood at the end of the study. But in truly double-blind randomized control trials of various fruit juices (i.e. blueberry [5], cranberry [6] and grape [7] juice), the effect of fruit juice intake on mood was at best small and non-significant [5], or missing altogether [6][7].

 

This is yet another example of where careful reading of the whole published paper, and looking at its results in the context of other research findings, is important for getting a more complete understanding of the research...

 

--Dean

 

 

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[1] Fedorikhin, Alexander and Patrick, Vanessa M., Positive Mood and Resistance to Temptation: The Interfering Influence of Elevated Arousal (2010). Journal of Consumer Research, Vol. 37, No. 4, 2010. Available at SSRN: http://ssrn.com/abstract=2086834

 

-----------

[2] Br J Health Psychol. 2013 Nov;18(4):782-98. doi: 10.1111/bjhp.12021. Epub 2013

Jan 24.

Many apples a day keep the blues away--daily experiences of negative and positive
affect and food consumption in young adults.

White BA(1), Horwath CC, Conner TS.

Full text: http://sci-hub.io/10.1111/bjhp.12021

OBJECTIVES: Prior research has focused on the association between negative affect
and eating behaviour, often utilizing laboratory or cross-sectional study
designs. These studies have inherent limitations, and the association between
positive affect and eating behaviour remains relatively unexplored. Therefore,
the objective of this study was to investigate the bidirectional relationships
between daily negative and positive affective experiences and food consumption in
a naturalistic setting among healthy young adults.
DESIGN: Daily diary study across 21 days (microlongitudinal, correlational
design).
METHODS: A total of 281 young adults with a mean age of 19.9 (± 1.2) years
completed an Internet-based daily diary for 21 consecutive days. Each day they
reported their negative and positive affect, and their consumption of five
specific foods. Hierarchical linear modelling was used to test same-day
associations between daily affect and food consumption, and next-day (lagged)
associations to determine directionality. Moderating effects of BMI and gender
were also examined in exploratory analyses.
RESULTS: Analyses of same-day within-person associations revealed that on days
when young adults experienced greater positive affect, they reported eating more
servings of fruit (p = .002) and vegetables (p < .001). Results of lagged
analysis showed that fruits and vegetables predicted improvements in positive
affect the next day, suggesting that healthy foods were driving affective
experiences and not vice versa. Meaningful changes in positive affect were
observed with the daily consumption of approximately 7-8 servings of fruit or
vegetables.
CONCLUSIONS: Eating fruit and vegetables may promote emotional well-being among
healthy young adults.

PMID: 23347122

 

---------------

[3] Blanchflower, D., Oswald, A., & Stewart-Brown, S. (2012). Is psychological well-being linked to the consumption of fruit and vegetables? Social Indicators Research. Advance online publication. doi:10.1007/s11205-012-0173-y. Free full text: 

 

Abstract

Humans run on a fuel called food. Yet economists and other social scientists rarely study what people eat. We provide simple evidence consistent with the existence of a link between the consumption of fruit and vegetables and high well-being. In crosssectional data, happiness and mental health rise in an approximately dose-response way with the number of daily portions of fruit and vegetables. The pattern is remarkably robust to adjustment for a large number of other demographic, social and economic variables. Well-being peaks at approximately 7 portions per day. We document this relationship in three data sets, covering approximately 80,000 randomly selected British individuals, and for seven measures of well-being (life satisfaction, WEMWBS mental well-being, GHQ mental disorders, self-reported health, happiness, nervousness, and feeling low). Reverse causality and problems of confounding remain possible. We discuss the strengths and weaknesses of our analysis, how government policy-makers might wish to react to it, and what kinds of further research -- especially randomized trials -- would be valuable.

 

------------

[4] Front Nutr. 2014 Jul 16;1:10. doi: 10.3389/fnut.2014.00010. eCollection 2014.

 
Positive effects of a healthy snack (fruit) versus an unhealthy snack
(chocolate/crisps) on subjective reports of mental and physical health: a
preliminary intervention study.
 
