NU_nutrition_TS

Obese people expend more energy than lean people

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on 25-07-2010 at 09:13 AM (481 Views)

The assumption that people become obese because they consume more energy than the expend is often tied to another myth: that obese people have slower metabolisms than lean people. However, studies have shown that, for the most part, this is not true.

Obese people generally have faster metabolisms (or at least expend more total energy than their lean counterparts). This is actually quite logical, because even walking (if you are carrying excess body fat) becomes a more energy intensive exercise. Also the resting metabolic rate is often faster. Again, if you think about it, having to do everyday activities with an excess weight load is likely to lead to increased hypertrophy of the working muscles over time, so the more muscle mass the faster the metabolism. So the fact that overweight and obese people eat more food than their lean counterparts is really no big surprise.

Is part of the reason for them over-eating (and over-eating on the wrong foods) the fact that they require more energy to do the everyday physical tasks that lean people can manage with less energy expenditure AND that the energy that could be provided by body fat is locked away, in their case, and so has to be provided by immediate and frequent food intake?

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jtwigg - 26-07-2010 10:17 PM

Why would the energy required to do everyday tasks be locked away?
NU_nutrition_TS - 26-07-2010 11:24 PM

Quote

Originally Posted by jtwigg

Why would the energy required to do everyday tasks be locked away?

Overweight and obese people are not breaking down their fat stores as effectively and liberating the free fatty acids that would normally furnish the bulk of the energy required for everyday metabolism (everyday tasks, for the most part, are not high intensity so most of the fuel would ideally be provided by fatty acids). Because of this they tend to use more glucose and serum glucose levels fluctuate more and cause them to snack more frequently. Also, when glucose oxidation takes precedent, the enzymes and other factors required for optimum entry of fatty acids into the mitochondria are down-regulated so even if they can release fatty acids from adipose tissue they cannot always be transported into the mitochondria to be oxidised and may end up being re-esterfied into triglycerides and stored back into adipose tissue.
jtwigg - 27-07-2010 10:49 AM

"Overweight and obese people are not breaking down their fat stores as effectively and liberating the free fatty acids that would normally furnish the bulk of the energy required for everyday metabolism"

Any references for this or is it just deduction? The fact that they have a higher metabolism when obese may indicate that obesity tends to be a vicious self-propagating cycle, given the behavioural effects likely to ensue (eating!) and the poorer hormone sensitivity etc, but does not explain why such people became obese initially. Also, if you adjusted for LBM and the bodyweight they have to carry around, I wonder if they would still have a faster metabolism lb for lb of lean mass ?
NU_nutrition_TS - 27-07-2010 04:40 PM

It is a deduction based on the gist of research I have been reading for several years. Clearly it is a reasonable deduction for one or both of two things are clearly happening for people to continue to get fatter - they are not oxidising enough of their stored fat and/or they depositing more than they oxidise. I don't have any specific references to hand but I can track some down and add them later. Here is one to be going on with:
Low ratio of fat to carbohydrate oxidation as pred... [Am J Physiol. 1990] - PubMed result
Basically the RQ indicates the ratio of fat to carbohydrate oxidation, the closer to 1 the greater the reliance on CHO oxidation. Fat oxidation tends to give an RQ of around 0.7.
Here's another:
Twenty-four-hour respiratory quotient: the role of... [J Clin Endocrinol Metab. 1998] - PubMed result
They conclude here that 'preceding diet does not fully explain variance of RQ', which I take to mean, aside from familial/genetic traits, etc., it does partially explain it. They go on to say that dietary preferences may encompass the 'familial similarities'.

You are correct that when corrected for FFM, lean and obese people tend to have similar total daily energy expenditures. However this feeds into my point that obese people may not simply be eating more than they expend out of greed, laziness or a simple mental inability to correctly reckon on how much food to eat. Nor are they cursed with a slow metabolism.

The self-reported food intakes are often less than expenditure as determined by a number of techniques, including doubly-labelled water, in a number obese people but that does not really quantify if they are actually eating significantly more than they expend - that is merely assumed in most studies because they are simply over-fat!

If Taubes is right and there is a hormonal glitch that causes fat to be preferentially stored rather than released and oxidised, then they may simply be eating enough (usually high CHO) to feed this process plus other other energy requirements not covered by fatty acids. It's just a matter of looking at cause and effect as possibly working in either direction.

Updated 27-07-2010 at 10:01 PM by NU_nutrition_TS