Knowledge for better food systems

FCRN long summary: Energy use, GHG and blue water impacts of scenarios where US diet aligns with new USDA dietary recommendations

This paper quantifies what the environmental impacts would be if the typical US diet were to shift in line with the USDA dietary recommendations. The paper has created a lot of interest and debate since it shows that shifting towards healthier diets in some cases can increase the energy, emission and water intensity of the diet. This is why we wanted to provide a more extensive summary and some commentary below. Please do read, share and add your own comments.

For a comment by the study's lead author Michelle Tom see here and for another comment by Professor Michael Hamm see here

The paper calculates a. energy use, b. greenhouse gas emissions and c. blue (irrigation) water use arising from typical US food consumption patterns.  It then looks at what would happen to these food related environmental impacts if diets were to change in one of three ways:

  • Scenario one: overall calorie intakes decline in accordance with USDA guidelines but the typical US food mix remains the same
  • Scenario two:  current food mix switches to conform to US dietary guidelines, but overall calorie intakes remain as they are (i.e. higher than the USDA’s recommendations)
  • Scenario three:  both the food mix and calorie intakes conform to USDA guidelines. I.e. this is the USDA’s ‘ideal’ diet.

The paper begins by assessing both average calorie requirements (for adults – children and teenagers are excluded from the analysis), and current average calorie intakes.  It estimates that people in the US consume on average 200 calories more than is recommended.  The researchers also estimate and take into account the amount of food that is wasted at the retail and consumer stages of the supply chain – this is important to measure the total environmental impacts of food supplied to US households, not just the fraction that is eaten.  The percentage wasted varies from food type to food type.  The authors then draw upon published data on blue water, energy use and GHGs associated with the production of foods in industrialised countries to make their calculations.

To conform to US dietary guidelines the following changes need to happen to typical US diets:

  • Meat intakes decline
  • Fruit consumption increases
  • Fat and oil intakes decline
  • Sugary food consumption decreases
  • Vegetable consumption increases
  • Dairy consumption increases (to our knowledge the US guidelines have the highest dairy recommendations in the world)
  • Fish consumption increases

 

The figure copied shows the amount, in terms of calories, that intakes of all these foods need to change by in order to conform to the scenarios described above.  As is shown, significant reductions in calorie intakes from sugars, oils and fats, and to a lesser extent meat, poultry and eggs, are compensated for by increases in fruit, veg and especially dairy consumption although the extent of compensation varies somewhat by scenario.

The study finds that, compared with current typical US diets:

  • Shifting to  Scenario 1 (reduction in overall calories) decreases energy use, blue water footprint, and GHG emissions by around 9 %. 
  • A shift to Scenario 2 (healthier food mix, but high calorie intakes) increases energy use by 43 %, blue water footprint by 16 %, and GHG emissions by 11 %.
  • Shifting to dietary Scenario 3 (recommended food mix and calorie intakes), increases energy use by 38 %, blue water footprint by 10 %, and GHG emissions by 6 %.

Why is this? There are a number of reasons, which we summarise below.

As regards GHG emissions, as noted, calorie intakes from sugars and oils/fats see the largest decline.  These foods may be unhealthy but they are also relatively GHG-efficient - which is why sugars and oilcrops are grown as biofuels.  So even large reductions do not lead to substantial GHG (or energy or water) reductions. 

Intakes of dairy products increase substantially in line with US recommendations – and since they are ruminant animal products they carry high environmental impacts.  Intakes of fruits and to a lesser extent vegetables also need to increase substantially to conform to US dietary guidelines.  These foods are not calorie dense.  So for a given quantity of calories, a significant volume of these foods needs to be eaten to compensate (even partially) for the reduction in sugar and oil and to a lesser extent meat consumption (e.g. 100g of apple has 40 calories, 100g sugar around 350 calories, butter around 800 calories, and 100g of lean beef has 175 calories – fatty beef has much more).  Although calorie intakes in scenario 3 are less than the US dietary baseline, there is still a need to compensate for the significant decline in sugars and oils.

Fish consumption also rises and salmon is taken as representative here.  Salmon is relatively energy intensive to farm, and energy translates into GHG emissions.  Increased energy use in the healthier scenarios is driven by increases in fruit and to a lesser extent fish consumption- as noted, fruits have low calories but high energy costs per calorie. Fish are often farmed or captured in energy intensive ways.

When it comes to water use, the main reason for the increase in blue water footprint is the increase in fruit and vegetable consumption.  The authors note the dependence of the US fruit and vegetable supply on the drought stricken Californian region and point out that the situation may be different in different parts of the world.  (NB – Europe is in a similar situation, as it sources produce from water stressed regions such as southern Spain and North Africa.  This UK focused paper in fact comes to very similar conclusions about the impact of healthier diet scenarios on blue water use).


It is important to note that the paper does not say that healthy diets are unsustainable, or that to address food related impacts diets need to be unhealthy.  What it does do is highlight the fact that the current US recommendations do not represent an alignment of health and environmental goals.  

The guidelines recommend about 700ml milk/dairy products a day – higher than anywhere else in the world.  They also recommend about 100g of meat, poultry or eggs / day – current average US intakes are only a little higher than that at 121g /day (the red meat component declines from 71 to 51 g day).   USDA Recommended intakes of grains (relatively low in GHGS, blue water and energy) are also lower than that recommended in some European countries, and that of fruits higher.  

