Aidan:
Welcome to episode 226 of the Ideal Nutrition podcast. My name is Aidan Muir, and I’m here with my co-host Torwen Eerkins. And we are going to talk about whether it is possible to boost metabolism. We will try to do this in the simplest way possible. One of the things we’re going to do as a starting point is define what we mean by metabolism.
Torwen:
I think that’s a great place to start. So, metabolism is the process in which your body converts what you eat and drink to energy. And if we look at that from a bigger picture, we know your body burns calories all the time, even at rest. That is your resting metabolic rate. Now this encompasses vital functions like breathing and circulation, and other small resting movements as well.
But, this is not a set number, and it can change, which we will get to. If we look at the total amount of energy you expend in the day, your total daily energy expenditure, this comes from your resting metabolic rate, an activity factor, non-exercise activity thermogenesis, and the thermic effect of food.
Aidan:
And that means we will be talking about a bit of both because it’s broad. If you just say metabolism, we could leave it at resting metabolic rate. I think when people say metabolism, they often mean a combination of that and total daily energy expenditure. So, we will cover a bit of both. And where we will start in terms of boosting metabolism is our thoughts on total calorie intake, both short and long term. I think we’ll start with short-term stuff.
Clearly, short-term calorie intake can impact things in either direction. The short version is if you eat more calories, it probably increases your energy expenditure. It probably increases your metabolism. But, if you decrease your calories, it probably decreases it a little bit.
Torwen:
And this is what we call metabolic adaptation as well. Potentially, we look at the physiological response of the body adjusting to reducing calories or eating more. It adapts compared to what you are giving it.
Aidan:
If we break down those variables you touched on with resting metabolic rate, that’s where metabolic adaptation comes into play. If the body has more calories available, it is more able or willing to spend calories on less important factors. The lower your calorie intake gets, the more it conserves energy. The common cliché is battery saver mode. There are heaps of things that downregulate. The body’s core temperature might drop a little.
Some hormones won’t work as well. A common one with women is they might lose their period. Fertility is no longer a priority if you don’t have enough calories to sustain yourself. There is heaps going on there. Going to incidental movement, people will often move less. Some of that is less willingness to move due to lower energy. Other stuff is incidental, like fidgeting less. People’s step count, if not actively tracked, will naturally decrease in many cases.
With exercise, people might burn fewer calories through exercise. That can be partly due to doing less exercise with fewer calories coming in. The thermic effect of food is the easiest one. We burn calories by eating and digesting food. If we eat more, we burn more calories through that process. If we eat less, we burn less through that process.
Torwen:
So, do we want to run through some studies that explain metabolic adaptation in longer-term situations? The biggest one people reference is the Biggest Loser study. It looked at participants on the show who lost a significant amount of weight, and their resting metabolic rate dropped significantly. Six years later, they regained a lot of the weight. Their resting metabolic rate remained suppressed, about 500 calories per day lower than predicted.
Aidan:
I’ve spoken about this heaps on the podcast, and it stands out. It was formative for me early in my career as a dietitian. People talked about yo-yo dieting being very detrimental to long-term metabolism. They argued we would need specific strategies to offset that. More research has come out on this topic.
It seems that study is a huge outlier. There are many reasons it could be an outlier. One is that it is very extreme. The protocol was extreme. What makes me think it’s less about the extreme protocol, outside the exercise component, is bariatric surgery. I don’t know if you’ve seen much on bariatric surgery and metabolism.
Torwen:
Not a whole lot, actually.
Aidan:
I had questions on this for a long time. I’m sure you’ve experienced this as well. Anecdotally, many clients report very low calories and no weight loss. I’m sure you’ve seen heaps. I see that more in the bariatric surgery space than anywhere else. It made me wonder what the research shows in people who’ve had bariatric surgery. I’ve put some studies in the show notes on this. The finding is there isn’t a difference versus what you would expect for metabolic rate using a prediction.
There is a study where six months after surgery there was a small increase in resting metabolic rate versus expected. That is after adjusting for body weight. It’s not an increase from where they started. They had lost heaps of weight. It’s an increase versus what a formula predicted, which is odd. They lost so much weight and were probably still losing weight. I won’t go too deep into that. There’s enough to say even with large weight losses, we aren’t seeing heaps of this. That brings us back to the Biggest Loser study. It has been re-interpreted.
One of the biggest points, from Kevin Hall, was the ongoing high exercise. Many participants still do heaps of exercise. If we do heaps of exercise, the body can downregulate in other areas. That includes resting metabolic rate. That was one explanation. Their resting metabolic rate was reduced by 500 calories. I don’t know if exercise alone accounts for that much of a drop. We have a combination of variables. It’s an extreme study, and they still do a lot of exercise. If we have one study showing one thing, and many other studies showing another, I lean to the others.
