How do personal trainers prescribe exercise for weight loss?
We’ve mentioned this before. Put 100 personal trainers in a room, and ask them to agree about a fundamental principle of exercise programming, and we will struggle to reach an agreement. Herding cats will be easier.
But ask about weight loss, and you will probably get agreement on a few principles. Most will agree that reducing your energy intake is part of a weight loss program. Most will agree that increasing your energy expenditure through exercise is also helpful.
Where does weights training come into this?
As part of an exercise program, trainers will also often recommend weights training. Cardio training can be pretty boring for some people, and trainers too. Rather than doing something repetitive like jogging, some of us would rather do something more varied and interesting.
Weights training fits the bill. The exercises change regularly, and the trainer can interact with the client more, providing feedback and guidance. And for some of us it feels a lot more comfortable than going for a jog.
And the reasoning is this: muscle mass requires energy to maintain it. So if we lift weights and increase our muscle mass, our resting metabolic rate (RMR) increases. Thus we burn more energy 24 hours a day, not just in the 30 or 60 minutes we have to do our workout.
But does weights training work for weight loss?
RMR is an important component of energy expenditure, that’s true. But our weights training probably doesn’t have the impact on weight loss that many trainers think it does.
Changes in RMR can absolutely lead to improved weight loss. But the opposite is also true, and our training can be counter-productive. If we train too hard, or severely restrict our energy intake, we can reduce our RMR, potentially having a negative impact on weight loss.
And changes in lean muscle mass don’t have as large of an impact as we seem to think (about 100 kJ per kg, per day). Muscle mass, while more metabolically active than fat mass, is less metabolically active than other lean tissues (such as organs), requiring less energy to maintain it.
Then consider the effect of our weight loss. When losing weight, we are usually in an energy deficit. This makes gaining extra muscle mass more difficult. So most of our clients (who are usually looking for weight loss) would only reasonably expect a small increase in muscle mass with their training.
Now there’s some new research…
In January 2019 some new Australian research was published that adds to what we know about this. The main aim of this research was to assess the accuracy of estimations of RMR in athletes – there are a number of calculations that we can use to predict this, but these are based on data collected from the general public.
These researchers used DEXA scans to determine the body composition of a group of 18 young elite rugby players. Their RMR was also assessed.
They were assessed again just before the start of their season, after 14 weeks of training. Players made no change to their usual training and diets during this period. This training involved weights training, running, and rugby skills.
The players had a significant increase in lean body mass (an average of about 1.5 kg), and a reduction of about 1/2 kg of body fat. But there was no change in RMR.
But this change in RMR is the main reason personal trainers use weights training as part of a weight loss training program. A lot of trainers. Myself included, back when I doing a lot of personal training! But it seems as though the impact of training (in this case a combination of weights and running) on metabolism is pretty small.
But these are rugby players, not my clients!
True, these participants were not the usual personal training client. But they are a convenient group to study for a number of reasons.
Firstly, they have a period where they don’t do a lot of training (their off-season). This is something in research we call a washout period, as it reduces the impact of other types of exercise. If you spend a lot of time starting or stopping different weight loss interventions, and they overlap or follow one another, it’s hard to measure the impact of any one.
Secondly, they train hard, frequently, and consistently. They will be more regular than many of our clients, because performance, selection and match payments may be influenced by their training effort and consistency. And they will train at a higher intensity. So the impact of the exercise program will be more noticeable than for many other populations.
So no, they are not the types of clients you will usually see in the gym, but that doesn’t mean we should ignore the results. That may be a positive in this case.
This research doesn’t change everything – it’s one study, with 18 participants. But it builds on an existing body of research. This research tends to be separated into that which looks at cardio training, or resistance training, something that wasn’t done in the study described above.
The evidence looking at cardio training is clear. While one study showed changes in RMR of up to 600 kJ, others show no change in exercise programs ranging from 3 months to 12 months. Body fat levels decreased, and muscle mass increased, in most of these studies, but the effect was small. And some exercisers compensate for the increase in structured exercise by being less active in the rest of their day!
Resistance training seems to be slightly better when you look at other research. Results have ranged from no change in RMR, to an increase of 1000 kJ per day, though the consensus seems to support changes of around 200-300 kJ (see here, here, and here).
Don’t get too excited about the 1000 kJ, otherwise you may be guilty of cherry picking evidence to suit your argument (we discuss this in a previous post). This seems to be an outlier, as I didn’t find any other studies which supported this number.
And even with this large change in RMR, body fat levels did not change. There’s more to weight loss than simple increasing energy expenditure!
So is weights training for weight loss a misconception?
Most exercise misconception come from a kernel of truth, that has been exaggerated or misinterpreted, and it appears that this is the case when discussing weights training for weight loss, too. While weights training will lead to a small increase in muscle mass for most people, this may not have a large impact in a weight loss program.
There are a couple of more things personal trainers should consider.
An added benefit of prescribing cardio training is the amount of energy they can expend during a session. This can have an impact on weight loss even when RMR doesn’t.
We can measure how vigorous an activity is by looking at the METs (or metabolic equivalents) expended. Sitting quietly will expend about 1 MET. Vigorous weights training gets us up to about 6 METs, while a steady jog gets us to 9 METs, and there are plenty of other options. So there’s a clear benefit from well programmed cardio training, when it is appropriate.
But it’s also pretty clear that we need to be providing nutrition advice, within our scope of practice. This is something not all trainers stick to, as discussed previously. If our clients are overweight, and have no medical complications, then fairly basic advice will do the trick. After all, if they were eating healthy foods in appropriate quantities, they probably wouldn’t be asking for our advice!
The take home message?
Exercise in general, and weights training has plenty of great benefits, but let’s not oversell it. For an effective weight loss approach, we need to take a more holistic view. Look at nutritional advice, and where appropriate, personal trainers should refer to a more qualified professional for extra guidance.