As the paper points out, research into the same issues, but based on the dietary guidelines of other countries show synergies between health and environmental goals – although depending upon region, blue water use can be a concern.

It is also important to emphasise that the health-environment trade off arises not so much because the US guidelines recommend lower animal product consumption – because while meat intakes decline dairy intakes increase – but because fruits, vegetables, dairy (and to a lesser extent fish) replace low carbon, but less healthy sugars and oils.  This French paper models similar changes and draws similar conclusions.  It would be interesting, for example, to see what would happen to emissions, blue water and energy use if the reduction in oils and fats were compensated for by increases in animal products instead (ie. along the lines of the much hyped Paleo diet).

When thinking about fruits and vegetables, not all produce carrries equally high impacts. There can be significant differences (which neither the paper nor the guidelines themselves discuss) in the GHG and water intensity of different fruits and vegetables. 

Finally – it is important to note that US production practices could be different – for example this blog – in a whole series of interesting blogs  by FCRN member Mike Hamm - discusses the need to re-localise the US food system, in part to avoid the need to ship embedded water about.

Abstract

This article measures the changes in energy use, blue water footprint, and greenhouse gas (GHG) emissions associated with shifting from current US food consumption patterns to three dietary scenarios, which are based, in part, on the 2010 USDA Dietary Guidelines (US Department of Agriculture and US Department of Health and Human Services in Dietary Guidelines for Americans, 2010, 7th edn, US Government Printing Office, Washington, 2010). Amidst the current overweight and obesity epidemic in the USA, the Dietary Guidelines provide food and beverage recommendations that are intended to help individuals achieve and maintain healthy weight. The three dietary scenarios we examine include (1) reducing Caloric intake levels to achieve “normal” weight without shifting food mix, (2) switching current food mix to USDA recommended food patterns, without reducing Caloric intake, and (3) reducing Caloric intake levels and shifting current food mix to USDA recommended food patterns, which support healthy weight. This study finds that shifting from the current US diet to dietary Scenario 1 decreases energy use, blue water footprint, and GHG emissions by around 9 %, while shifting to dietary Scenario 2 increases energy use by 43 %, blue water footprint by 16 %, and GHG emissions by 11 %. Shifting to dietary Scenario 3, which accounts for both reduced Caloric intake and a shift to the USDA recommended food mix, increases energy use by 38 %, blue water footprint by 10 %, and GHG emissions by 6 %. These perhaps counterintuitive results are primarily due to USDA recommendations for greater Caloric intake of fruits, vegetables, dairy, and fish/seafood, which have relatively high resource use and emissions per Calorie.

Citation

Tom, M. S., Fischbeck, P. S., Hendrickson, C. T., (2015). Energy use, blue water footprint, and greenhouse gas emissions for current food consumption patterns and dietary recommendations in the US, Environment Systems and Decisions, DOI: 10.1007/s10669-015-9577-y

Read the full paper here.

We are happy to also present a blog-post from one of the authors on this paper.  We have also had a commentary on this in a blog-post by Prof. Michael Hamm

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Comments

Miguel Astudillo's picture
Submitted by Miguel Astudillo (not verified) on

Yesterday I had a look to the article as the headline “Lettuce is ‘three times worse than bacon' for emissions and vegetarian diets could be bad for environment” didn’t appear to me as very accurate.

I haven’t seen the reference to bacon in the study but I guess is because they compare it in a caloric base, which makes little sense as -unless you wan't to hurt yourself- diets don’t meet caloric needs with lettuce.

I found the article a bit misleading, especially when they say: “In addition, this study’s results demonstrate how the environmental benefits of reduced meat consumption may be offset by increased consumption of other relatively high impact foods, thereby challenging the notion that reducing meat consumption automatically reduces the environmental footprints of one’s diet.”

I don’t think it’s fair to conclude that and it could easily lead to wrong interpretations. As previously pointed out the big changes in the studied diets are the reductions of fats and sugars, not meat. Besides, according to their own results “meat” products have the highest emissions per calorie and most of the increase in impact comes from… dairy products!

"Although meat products have the highest emissions per Calorie, overall GHG emissions increase due to increased Caloric intake of dairy, seafood, fruits, and vegetables, which collectively offset emission reductions resulting from decreased meat consumption as well as reduced sugars, fats, and oils, which again have relatively low emissions per Calorie. Dairy, by far, has the greatest impact on increased GHG emissions"

John Kazer's picture
Submitted by John Kazer (not verified) on

I agree that it is a bit strange to compare lettuce and pork (nee bacon) on a calorie basis.

In addition, the report concludes that the current US diet has a lower carbon footprint than the diet implied by the current (not updated) US dietary guidelines.  These guidelines apparently provide recommended amounts of total energy provision per food group - so the report authors have scaled up/down the GHGs to meet these proportions accordingly.

Reading through some of the supplementary material, the implications of this energy-intake based switch between current and recommended diet (see Table 3 of supplementary info) are:

A doubling of fruit, vegetable and dairy consumption
A ~30% reduction in meat consumption
No real change in grains consumption

My first thought is that the large increase in energy intake from fruit, veg and dairy rather than grains is the crucial factor here.  Food for thought for the US dietary guidelines perhaps?  I wonder where the current discussions on the 5 year update will go now that they’ve decided not to include sustainability as a consideration…

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