Smith AP(1), Rogers R(1).
 
Author information: 
(1)School of Psychology, Cardiff University , Cardiff , UK.
 
BACKGROUND/AIMS: Recent research has shown associations between type of snack and
wellbeing. These studies have been cross-sectional and the aim of the present
research was to examine this topic using an intervention study.
METHODS: A between-subjects intervention study was carried out. Volunteers (100
students, mean age = 19.00 years; 27 male, 73 female) completed online
questionnaires measuring anxiety and depression, fatigue, somatic symptoms,
cognitive difficulties, and distress at baseline. They were then randomly
assigned to one of two snacking conditions - chocolate/crisps or fruit.
Volunteers consumed one snack item in the mid-afternoon each day for 10 days. At 
the end of the intervention, the volunteers completed the questionnaires again.
RESULTS: Analyses of the baseline data confirmed that consumption of chocolate
was associated with greater emotional eating and depression. Analyses of
covariance, with the baseline data as covariates, were carried out on the
post-intervention responses. The results showed that consumption of fruit was
associated with lower anxiety, depression, and emotional distress than
consumption of crisps/chocolate. Similarly, scores for somatic symptoms,
cognitive difficulties, and fatigue were greater in the crisps/chocolate
condition.
CONCLUSION: These results extend findings from cross-sectional studies and give a
clearer indication of causal effects of different types of snacks on wellbeing.
 
PMCID: PMC4428353
PMID: 25988113
 

------------

[5] J Agric Food Chem. 2010 Apr 14;58(7):3996-4000. doi: 10.1021/jf9029332.

 
Blueberry supplementation improves memory in older adults.
 
Krikorian R(1), Shidler MD, Nash TA, Kalt W, Vinqvist-Tymchuk MR, Shukitt-Hale B,
Joseph JA.
 
Author information: 
(1)Department of Psychiatry, University of Cincinnati Academic Health Center,
Cincinnati, Ohio 45267-0559, USA. robert.krikorian@uc.edu
 
The prevalence of dementia is increasing with expansion of the older adult
population. In the absence of effective therapy, preventive approaches are
essential to address this public health problem. Blueberries contain polyphenolic
compounds, most prominently anthocyanins, which have antioxidant and
anti-inflammatory effects. In addition, anthocyanins have been associated with
increased neuronal signaling in brain centers, mediating memory function as well 
as improved glucose disposal, benefits that would be expected to mitigate
neurodegeneration. This study investigated the effects of daily consumption of
wild blueberry juice in a sample of nine older adults with early memory changes. 
At 12 weeks, improved paired associate learning (p = 0.009) and word list recall 
(p = 0.04) were observed. In addition, there were trends suggesting reduced
depressive symptoms (p = 0.08) and lower glucose levels (p = 0.10). We also
compared the memory performances of the blueberry subjects with a demographically
matched sample who consumed a berry placebo beverage in a companion trial of
identical design and observed comparable results for paired associate learning.
The findings of this preliminary study suggest that moderate-term blueberry
supplementation can confer neurocognitive benefit and establish a basis for more 
comprehensive human trials to study preventive potential and neuronal mechanisms.
 
PMCID: PMC2850944
PMID: 20047325
 
--------
[6] J Altern Complement Med. 2005 Apr;11(2):305-9.

A double-blinded, placebo-controlled, randomized trial of the neuropsychologic
efficacy of cranberry juice in a sample of cognitively intact older adults: pilot
study findings.

Crews WD Jr(1), Harrison DW, Griffin ML, Addison K, Yount AM, Giovenco MA, Hazell
J.

