Feb 23 2018

More on Weight Loss

Whenever I post about the topic of diet or weight management, there is always a lively discussion in the comments. There are also always many comments that are based on ideology, rather than evidence, and the level of emotion is significantly higher than for many similar topics.

I think this is because the topics of food, eating, and weight management seem very personal. Also, everyone has a lifetime of experience with eating, and so everyone feels like their own anecdotal experience makes them an expert (even if they intellectually know they aren’t). The topic is also a setup for confirmation bias – we all have many friends, co-workers, and family with their own dieting experience, and we can even look around and see what strangers are doing and how they look. It is therefore really easy to see the evidence for whatever your opinions are, to have the illusion that your beliefs are confirmed.

To add further fuel to the confirmation bias fire, there are a host of gurus and fake experts out there, and even genuine experts but who only see a slice of the science. There are enough expert and pseudoexpert opinions out there to confirm whatever position we want to take.

For all these reasons dieting is one of the more challenging issues for the average person to find objective reliable information. That is all I am trying to do here, adding my perspective as an advocate for science-based medicine. Interpreting scientific medical data is complicated, and there are lots of opportunities to be mislead. The weight-management literature is particularly complex and heterogeneous, so again, you can support pretty much any position unless you take a really careful look at the literature.

So to follow up my post from yesterday, which prompted a flood of different opinions, let’s take a further look at the evidence.

Why Basic Science Evidence is Unreliable for Clinical Claims

I frequently recommend that in order to resolve differences of opinion on factual matters it is best to establish common ground and solid premises from which to proceed. One of the frequent premises that results in differences in interpreting the medical literature is the relationship among the various types of scientific evidence.

One clear principle, that I have stated many times both here and at SBM, is that we cannot reliably infer clinical outcomes from basic science alone. In other words, we cannot study what happens in biochemistry, physiology, and to cells in a petri dish and then reliably make clinical claims about what will happen in people. The history of medicine is clear – we just can’t.

The primary reason for this is that the body is a complex web of interacting mechanisms. Predicting what happens when we pull on one thread of this web is like predicting the weather – with any complexity it rapidly become impossible.

Sometimes our predictions work out, but often they don’t because there is a compensatory mechanism in play. Our interventions may even have the opposite effect of what we want. Scientists predicted that alpha interferon would reduce inflammation in Multiple Sclerosis. It increased it and made the disease worse. Beta interferon, however, worked. Antioxidants were supposed to cure everything, but ended up curing nothing, because of all the homeostatic mechanisms in place.

Even when the direction of the effect is successfully predicted, we may not be able to estimate the magnitude of the effect. This is often the case, where the clinical claims are correct, but the effect is simply insignificant.

It is just not the role of basic science to determine clinical claims. Basic science helps us make hypotheses, to find new mechanisms, to know how plausible claims are, and to understand why things work the way they do. But clinical claims require clinical evidence – we always need to see what the net effect is of specific interventions.

Different Types of Clinical Evidence

There are also different types of clinical evidence and often these different types of evidence give different answers. There are observational studies, which have the advantage of looking at many people over long periods of time, but are not controlled. There are experimental studies, which have the advantage of controlling variables, but the trade off is that they are limited in size and often contrived and therefore hard to generalize to the “real world.”

Neither kind of evidence is perfect or gives us the final answer, but neither are they worthless. They can be used in a complementary way to infer the most likely answer.

What Strategies Work for Long Term Weight Loss?

With that as background, let’s take a look at what the clinical evidence shows in terms of what strategies are likely to be best for maintaining a healthy weight long term. We are also interested in being overall healthy long term, but if we go beyond weight management we will rapidly get way beyond what I can cover in one blog post. So I will only make general comments about overall health effects.

For the reasons I stated above, I am going to focus on clinical evidence in humans. The basic science informs the clinical evidence, but does not trump it. Also, animals studies are very difficult to extrapolate to people with things like diet. So I will stick to human clinical trials.

I also want to reinforce the question above – the goal is long term weight management, not short term weight loss only. One big question is – is calorie control more important, or is exercise more important? This is an endless debate, but I think the best current answer is – both.

Calorie restriction does result in weight loss, but also can reduce metabolism, reduce overall activity levels, increases hunger, and even cause depressed mood. You can blame evolution for this – it seems that our bodies interpret calorie restriction as famine, and responds by conserving calories and increasing hunger. When food is available, we want to pack on the fat stores to prepare for the next famine. This adapted us to pre-industrial life, but not our current stable abundance.

Exercise is a way to counter some of these effects. It can maintain metabolism, activity, help with mood, and also have other health effects like maintaining muscle mass. But – exercise also has a down side. Exercise alone is not a successful strategy for most people, and also can result in increased hunger, and compensatory decrease in baseline activity.

