[Sam Brown (OP)]

Peep has asked:

"How much more methane do humans produce when they change from a meat to a vegetarian diet? Does this negate the methane produced by cattle?"

What do you think?

[chris]

It's widely claimed that vegetarians fart more; indeed, Giles Yeo endorsed this on the programmed back in March when he explained that switching to a vegan diet for the purposes of a TV programme made him "more windy"; however, it's also claimed that this effect tends to be an acute response to a dietary switch and that, after a brief "settling down period", the gut adapts to its new calorie source and the excess flatulence resolves.

This probably reflects a shift in the intestinal microbiome. The bacterial populations of the gut are a product of what we feed them, so if we radically alter our diet, the bugs are being force fed on foodstuffs that they are ill-adapted to handle and the results can be unpleasant for a while. But if the dietary change persists, the population of bacteria re-configures to accommodate the new sources of calories and the different composition of the supplied foodstuffs (more fibre etc) and the volumes of gaseous products drops.

Thus, I think that a human going vegetarian is never going to counterbalance the methane cost of a cow.

[Colin2B]

Thus, I think that a human going vegetarian is never going to counterbalance the methane cost of a cow.

Right on!
Two reasons. First, the cow burps out most of it’s gas which is mainly methane and part of the ruminant process of breaking down the grass/fodder. Second, very little of human gut gas is methane, biggest constituent is hydrogen.
Certainly there are a lot of foods that that vegetarians eat more* of eg legumes, brassicas, etc that produce more gas.

*our experience of our children’s vegetarian school friends who came to tea was that they didn’t eat vegetables either. Offers of broccoli, carrot, peas etc were refused with “I only eat chips and baked beans”. Rarely lasted into adulthood.

[alancalverd]

Children's diet seems to have no effect on their health and fitness. Two neigboring lads who grew up with my kids, only ever ate jam sandwiches on white bread until the age of about 15. A few years later, at 6 ft and 15 stone, they both played rugby for Scottish Universities.

Nearly all the vegetable matter consumed by large ruminants, goes into maintaining their body temperature to keep the digestive system working: very little is converted into meat.

Humans do not use anerobic fermentation to predigest their food, which is why we can't eat grass and don't fart much methane

[Bored chemist]

Nearly all the vegetable matter consumed by large ruminants, goes into maintaining their body temperature to keep the digestive system working: very little is converted into meat.

That doesn't make much sense
They are bigger than us  and have comparable body temperatures, so they should find it easier to maintain their core temperature
https://en.wikipedia.org/wiki/Allometry

Also, the "fermentation" processes that take place in the gut release heat.
(Large breweries need serious cooling systems)

Children's diet seems to have no effect on their health and fitness.

No
https://en.wikipedia.org/wiki/Dutch_famine_of_1944%E2%80%9345#Legacy

Two neigboring lads

You should know better than to equate anecdotes with proper studies.

Humans do not use anerobic fermentation to predigest their food,

In a very real sense, we do.
Cows use bacteria to do it.
We use cows.

[evan_au]

Humans do not use anerobic fermentation to predigest their food

We don't have multiple stomachs like a cow.

But much of our digestive system is anaerobic, and populated by bacteria which have a severe allergy to oxygen. This makes it hard to grow them in the lab or in a probiotic factory.

Some of our microbiome are yeasts, which do ferment parts of our food - they cope with fruits and cooked vegetables, but they don't cope with the very tough cell walls of grass.

It's a bit mysterious how these helpful bacteria can transmit from one person to another, but some of them form resistant spores which can survive the journey from one person to another.

See: https://en.wikipedia.org/wiki/Gut_flora#Classifications

[alancalverd]

« Reply #4 on: Today at 00:13:09 »                  
                              
                           
                              

Quote from: alancalverd on Yesterday at 23:13:38Nearly all the vegetable matter consumed by large ruminants, goes into maintaining their body temperature to keep the digestive system working: very little is converted into meat.

That doesn't make much senseThey are bigger than us  and have comparable body temperatures, so they should find it easier to maintain their core temperature

Most of the energy consumed by mammals is used to maintain body temperature or other "background" functions such as circulation. Adult humans consume around 10 MJ per day 90% of which is "maintenance". Cattle actually work at a somewhat higher body temperature and beef cattle convert less than 5% of their input into meat - the rest ultimately goes into forming combustion and fermentation products (CO2, H2O, CH4).

So on a simple energy balance, of two adult humans the vegetarian is responsible for less than 10% of the CO2 and CH4 attributable to the carnivore. 

[chris]

It's a bit mysterious how these helpful bacteria can transmit from one person to another

Most of them are acquired in the few minutes of life when a baby gets a taste of the outside world - quite literally - as it is being born. As some people like to say, a person born the normal way's first taste of life is a mouthful of muck - their mum's muck; tasty it might not be, but it's probably the most important meal they'll ever eat...

[Bored chemist]

So on a simple energy balance, of two adult humans the vegetarian is responsible for less than 10% of the CO2 and CH4 attributable to the carnivore.

If that was true then the vegetarian's breathing rate would only be 10% of the carnivore's.

Trying to include the energy used in producing food is much more complicated. A vegetarian who eats food grown on the other side of the planet and delivered by plane may well be responsible for more CO2 than an omnivore who keeps their own chickens to add eggs + occasional meat to locally grown crops.

Most of the energy consumed by mammals is used to maintain body temperature or other "background" functions such as circulation.

Yes, whether they are vegetarians or not.

If I'm maintaining my heart, lungs , kidneys + liver etc at 37C then I maintain the gut at 37C  without significant extra effort (exactly zero extra if my food is at 37C when I swallow it).

In fact, as I pointed out, to keep the gut at 37C, I have to remove heat produced by fermentation (and I'm using that word in the original, rather loose sense rather than restricting it to yeast )