The Science Behind How Weight Loss Actually Works
Forget the marketing hype and diet gimmicks. When it comes to losing weight – listen to Newton. Here are the scientific facts on how weight loss works.

June 28, 2022

A popular topical discussion point during the festive season (and, pretty much the rest of the year, too): “Doc, what are your thoughts this diet – do you think it works?”

This conversation normally continues with me explaining that I’m not a dietician or a medical doctor. I’m a sport scientist, and therefore not an expert on the topic. However, I do consider myself well trained in sifting through information and differentiating fact from fiction. I would also like to think  my education has provided me with that a fair amount of common sense.

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My answer to the diet question always involves a refresher on Newton’s laws of physics, and two simple words: calorie deficit. Now, let me explain further.

Back to Basics

The first law of thermodynamics states that energy cannot be created or destroyed in a closed system. What this means is that if the calories you consume (even those snacks you may consider to be healthy) are more than the calories you burn (your resting metabolic rate and all your activity in a day), you’re going to gain weight. Simple. Other way around? You’re now in a calorie deficit, and on track to lose weight. Simply put, all diets—no matter if they’re low-carb, Banting, Paleo (I don’t even know if that’s the same as Banting any more?), low-fat, Weigh-Less, vegan, Mediterranean or intermittent fasting—work through creating a calorie deficit. If they don’t, you’re not going to lose weight.

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When you cut down one macronutrient (whether it’s fat or carbohydrate), or you restrict yourself from eating for 16 hours in a particular day, your total calories for that day will be lower than for an unrestricted diet. Choosing a diet to follow to lose the excess kilos around your belly should be less about the magic each diet inevitably claims, and more about which diet will be more sustainable in helping you create a long-term calorie deficit. Look at the food you’re consuming, and calculate the number of calories in your meals. I don’t advise counting your calories each day, but do the odd stocktake, to make yourself aware of what you’re eating. This will help you make better choices around food and meal selection.

The Superfood Myth

smoothie bowl

We’re often fooled into thinking we’re doing well by selecting a smoothie under the ‘superfoods’ category at our favourite smoothie bar; however, some of these smoothies may contain as much as 740 calories per 500ml. The popularity of ketogenic approaches may have us believe that we should ask for the fat to be left on our steak if we want to lose weight. But 100g of fat may contain 900 calories. Unless this practice is reducing hunger significantly enough to still be creating a calorie deficit (which is how ketogenic diets work), you’re not going to lose weight.

Now, I know that many readers are not going to agree with everything I’ve said. And yes, of course hormones play a critical role. But hormones are not more important than kilojoules. They may alter the number of calories you burn, how many you eat, or where you store them. But a hormone is not going to help you defy the first law of thermodynamics.

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Choose an approach to eating (don’t call it a diet—it’s a way of life) that creates a calorie deficit and that is going to be sustainable (by including foods that you enjoy eating). Ensure that your meals leave you satisfied and not hungry. There are several approaches that may work for you; but they don’t work through magic, and they don’t work through some secret bio-hack—they work through creating a calorie deficit. If you continue to battle to lose weight, see a dietician who is specifically trained to assist by creating an approach to sustainable weight loss.

Oh, and to answer the original question: “Diet X will only work if it makes you eat less than you burn.”