Por que o exercício físico também não funciona?

Poucas coisas dão tanta fome quanto o exercício. E é natural que seja assim. Afinal, quando perdemos água ficamos com sede, porque não ficaríamos com fome quando “perdemos” calorias? No entanto, a sabedoria convencional é a de que precisamos criar um défcit calórico, ou seja, não apenas deveríamos fazer mais exercício, mas ao mesmo tempo comer menos. Pense um pouco: qual a chance de isso dar certo? Qual a chance, no logo prazo, de que alguém com fome vá ter energia para se exercitar, ou de que alguém que se exercite consiga comer pouco, continuamente, por anos?  Já ouviu falar em homeostase?? O nosso corpo buscará o equilíbrio, com mais fome ou mais “preguiça”.

É evidente que existem inegáveis benefícios à saúde com a prática regular de exercícios. Não é disso que estamos falando. O fato é nunca tantas pessoas fizeram tanto exercício, nunca houve uma proliferação tão grande de academias, nunca tantos aparelhos de exercício foram vendidos, e nunca tivemos tantos obesos. O exercício físico simplesmente não parece resolver o problema, ao menos para a maioria das pessoas. As evidências? Lembra do post sobre o fato de que pessoas que exercem funções manuais pesadas são em média mais obesas? Há uma metanálise finlandesa, do ano 2000, sobre 12 estudos avaliando o exercício como mecanismo de perda de peso. A conclusão? O exercício não teve efeito sequer para previnir ganho de peso, e em alguns estudos acelerou este ganho em relação ao grupo controle.

Um estudo de 2006, da Universidade da Califórnia em Berkeley exemplifica a situação. Foram estudados 13.000 corredores, que foram cuidadosamente acompanhados para comparar as distâncias percorridas por semana e o peso no decorrer dos anos. Os que corriam mais tendiam a pesar menos no início do estudo (correlação, não necessariamente causa). Contudo, TODOS tendiam a ganhar peso a cada ano que se passava, mesmo os que corriam mais de 40 milhas por semana. Os autores, que acreditavam no paradigma do balanço calórico, afirmaram que, para manter o peso, os corredores deveria aumentar em algumas milhas por semana a sua quantidade de corrida a cada ano. Se fôssemos levar às últimas consequências este pensamento, uma mulher de 20 anos que corresse 3 milhas por dia 5 dias por semana, teria de aumentar para 15 (quinze) milhas por dia 5 dias por semana para manter, aos 40 anos, o peso que tinha aos 20. O absurdo de ter de correr uma meia-maratona 5 vezes por semana para manter o peso é mais um motivo para questionar se é realmente a falta de exercício que leva ao acúmulo de gordura com o passar dos anos.

A falha de pensamento neste caso é acreditar que a o gasto calórico induzido pelo exercício não será inconscientemente compensado pelo aumento da ingesta. Seria como supor que, após suar bastante, não haveria um aumento da sede. Além disso, exercício moderado queima uma quantidade irrisória de calorias. É necessário subir 20 lances de escada para queimar as calorias de uma fatia de pão. E o que garante que uma pessoa que decida subir 20 lances de escada todos os dias, não vá sofrer um diminuto aumento de sua fome, equivalente a uma fatia a mais de pão em 24 horas?

Reproduzo abaixo um posto relevante referente ao assunto acima:

Exercise boosts the metabolism? It seems the reverse might be true

I want to preface this post by
saying I am a huge advocate of exercise, preferably outside if weather
conditions allow. I believe there are physical and psychological
benefits to being active, and I even ‘walk the talk’: although I do not
run any more, I am a notorious walker and swim regularly. I also do a
quick resistance-based exercise regime at home (or in a hotel room, say)
most days.

But while I strongly advocate
activity and exercise, I have more than once written about the limited
role that exercise, as is commonly advised, has in weight control. Those
seeking to attain or maintain a healthy weight are often advised to
walk, jog or cycle regularly. The idea here is that burning extra
calories through exercise will assist weight loss, and the theory
certainly seems to make sense. The problem is, when researchers have
studied the impact of regular exercise on weight loss, the results have
been pretty dismal. When added to dietary change over the medium term
(e.g. few months), regular exercise boosts weight loss by about 2 lbs on
average. In other words, if someone were to lose 20 lbs over 4 months
through dietary change, adding regular aerobic exercise to this would,
generally speaking, lead to a loss of 22 lbs. (Remember, though, there
are other benefits to be had from exercise).

The idea that activities such
as running and cycling (aerobic exercise) are not particularly effective
for weight loss is counter-intuitive. Some people imagine that
individuals must be losing fat and gaining muscle. But aerobic activity
will build minimal muscle, if any at all. So, what rational explanations
exist for the observation that aerobic exercise does not translate into
significant weight loss for many?

Well, the first thing is that
exercise does not burn much in the way of calories unless we’re doing it
in very significant quantities. Let’s say you jog for half an hour and
burn about 200 calories more than you would have burned sitting down.
That’s obviously better than nothing, but this is not a ton of calories,
and as there are about 3,500 calories in a pound of fat, theoretically
you’d have to do 17 or 18 of those half-hour runs before you’d lose a
pound of fat from your body. That, for many, would not seem like a
particularly worthwhile return on investment.

