If we forget about things like hypothyroidism, polysystic ovarian syndrome, insulin resistance and other conditions that make it difficult to lose weight, shedding kilos boils down to one thing called calorie deficit: you need to expend more calories than you take in. Or, as they say, "burn" more than you "eat".

The simple act of being alive expends most of our calories. Just being means you’re burning fuel. This is called your basal metabolic rate or BMR. When people talk of having a slow or a fast metabolism, this is what they are referring to.

Read more: See how much you should be eating

But besides the calories we use simply by staying alive, we also expend energy by doing things that living people do – walking, working, sitting, driving, cooking, showering, kissing and so forth. This, along with your BMR and whatever added activities or exercise you do, count as your total energy expenditure.

How big must the calorie deficit be?

According to Lifehacker, in order to lose half a kilogram a week you need to create a 3500 calorie deficit. That’s 500 calories you have to “lose” per day for a week.

If you’re an average woman and you want to maintain your weight, you should be eating about 2000 calories per day, according to Authority Nutrition. In order to lose a pound (half a kilogram) a week, that number must be brought down to 1500 calories per day.

So to bring that number down you can either “burn” through added activity, or not ingest by making dietary changes.

Read more: Britain's diet craze: Lose 3 kgs in 7 days?

If you take the exercise route, that works out to the equivalent of running 6 kilometres every day. So that means if you run 42 kilometres – the equivalent of a marathon – every week, you will lose 0.5 kilograms per week.

Now I don’t know about you, but that seems a lot of effort for 500 grams lost?

Getting the same effect by making a dietary change would be equivalent to swapping out a Big Mac and Coke (860 calories) for a tuna and mayo sandwich on rye with sparkling water (270 calories), which seems much more reasonable. Well, to me, at least.

People overestimate exercise and underestimate food

One of my favourite studies (perhaps because I could relate?) involved subjects being asked to do an hour’s worth of exercise, and then being asked to eat from a buffet in order to replenish the calories they think they had expended. Without fail people ate two to three times more calories than what they actually burned.

Similarly, people tend to over-reward themselves for exercise. You know what I’m talking about. When going to a restaurant, you’d order dessert because you know you’ll be walking the ten blocks home.

If you break it down though, the walk will only burn about 70 calories and the chocolate mousse you ate was around 400…

Read more: Check out this handy caloric chart for alcoholic beverages

But what if you combine the two?

The most interesting article I’ve read on this topic was in the NY Times. It delves quite deeply into various studies and various reviews of various studies and if you’re interested in this topic I urge you to read it. In this same article they refer to a meta-analysis that was done that found that the combination of diet and exercise over a long period of time did yield more sustained weight loss than diet alone.

But the key here is long term.

So I can skip exercise then?

Please don’t. Yes, dieting is more effective for weight loss than exercise. And for many people – especially young women – being slender is the main goal. But this is where dangerous eating patterns, stupid fad diets and lifelong bad habits come from.

Overall health, well-being, and general enjoyment of life should be your main goal. Food is not your enemy, and exercise is not a tool to punish yourself for eating.

An exercise regime will make you feel and look better and will help a great deal towards heart, bone, muscular and pulmonary health. It will also combat muscle loss that often goes hand in hand with diet, and will keep your body toned and strong.

You might not lose as much weight - remember, muscle is denser than fat, so you might weigh more - but your body will thank you. Now and in the decades to come.