Author information:
(1)Virginia Polytechnic Institute and State University, Blacksburg, Virginia,
USA. wdcrewsjr@aol.com

OBJECTIVES: The aim of this research was to conduct the first known clinical
trial of the short-term (i.e., 6 weeks) efficacy of cranberry juice on the
neuropsychologic functioning of cognitively intact older adults.
PARTICIPANTS: Fifty (50) community-dwelling, cognitively intact volunteers, > or
= 60 years old, who reported no history of dementia or significant neurocognitive
impairments, participated in this study.
DESIGN: A 6-week, double-blind, placebo-controlled, randomized, parallel-group,
clinical trial was utilized. Participants were randomly assigned to receive
either 32 ounces/day of a beverage containing 27% cranberry juice per volume (n =
25) or placebo (n = 25) for 6 weeks, and administered a series of
neuropsychologic tests at both pretreatment baseline and again after 6 weeks of
either cranberry juice or placebo treatment to assess treatment-related changes.
OUTCOME MEASURES: Efficacy measures consisted of participants' raw scores on the
following standardized neuropsychologic tests: Selective Reminding Test, Wechsler
Memory Scale-III Faces I and Faces II subtests, Trail Making Test (Parts A and
B), Stroop Color and Word Test, and the Wechsler Adult Intelligence Scale- III
Digit Symbol-Coding subtest. A subjective Follow-up Self-report Questionnaire was
also administered to participants at the conclusion of the end-of-treatment phase
assessments.
RESULTS: Two-factor, mixed analyses of variance (ANOVA) revealed no significant
group (cranberry juice and placebo) by trial (pretreatment baseline and
end-of-treatment assessments) interactions across all of the neuropsychologic
tests and measures utilized in this study when a Bonferroni corrected alpha level
was used to correct for multiple comparisons (i.e., .05/17 group by trial
comparisons = .003). Pearson Chi-Square analyses of the groups' self-reported
changes over the 6-week treatment phase in their abilities to remember, thinking
processes, moods, energy levels, and overall health on the Follow-up Self-report
Questionnaire revealed no significant relationships. However, a nonsignificant
trend (X2(1) = 2.373, p = 0.123) was noted for participants' self-reported
overall abilities to remember from pretreatment baseline to the end-of-treatment
assessment. Specifically, more than twice as many participants in the cranberry
group (n = 9, 37.5%) rated their overall abilities to remember by treatment end
as "improved" as compared to placebo controls (n = 4, 17.4%).
CONCLUSIONS: Taken together, no significant interactions were found between the
cranberry and placebo groups and their pretreatment baseline and end-of-treatment
phase (after 6 weeks) standardized neuropsychologic assessments.
A nonsignificant
trend was noted, however, on a subjective, self-report questionnaire where twice
as many participants in the cranberry group rated their overall abilities to
remember by treatment end as "improved" compared to placebo controls.

PMID: 15865497

 
-----------
[7] Br J Nutr. 2010 Mar;103(5):730-4. doi: 10.1017/S0007114509992364. Epub 2009 Dec

23.

Concord grape juice supplementation improves memory function in older adults with
mild cognitive impairment.

Krikorian R(1), Nash TA, Shidler MD, Shukitt-Hale B, Joseph JA.

Author information:
(1)Department of Psychiatry, University of Cincinnati Academic Health Center,
Cincinnati, OH, USA. robert.krikorian@uc.edu

Concord grape juice contains polyphenol compounds, which have antioxidant and
anti-inflammatory properties and influence neuronal signalling. Concord grape
juice supplementation has been shown to reduce inflammation, blood pressure and
vascular pathology in individuals with CVD, and consumption of such
flavonoid-containing foods is associated with a reduced risk for dementia. In
addition, preliminary animal data have indicated improvement in memory and motor
function with grape juice supplementation, suggesting potential for cognitive
benefit in ageing humans. In this initial investigation of neurocognitive
effects, we enrolled twelve older adults with memory decline but not dementia in
a randomised, placebo-controlled, double-blind trial with Concord grape juice
supplementation for 12 weeks. We observed significant improvement in a measure of
verbal learning and non-significant enhancement of verbal and spatial recall.
There was no appreciable effect of the intervention on depressive symptoms and no
effect on weight or waist circumference. A small increase in fasting insulin was
observed for those consuming grape juice. These preliminary findings suggest that
supplementation with Concord grape juice may enhance cognitive function for older
adults with early memory decline and establish a basis for more comprehensive
investigations to evaluate potential benefit and assess mechanisms of action.