But another problem with
exercise is its tendency to stimulate the appetite. And even if it does
not do that, some may ‘reward’ themselves with food or drink (e.g.
alcohol) after exercise. And it doesn’t usually take too many additional
calories to undo the calorie deficit induced by exercise.

I’ve written about these
factors before, and write about them again here as a prelude to writing
about something I learned when I was on the ‘low-carb cruise’ in May.
One of the other speakers on the programme was diet and exercise
researcher Dr Jeff Volek from the Neag School of Education at the
University of Connecticut. Jeff’s presentation on the cruise included
details on how a low-carbohydrate diet can stimulate fat-burning during
exercise (more about that in another post, perhaps). During his
presentation, he remarked that (as we know), exercise is not a very
powerful weight loss tool.

However, he went on to talk about a mechanisms here that came as quite a surprise to the audience, I think: aerobic exercise can suppress the metabolic rate.
We’re often told that exercise not only increases calorie burn while
we’re exercising, and also for some time after. It turns out, that may
well not be the case for many people. In fact, according to research,
the opposite is quite likely to be the case.

Jeff has written a book with his colleague Dr Stephen Phinney called the Art and Science of Low Carbohydrate Performance which contains this passage:

There are 4 well-controlled,
inpatient, metabolic ward studies (the gold standard for human research)
published from 1982 thru 1997 that showed statistically significant
reductions in resting metabolic rate when overweight subjects performed
300-600 Calories per day of endurance exercise for weeks at a time
[1-4]. There are no equally rigorous human studies showing the opposite.
There are animal (rat) studies that show the opposite, and there are
human studies done under less controlled conditions that show the
opposite. However there are also similarly less rigorous studies that
agree with the above four gold-standard studies. When the quality/rigor
of the studies is taken into account, the weight of the evidence
supports two main conclusions:

1. Humans vary
one-from-another in how their metabolism responds to endurance exercise,
and much of this inter-individual variation is inherited (genetic).
Given this wide individual variance, studies involving small numbers of
subjects could get differing results based on random chance.

2. Although genetically lean people as a group may respond differently,
when overweight humans do more than one hour of endurance exercise
daily, resting metabolism on average declines between 5 and 15%.

The fascinating question is,
if our interpretation of this published literature turns out to be
correct, then how come most doctors, dietitians, and sports scientists
think the opposite? Part of the answer is that there is a lot of simple
logic suggesting that exercise speeds resting metabolism. First,
exercise builds muscle, and muscle burns energy even at rest. Second,
there are a lot of skinny athletes out there who think they are skinny
because they train hard (as opposed to being able to train hard because
they are skinny). Third, it is a common observation that heavy people
tend not to exercise much, so it is easy to blame their weight problem
on a lack of exercise. And finally, everyone loves a
‘2-for-the-price-of-one’ sale. It’s just way too tempting to think that
you could burn 600 Calories during a 1-hour run and then, as a result,
burn another 600 Calories over the course of the next day?

They go on to say that:

We are not saying that exercise
isn’t good for people. Both of us are personally committed to leading
vigorous lives, and encouraging others to consider doing the same. What
we object to, however, is mis-informing the public as to what and how
much benefit they can expect from exercise, particularly as it pertains
to weight loss. From our perspective, telling heavy people to exercise
because it speeds resting metabolism (and thus markedly increasing one’s
rate of weight loss) is about as credible as selling them the Brooklyn
Bridge.

The idea that the body would
down-regulate the metabolism in response to exercise makes, I think,
intuitive sense. We know, for example, that when people consciously cut
calories to lose weight, it very often puts a sizeable dent in the
metabolism. This is probably part of a survival mechanism (the body
doesn’t know we’re not going to starve ourselves to death, and will put
into play mechanisms which help the body preserve its weight and fat
stores). It’s not too difficult to imagine that the body would have a
similar response to increased calorie expenditure (through exercise).

None of this should put you
off taking exercise if that’s what you like to do and are physically
able. However, these observations may go some way to explain why all the
effort you may be putting in pounding the streets or exercising on a
treadmill or cross-trainer are not causing the pounds to melt away. My
experience tells me that most bang for the buck for weight loss is had
by getting the diet right. For me, that means a diet based on real food
that it generally higher in fat and lower in carbohydrate than the diet
we are traditionally advised to eat. The scientific rationale for such a
diet is explained in my book Escape the Diet Trap.

References:

1. Bouchard C, et al. The response to exercise with constant energy intake in identical twins. Obes Res 1994, 2(5):400-410.

2. Heymsfield SB, et al. Rate
of weight loss during underfeeding: relation to level of physical
activity. Metabolism 1989, 38(3):215-223.

3. Phinney SD, et al. Effects
of aerobic exercise on energy expenditure and nitrogen balance during
very low calorie dieting. Metabolism 1988, 37(8):758-765.

4. Woo R, et al. Voluntary food intake during prolonged exercise in obese women. Am J Clin Nutr 1982, 36(3):478-484.