How much is a "pound of flesh", that brutal payment demanded by Shylock in Williams Shakespeare's The Merchant of Venice

In 1958, we got our closest answer, and the health industry got a new standard against which to predict and measure weight loss. 

That year, researcher Max Wishnofsky published a study that would form the basis of how organisations like Livestrong measure weight loss, writing that a pound of human fat represents about 3,500 calories. The logic goes that cutting 500 calories per day, through diet or physical activity, would result in about a pound of weight loss per week, and vice versa. 

Now viewed by many to be too simplistic an equation for predicting weight loss, the health industry is flooded with theories, studies and fads, eagerly devoured by a baffled public. 

For all our information and technology, the vast majority of us are none the wiser when it comes to managing our weight. Reduce this dilemma to its core and you find a fundamental lack of understanding of how our body uses food, or burns calories. 

So, we spoke to two experts to get to the heart of the matter. 

What is a calorie? 
As Karl Henry - Operation Transformation’s in-house personal trainer - explains, a calorie is a "unit of energy", adding "I think people get fixated upon [it], particularly in health, which has currently become quite extreme". 

"It’s how many units of energy your body has to burn up to keep your body alive during the day, as well as your activity on top of that."

Typically, a man requires 2,500 calories per day, while a woman needs 2,000, but both amounts vary depending on your build or lifestyle factors. 

How are they burned? 
So if you want to get into it, there is an equation you can use to calculate exactly how many calories you burn. And we said we’d never need math after secondary school. 

It revolves around a measurement called MET - meaning metabolic equivalent - which is "roughly equivalent to the energy cost of sitting quietly", according to Compendium of Physical Activities, a website created by the National Institutes of Health and designed to help people calculate their calorie output. This is roughly 1 kcal/kg/hour.

Calculate your weight in kilograms, then using the Compendium search for your activity from the dropdown menu. Take the MET value of that activity and multiply it by your weight to find the amount of calories burned in an hour. 

This is only an estimate, and frankly is more mental acrobatics than we’re willing to spend after an hour in the gym, but it works in putting some definable shape on calories and how they work in our bodies. 

The problem with calorie tracking
But with all the apps, articles and studies on intermittent fasting and ever more inventive gym classes designed to torch calories in the shortest amount of time, there is comparatively little attention paid to how our bodies use up calories without us trying. 

"What I don't think people realise is that the body requires the majority of calories for the daily maintenance of the body", Michelle Loughlin, a dietitian with Spectrum Health, says. "This is your basal metabolic rate or the amount of energy your body needs at rest, so to keep your brain functioning, your lungs functioning, your heart pumping." 

Where this can lead to trouble, Michelle notes, is when a person is calorie-tracking and starts eating only to train or exercise, ignoring the fact that their body needs upwards of 2,000 calories a day for a reason. 

"Many people assume that if they don't exercise for a day that they do not need calories or energy and this is a major misconception. Around 70% of the energy you require is to simply keep the body ticking over."

A further 10% of calories go towards breaking down food, a process called thermogenesis. Have you been told that eating one food over another will use up more calories? Yeah, Loughlin is pretty blunt about this: "Please ignore claims that eating certain foods will make you burn more calories."

Clickbait confusion
Fasting. Intermittent fasting. Alternate-day fasting. Celery juice. Ketogenic diets. Gluten-free diets. You can’t move for the new ways of dieting and the studies being published about them. If the proliferation of hot-takes, takedowns and clickbaity articles about diets mean anything, it’s that people are more confused than ever about how to best look after their bodies. 

And that’s with the most advanced technology, apps, and information we’ve ever had, right at our fingertips. 

For Loughlin, the main stumbling block with clients is how many of them do not understand the role exercise plays in how our body uses calories. "Most people completely overestimate how many calories are burned in a 45-minute session at the gym", she says. "It’s roughly 300-500 calories, which is an average sandwich."

She explains that due to this misunderstanding, many people then eat possibly double the amount of calories after exercise. 

And who hasn’t walked out of a gym class, red-faced and glinting with sweat, and instantly treated themselves with a burger - protein is necessary after exercise, right? - smothered in cheese, bacon and barbeque sauce? Or, exercised a little purely to justify that second drink on your girls’ night out? 

This mentality - where you need to preempt any indulgence or "bad food" with exercise - is one of the more pernicious symptoms of a diet-obsessed culture, and one that Loughlin actively tries to break down with her clients. 

Over-eating or under-exercising?
Due to more studies being published on the importance of movement in our general health a tide has somewhat turned in the dieting world, with more experts pushing for more activity over restricted diets. Fitness trackers support this, encouraging you to find little ways to move more rather than overhauling your entire eating style. 

But research poses more questions than answers sometimes. Take the study by anthropologist Herman Pontzer, carried out over 2009 and 2010, which saw him study the lifestyles of the Hadza, a hunter-gatherer tribe in Tanzania with extremely active physical lives. 

Expecting to find that due to their more active lifestyles - climbing trees, hunting and killing animals, foraging for nuts and berries and digging for crops - the Hadza torched through calories, proving his theory that Western bodies have become sluggish and sedentary, he found something more complex. 