Torwen:
And like you mentioned, it is extreme. They entered an extreme calorie deficit for a long time. They increased exercise to an extreme as well. So the results may not reflect a modest deficit over a longer period. Another study challenges this. It is titled “Metabolic adaptation is an illusion.” It shows metabolic adaptation depends on energy balance. It reduces as soon as weight stabilises again.
Aidan:
In that study, four weeks after they finished the fat loss phase, they were at maintenance. They were re-measured, and there was no difference in metabolic adaptation. This is a common finding. I term this as the body caring more about recent actions than old history.
And that is fortunate. Broad research on metabolic adaptation shows this. You could point to a study and say maybe they didn’t lose heaps of weight. My interpretation is that for larger weight loss or severe deficits, it might take longer. I don’t think it takes much longer from what I have seen. I view that as positive.
Back when the Biggest Loser study was the main reference, it felt bleak. It seemed like what we did five years ago could destroy metabolism today. The rest of the research is more positive. What we are doing now matters far more than what we did back then.
Torwen:
And people often talk about metabolism in relation to weight. In that same study, change in metabolic adaptation did not predict weight regain after a year. That is positive too. A modest calorie deficit does work. You are not affecting resting metabolic rate longer term.
Aidan:
But, even in the Biggest Loser study, those who maintained the most weight loss were most adapted. They were also doing the most exercise. Those people are more likely to maintain weight loss. It is good to see that. That’s exciting. This leads into strategies like reverse dieting. I don’t know if it’s as common these days. It used to be more common. Have you had clients talk about reverse dieting before?
Torwen:
Somewhat. I work less in weight loss alone. People ask how to get out of a deficit and slowly increase calories.
Aidan:
So, the logic for reverse dieting is this. If somebody is metabolically adapted, slowly increasing calories could help. That is instead of one big jump. They might increase calorie intake over time. There are many case studies where people add 100 calories per week for a long time. They end up on surprisingly high calories without gaining weight. A few thoughts here. To reverse adaptation quicker, you could jump straight to higher calories.
Like, you don’t need to go 100 at a time. The case studies can be misleading. Often people are still in a deficit at the start. They get leaner at the start. In most cases, they are in a surplus at the end. They end at the same weight. It looks like maintenance, but they are in a surplus at the end. They haven’t boosted metabolism by 500 calories.
They have simply ended in a surplus. Research on metabolic adaptation makes it clear it doesn’t need to be a slow journey. Another perspective I heard on a podcast stuck with me. Say you build your metabolism through this approach. Within four weeks back in a deficit, you adapt the other way.
Torwen:
It may not make sense to take it slower if that is your concern.
Aidan:
And the only exception is behavioural. If somebody overshoots coming out of a deficit, I would go slower.
Torwen:
So, that leads into yo-yo dieting. Going from very low to maintenance or higher, up and down. This can be intentional or unintentional. Your body adapts to this. How long you take between lows and highs makes a difference. In a significant deficit, TEF decreases. Exercise expenditure decreases, and steps or non-exercise activity decrease. This can increase hunger or the reward value for food. It makes weight maintenance harder. Generally, we don’t recommend yo-yo dieting.
Aidan:
Absolutely not. I’ve thought about this in the bodybuilding space. There is bulking and cutting. I wouldn’t call that yo-yo dieting. Bulking should be a very small surplus for a long time. Then the cut is shorter and controlled. Yo-yo dieting is extreme in either direction. It’s rarely a small surplus. It’s usually large.
I’m massive on adding a maintenance phase for a history of yo-yo dieting. That could be slightly below, at, or slightly above maintenance for an extended period. We have RMR testing equipment at Ideal Nutrition. And we see people with suppressed metabolism and that history. There are many variables. It’s hard to say if that caused it. In those cases, I’d spend time at maintenance. We do see it go up after that.
Torwen:
That is for a few reasons. Having a diet break helps if someone wants to lose a larger body mass over time. Mentally, it can be good. For metabolism, it can be good. And it can also help hormones and general appetite and hunger levels.
Aidan:
Now we’ll talk about something more practical. What can we do outside of calories? What foods can help? I don’t know if we will have the same list. I’ve got three things. These are novel strategies, and we will talk about use. I’ve got chilli or capsaicin, caffeine, and green tea or green tea extract. Have you got anything else?