The best strategy appear to be both – moderate calorie restriction and moderate regular exercise maintains a caloric deficit (or balance if you are just trying to maintain weight) and mitigates the negative effects of each.

But again there are lots of ways to ask the same question. One study looked historically at physical activity and total calorie consumption by society to see the relative contribution of each to increasing obesity. They found that the dominant factor was an overall increase in caloric intake (not a decrease in exercise). In fact, the increase in calorie consumption was enough by itself to explain rising obesity levels.

The Characteristics of Successful Dieters

Another approach to the question is to look at people who lost weight and kept it off for a significant amount of time, and see what features they have in common. There are many studies looking at just this, and here is a recent (2016) review of what those studies have found:

However, many people report that they have successfully managed weight loss maintenance in the long term. Several factors have been associated with better weight loss maintenance in long-term observational and randomized studies. A few pertain to the behavioral area (eg, high levels of physical activity, eating a low-calorie, low-fat diet; frequent self-monitoring of weight), a few to the cognitive component (eg, reduced disinhibition, satisfaction with results achieved, confidence in being able to lose weight without professional help), and a few to personality traits (eg, low novelty seeking) and patient–therapist interaction.

The key factors that predict long term success are monitoring your caloric intake and measuring the net effect by weighing yourself regularly (once a week). People tend to underestimate their caloric intake, so tracking it helps. Also, the scale does not lie, so monitoring the effect is key.

But also, regular exercise predicts success. This fits the data I reviewed above about the effects of dieting alone.

Psychologically, support is very helpful. Shaming and negative emotions are detrimental. If people see the benefits of changing their lifestyle, they are more likely to keep it up. Confidence that they can lose weight, also helps. Using a buddy system is also of value – make the people around you your partners in your attempts at weight loss and maintenance.

Interestingly, while there are various results, there appears to be a consistent signal that a low fat diet is a better predictor of long term success than a low carb diet. These are uncontrolled studies, however, so it is hard to know how to interpret this. What is clear, however, is that low carb diets were not a predictor of long term success.

Low Carb vs Low Fat in Experimental Studies

This is where there is the most controversy – experimental studies where low carb vs low fat are compared head-to-head. Overall the results are very heterogeneous, and there are always lots of details to criticize if you don’t like the outcome. That was not low carb enough, or they had the wrong carbs, or they were or weren’t calorie restricted, etc. There are lots of study design choices that potentially affect the outcome, and the applicability to the real world is always doubtful.

But – having said all that, we can summarize the overall trends in these studies. I always go first to the most recent systematic reviews. This review concludes:

Both types of macronutrient-centered weight loss diets produced weight loss. Manipulation of macronutrient composition of weight loss diets does not appear to be associated with significantly different weight loss or metabolic outcomes.

However, other reviews conclude that low carb diets work better than low fat, so why the difference in reviews? Well, it depends on what features of the studies you think are important.

What the studies show overall is there, in the short term (usually six months) the low carb arm of the study loses more weight than the low fat arm. If you focus on this fact, you might conclude that low carb is better. It further seems that low carb produces less hunger than low fact, and that is often offered as the explanation for the benefit.

However, we can’t stop there. Longer terms studies, one or more years, give mixed results with the overall best conclusion that there is no real difference between macronutrient ratios. Further, overall weight loss is always modest. The charts also tell an interesting story in the long term studies, with all of the weight loss occurring in the first six months (where low carb does have a small advantage) but then subjects tend to gain weight and plateau, with either a small or no difference in ultimate outcome at 1-2 years.

How Do We Put This All Together?

That is the real question – how do we make the best recommendations from all the various kinds of evidence available. I think this study gets it right – they compared low carb to low fat diets for two years, but also with behavior intervention. Both groups lost equal amounts of weight at one and two years.

What I think the evidence shows overall is that, for long term weight loss and management, what matters is the behavioral and psychological aspects of eating and exercise, not the portion of macronutrients. I think the studies that find commonalities among successful weight losers is probably the best evidence we have.

Having said that, it also seems that there are many individual factors involved, mostly with personality, behavior, resources, and medical condition. So everyone has to take these general principles, and apply them to their individual lives.

And again, as I said, there are a host of other health issues dealing with diet, weight is not the only one. But there I think the consensus of evidence is that you just need to eat a variety of food with plenty of plants. Eat your vegetables. Don’t overdo anything.

I think obsessing with carbs vs fats is the wrong approach. The evidence simply shows that focusing on this factor does not help with long term success. At best there is a short term advantage, but that may actually be counterproductive, because it convinces a lot of people that low carb is the answer.

In any case, that is my reading of the evidence. I am open to other evidence and perspectives, but I would ask that you spare me your anecdotal evidence, spare me your personal attacks or conspiracy theories, don’t bother with the “more skeptical than thou” posturing, and just discuss the science. Please.


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