In reality, the Hadza burned the same amount of calories as people in the West, but remained lean and incredibly strong. How was this possible? 

While more research would need to be done, Pontzer theorised that it was due to their controlled diets. They don’t overeat. 

How can we track what we burn? 
Given that so many factors go into how many calories your body burns on any day - activity levels, sleep levels, stress, preexisting health conditions to name a few - Henry suggests that the only way to accurately test this is to go to a professional to have your basal metabolic rate (BMR) tested. 

The Human Performance Laboratory in UCD offers a range of science-backed tests priced between €40 and €120, including tests for lung functioning and body composition including body fat levels. As well as this, Health Matters offer two kinds of metabolic testing, one for weight management and one for building muscle mass. 

But how accessible are these tests? Henry says they’re "fairly accessible" and "not that expensive", and if you’re serious about your health or about tracking performance like he is, he’s not wrong. 

For professional athletes, for example, fine-tuning your training and diet to the particulars of your body is essential for reaching goals healthily and steadily. 

However, Loughlin says: "I don't think there is a truly accurate way of measuring calories burned in a day readily available to the public."

She has an alternative method, one that might require more work and food mindfulness but can be done by anyone, any time. 

"What you can do is to weigh yourself, eat the suggested amount of calories for weight maintenance for 7-14 days, then weigh yourself again after 7-14 days and if the scales go down then you know this is an underestimation of your calorie requirement. 

"If they go up then maybe you overestimated your calorie requirement and if they stay the same then you have found your maintenance calorie requirements and you can then alter as needed."

"Most people who are trying to change their habits either to lose weight or to gain weight can be too focused on minor details instead of looking at the entire week as a whole", she adds. "It really does take at least a week to see a difference when it comes to making adjustments to your calories."

Can a fitness tracker help me lose weight? 
Yes, and no. 

Fitness trackers are so ubiquitous now that they have been stylised into the chicest of fashion accessories, with Prince Harry even sporting the sleek Oura ring, which looks as unassuming as a simple silver ring. 

The most common fitness trackers - including those made by FitBit, Garmin and the Apple Watch - typically work based on your heart rate, which is why they’re worn on the wrist’s pulse point. Through this - and the information you add in about your weight, gender and the like - they monitor sleep, activity and calories burned. They’re everywhere. Your mum probably has one. I bet you have three. 

However, both Henry and Louglin caution that these gadgets are not as accurate as many take them to be. "They give a really good guideline, which is the benefit", Henry says. 

More than that, a fitness tracker is an "awareness piece", he says, guiding those who may be new to exercise or need a helping hand towards achievable goals. "It gives you that 10,000 steps target to work towards, which is beneficial once your intensity is high enough." 

Loughlin agrees that they can be a tool for building good habits and consistency, but adds "I think fitness trackers can cause a lot of confusion. They are highly variable and there are so many on the market that it is difficult to give a blanket yes or no answer."

"I have often run on a treadmill and been told I have burned 800 calories and then look at my fitness tracker to see that it says that I have burned 350 calories. So, often there is huge variance."

Indeed, a Stanford study, carried out in conjunction with the Swedish School of Sport and Health Sciences and published in 2017, found that fitness trackers do not track calories burned as accurately as they track heart rate. More than this, the study found significant differences in how the seven trackers they analysed monitored calories burned, with the worst offender off by 93%. 

This is disappointing news at best, and a concerning insight at worst: considering how many people use fitness trackers to manage their weight and lifestyle, using this data to make life decisions, one can reason that this misleading information could lead people towards adopting unhelpful or even dangerous habits. 

"I think if you are going to rely on fitness trackers then you need to do so with a flexible attitude as opposed to being ruled by the number that they give you and also use a bit of common sense", she says. Simple stuff such as, if you’re hungry after lunch, eat a little more food. 

What about machines in the gym? 
Again, yes, and no. 

As Henry states again, "Once they’re linked into you via heart rate monitor or something like that, then they’re fairly accurate. If there’s no heart rate monitor component, it’ll be some kind of guestimate based on scientific data."

Tracking accurately for you
Heart rate and oxygen consumption are two of the most reliable markers for measuring exertion, and therefore calories, Henry says. 

"One of the things about exercise is the fact that in order for it to be beneficial, you have to challenge the body a little bit. The intensity of that workout is important and you’ll generally burn more calories." 

"Once you’re slightly out of breath or just barely able to hold a conversation, the body’s working really hard. The heart’s beating really fast, the lungs and lung capacity are breathing hard, your muscles are being challenged."

For Loughlin, also, individual circumstances play a huge role in how she advises her clients. "If someone has a history of an eating disorder*, emotional eating or binge eating then I would be more likely to ask them to focus on healthy habits as opposed to tracking calories."

For some people, having a fitness tracking app that sends you notifications and logs your calories can create a maelstrom of pressure where the person feels tied to a number of calories and overly fixated on food. It’s in situations like this that harmful habits with food and eating can develop. 

"As soon as the calorie tracker seems to have power over you instead of guiding you, then it is time to take a break from using them."

It is important to note that any lifestyle changes will be unique to each person and anyone considering a new diet plan should contact their GP beforehand.

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