Torwen:
I’ve got similar. I’ve also got general recommendations. So, maybe we start with that, then look at your specifics. To bring it back, the thermic effect of food is our topic here. It is the energy your body uses to break down nutrients in food. When you eat certain foods, or larger meals, TEF increases. Some foods use more energy to be broken down. In general, protein and high-fibre foods do this. Whole grains or vegetables are the biggest groups. They contribute to increased TEF. As Aidan mentioned, there are some niche specifics as well.
Aidan:
On protein and fibre, they increase the thermic effect of food. Another underrated one, not directly metabolism, still indirectly helps. A higher intake of minimally processed foods versus processed can increase digestion costs. Calories required in digesting will increase a bit. Calories absorbed can decrease a little. It’s not huge. If someone goes from mostly processed to mostly minimally processed, you can see a swing.
That can be around 100 to 200 calories in that direction. That is more than the novel strategies I’ll now mention. I’ve included research in the show notes. It covers capsaicin, caffeine, and green tea. With capsaicin, found in chilli, supplementation shows modest increases. It’s about 50 calories per day when consumed regularly. With caffeine, going from none to a fair bit shows a small increase. Around 50 calories per day.
Regular usage probably decreases that a little. And green tea extract is similar, around 50. Some research shows 50 to 150. I had an interesting chat with Tyler on our team. He went through this research. Many dietitians rip on fat burners, saying they don’t work. Those often contain ingredients like this. A supplement could increase energy expenditure by 150 calories. Why is that a bad thing? I think it’s worth knowing these exist. Do they matter? A few things.
If you do this every day for a long time, benefits likely reduce. I also think appetite adjusts based on energy expenditure. If you increase expenditure by 150 calories, appetite may rise. It won’t match exactly, but it balances out over time. Anything creating a deficit for a long time typically makes you hungrier. Because it’s a defence mechanism. Otherwise, you could waste away. These things exist. And I don’t go out of my way to recommend them for that.
Torwen:
Neither do I.
Aidan:
And on caffeine, I can’t imagine going very high specifically for this. Not if you don’t otherwise want that much caffeine.
Torwen:
So, should we chat about other contributors then? Food is one aspect. We also know muscle mass, genetics, fidgeting or general activity, and exercise play roles. Age and gender also influence resting metabolic rate.
Aidan:
Did you get interesting research on this? I know we spoke off air.
Torwen:
Not a whole lot. I’d love to hear what you’ve got.
Aidan:
So, I looked into research. Genetics matter here. It’s hard to say how much because there are many moving parts. It would matter even more beyond metabolism. That includes how much people naturally want to eat. Going specific to total daily energy expenditure, I have a loose estimate. After accounting for fat-free mass and common variables, differences remain. There is often a 200 to 300 calorie difference in RMR between people. Genetics likely influence that. It’s not massive, but it’s meaningful. Numbers get larger when we include incidental movement like fidgeting.
Torwen:
And I think people underestimate that. Would you put fidgeting in non-exercise?
Aidan:
Yeah, absolutely.
Torwen:
When you see clients who fidget, or ask about that, you see higher needs. They often have higher energy requirements or expenditure. It can make a big difference.
Aidan:
This is not exactly the same space, but I think about Parkinson’s research. There is often heaps of shaking, so energy expenditure is much higher. But bringing it back to natural fidgeting. Compare a very still person to someone constantly fidgeting. It adds up. It makes a big difference.
Torwen:
Practical recommendations then. Increasing exercise makes sense. Increasing muscle mass can also make a difference. Muscle tissue requires more energy to maintain than body fat. That matters. What do you recommend there, Aidan?
Aidan:
So, on muscle mass, that brings me back to bariatric surgery. I mentioned an increase earlier. I don’t think it was a random increase. They lost a large percentage of fat relative to muscle. When they redid things based on the formula, it looked more favourable. Muscle burns more calories than fat. I don’t know how practical the muscle thing is in many cases. Building muscle is hard. The effort can outweigh the metabolic benefit.
Everyone should still put effort there. And other practical things. I don’t think you should be in a deficit year-round. Spending time out of a deficit makes sense. Having a good baseline of general movement wherever possible helps. It’s not technically in the RMR category. It gives you a bigger calorie budget. That makes it easier to eat slightly more food. And a mostly minimally processed diet helps. Because you will naturally take care of fibre and protein. Add those up and you get hundreds of calories into energy expenditure.
Torwen:
That makes a huge difference. Is there anything else you wanted to add?
Aidan:
That covers everything from me.
Torwen:
Amazing. Well, this has been episode 226 of the Ideal Nutrition Podcast. If you enjoyed this episode, we’d love a positive review. We will be back next week.