The Science Behind How Weight Loss Actually Works

Forget the marketing hype and diet gimmicks. When it comes to losing weight – listen to Newton.

Dr. Mike Posthumus |

A popular topical discussion point this past festive season: “Doc, what are your thoughts on diet X – 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 truth from fiction. I would also like to think that my education has provided me with a fair amount of common sense.

Related: Our Ultimate Match Day Muscle Meal. Plus, The Best Low-Calorie Beers

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.

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.

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.

Related: Smash This Low-Calorie Protein Feast In Minutes

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 740kCal 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 900kCal. 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.

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.

Related: Our 14 Step Weight Loss Guide To Losing Fat In 24 Hours

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

What the Doc finds super-useful right now:

This month I thought I’d highlight some very useful and educational Instagram accounts. Because Instagram doesn’t have to be filled only with selfies and quacks. There are some really knowledgeable experts out there who share their knowledge with their followers, and I highly recommend giving them a follow.

Related: The Truth About Sweet Potatoes: Are They Good or Bad for Weight Loss?

@DRNADOLSKY Dr Spencer Nadolsky is a physician who focuses on lifestyle changes and fat loss. Includes funny memes, and has a strong emphasis on debunking diet misconceptions.

@DR.JACOB.HARDEN Dr Jacob Harden is a doctor of chiropractic and shares very useful strengthening exercises for prehabilitation and rehabilitation of injury. Helps you avoid injuries.

@JOETHERAPY Joe Yoon is a massage therapist, and he shares content that will assist you to stretch and improve mobility around certain joints and structures.


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