PMID: 20028599

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All,

In addition to the problem I discuss above with the study of the impact on happiness of fruit & vegetable (FV) consumption (PMID 23347122), namely the very small positive effect fruit & vegetable (FV) intake had on happiness, there is another, less obvious problem. Here it is.

We all know the rewarding feeling we get when we make choices that we believe will benefit, rather than undermine, our health. It gives us a sense of empowerment, and makes us feel good about ourselves. In short, it makes us happier. So even if the causality link does go from FV intake to happiness (rather than the other way around), the way happiness results from FV intake may be purely psychological, rather than something beneficial in the FVs themselves. In fact, the happiness-boost resulting from the good feeling of simply making healthy choices (by eating more FVs) may have even been exaggerated by the design of this study, which had subjects report both their mood and their FV consumption at the same time using the same online survey, forcing subjects to recount and share whether they ate well or poorly that day. The effect of this exercise on one's mood might even be expected to carry over (at least in a small way) to the following day, which is what the researchers observed. 

 

Don't get me wrong, FVs may (and probably do) have some beneficial physiological effects that either directly or indirectly benefit our mood through non-cognitive means, just as they benefit other aspects of health. But this study in no way teases apart the psychological vs. the physiological effects of FV intake on happiness. That's the major reason why double- (or at least single-) blind experiments are so important. If the person doesn't know whether they are getting a treatment or a placebo, it eliminates the possibility of a psychological cause for an observed effect.

 

Of course blinded studies are very difficult to do with something like fruits and vegetables, or whole foods in general. That's why double-blind nutrition experiments are relatively rare and hard to conduct.  Those that try (e.g. with fruit juice vs. sugar water faux juice) don't always succeed in mimicking the flavor and texture of the real thing with the placebo, making positive results hard to trust.

 

Plus subjects are missing out on some of the important nutrients (at least the beneficial fiber) when researchers feed them juices instead of whole fruit, as the three studies of blueberry (PMID 20047325), cranberry (PMID 15865497) and grape (PMID 23347122) juices did. So negative results (as those three studies found) are hard to put too much stock in either... Although (famously) the cited blueberry study did find an improvement in memory functions, so the negative results regarding mood might carry a little more weight. Although on the flip side, the blueberry juice study they did observe a NS trend (P = 0.08) toward reduced depression in the blueberry juice group relative to placebo. So who knows...

 

In summary, it doesn't look like there is whole lot of compelling evidence that increased FV intake can significantly increase one's happiness. If they do, until/unless larger, credible, randomized double-blind control trials are done, it's not possible to rule out that the benefits are "just" psychologial, although such placebo-like effects resulting from positive thinking shouldn't be dismissed as unreal, as discussed in this thread.

--Dean

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There is a new study out [1] which found a quite similar (i.e. tiny) effect of eating more fruits and veggies (F&V) on happiness as did the study I opened this thread with (PMID 23347122) in February. Not surprisingly, such a "feel good" story is making the rounds, with such exaggerated headlines as It's Been Proven: Eating Fruits and Veggies DOES Boost Your Happiness and Happiness Boosted To Max By This Amount Of Fruit And VegetablesThe story even caught Gordo's attention. Welcome back Gordo - I hope you're happy after your epic road trip!

 

It turns out once again (like in PMID 23347122) you have to eat a butt-load of extra F&Vs to "Boost Happiness to the Max". 

 

The researchers in [1] surveyed 12K Australians repeatedly over several years and correlated their reported F&V consumption with their reported subjective happiness & well-being. The F&V questions on the survey included:

 

“Including tinned, frozen, dried and fresh fruit, on how many days in a usual week do you eat fruit?” and “Including tinned, frozen and fresh vegetables, on how many days in a usual week do you eat vegetables?” with possible responses ranging from 0 (“do not eat any fruit or vegetables in a usual week”) to 7 days per week. For individuals who responded with some positive frequency to these questions, the following was also asked: “On a day when you eat fruit, how many serves of fruit do you usually eat?” and “On a day when you eat vegetables, how many serves of vegetables do you usually eat?” 

 

They also calibrated their understanding of serving sizes by showing them pictures of single servings of different F&Vs. Overall, it sounds like a reasonable enough methodology, as questionnaire studies go... 

 

What they found was that F&V consumption in year N was indeed positively correlated with reported happiness and life satisfaction in the subsequent year (i.e. as reported when the survey was administered again to the same individuals a year later).

 

 Interestingly, to rule out reverse causality, they tried to predict in the other direction, and failed. In other words, you could use a person's reported F&V consumption in year N to (weakly - see below) predict their happiness in year N+1. But you couldn't use a person's happiness in year N to predict their F&V consumption in year N+1.

 

The sad, but perhaps not surprising (given the previous result), thing was how weak the effect was. For each additional portion of F&V a person consumed in year N, a person's reported "happiness" and "life satisfaction" (which were surveyed separately) a year later was 0.02 points higher, on a scale of 1 to 6! Big whoop...

 

So while it was statistically significant across a large population, the association between F&V intake and happiness once again does not appear to be very viable means of boosting happiness.

 

For comparison, here is Table 1 from the full text (pdf) of [1], which compares the magnitude of the effect of various diet and lifestyle changes on reported "life satisfaction", after controlling for the following potential confounders - "household income, age, education, whether working, marital status, health, children, alcohol and food patterns, BMI, and exercise". I've highlighted a few interesting items:

 

7eZUidm.png

 

So it looks like being (or getting, it's not clear from the text exactly which) separated, divorced or widowed had the biggest predictive value on subsequent happiness, with a negative score of -0.33 to -0.58, compared to a positive score of +0.02 for eating an addition serving of F&V. Also larger in influence than F&V were "having a long-term health condition" (-0.14), "eating breakfast regularly" (+0.11), and "exercising regularly" (+0.09).

 

Of course that +0.02 life satisfaction boost from F&V was per serving. In theory you could get a relatively big boost by say, increasing your F&V from 0 servings per day to 8 servings per day (0.02 * 8 = 0.16). But since presumably almost nobody in the 12K people they surveyed actually made such a drastic change, it isn't very legitimate to extrapolate linearly like that.

 

If you could extrapolate like that, it would be possible to eat 30+ servings of F&V per day like some of us do, and be positively giddy with happiness and life satisfaction! Wait a minute... Maybe it's not so unbelievable after all ☺.

 

--Dean

 

---------

[1] Am J Public Health. 2016 Aug;106(8):1504-10. doi: 10.2105/AJPH.2016.303260.

 
Evolution of Well-Being and Happiness After Increases in Consumption of Fruit and
Vegetables.
 
Mujcic R(1), J Oswald A(1).
 
 
OBJECTIVES: To explore whether improvements in psychological well-being occur
after increases in fruit and vegetable consumption.
METHODS: We examined longitudinal food diaries of 12 385 randomly sampled
Australian adults over 2007, 2009, and 2013 in the Household, Income, and Labour 
Dynamics in Australia Survey. We adjusted effects on incident changes in
happiness and life satisfaction for people's changing incomes and personal
circumstances.
RESULTS: Increased fruit and vegetable consumption was predictive of increased
happiness, life satisfaction, and well-being. They were up to 0.24
life-satisfaction points (for an increase of 8 portions a day), which is equal in
size to the psychological gain of moving from unemployment to employment.
Improvements occurred within 24 months.
CONCLUSIONS: People's motivation to eat healthy food is weakened by the fact that
physical health benefits accrue decades later, but well-being improvements from
increased consumption of fruit and vegetables are closer to immediate.
POLICY IMPLICATIONS: Citizens could be shown evidence that "happiness" gains from
healthy eating can occur quickly and many years before enhanced physical health.
 
DOI: 10.2105/AJPH.2016.303260 
PMID: 27400354

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