When I was younger, I made a deal with my body. For my part, I would eat whatever I liked in whatever quantities I deemed appropriate, take only minimal exercise and generally try not to stress the poor thing out unduly. In return for this largesse, my body agreed not to change its size and shape in any way.

Aged about 32, my body welched on the deal.

Now, I should say, that although not very tall, I do have quite a slender frame and as a boy or even a young man, I was positively skinny. Through my thirties, however, to this slim physique was added the gentle and yet unmistakable curve of a spare tire around my abdomen and even, yes, a little extra flesh around the jawline. However – what’s a boy to do? I do like my food (at least some food), especially red meat, potatoes (especially in the form of fries, creamy mash or roasted in goose fat) and glorious, blessed, holy cheese. Not only that, but as I do most of the cooking, I am in the habit of cooking for two – whether there is anyone else home or not. Clearly, a big part of the problem was portion control.

In 2008 I put myself on a calorie-controlled diet and I’m repeating the experiment currently. Ever looked at those two ton Americans who end up having to be winched out of their homes and taken to hospital to be humanely destroyed, and wondered how they got like that? Because they didn’t start managing their diet when they were still only a bit chunky. If you wait until you are already morbidly obese, it’s too damn late.

So, below I’m going to lay out how I’m doing it and how well it’s working. It’s not the only way, I’m sure, but it’s entirely in line with most findings about weight loss, so I want to take the time briefly to explode a few myths. Before that though, a brief moment of exculpation.

Weight gain and loss is a sensitive issue for some people, and I don’t pretend to understand what it’s like to be anorexic or to have suffered decades of taunting about my weight or to define myself in terms of how skinny I am. My self-esteem is – thankfully – not tied to whatever the scales tell me today. I am not “battling my weight”, I am not crash-dieting. I am a little bit heavier than I think is ideal and I’m doing something about it. This isn’t an instruction to anyone else. It’s just a description of my thought process, my actions and their outcomes. ‘Kay? ‘Kay.

So, to begin with calories are king. Energy can neither be created nor destroyed. Every calorie you ingest has to go somewhere. Some will be used to build muscle. Some will be used to keep your heart pumping and your blood oxygenating. Some will be lost through excretion. But they all go somewhere. Generally speaking, if you burn up more calories than you take in, your body will start unlocking the extra calories it has stored in the form of fat in order to keep the show on the road. And if you use fewer calories than you ingest, your body will start adding to those fat stores.

Now, metabolic rates (how fast your body goes through calories) do vary, but they vary much more from person-to-person than they do for the same person from day-to-day. Your metabolic rate is your metabolic rate, and although it will change a bit according to diet, disposition, overall health and so on, it won’t change a lot. You want to lose weight? You need to shift that calories balance.

And here’s where two different concepts tend to get conflated. There is a world of difference between a “healthy” food and a “low-calorie” food. A glass of water is the perfect low-calorie food, since it contains no calories at all. But it also contains no nutrients. If you ingest nothing but water, you will die (although not for 2-3 weeks).

A McDonald’s cheeseburger contains about 295 calories. A Pret A Manger Chicken Avocado sandwich contains 462 calories. The Pret sandwich is probably more nutritious – it contains a wider variety of nutrients than the cheeseburger – but if you wanted to lose weight, you would be better off with the burger!

So, it’s important to be clear about your goals before you start modifying your eating habits. It’s actually very, very difficult to hurt your body by not giving it enough of the things it needs. If you don’t eat enough fibre, you’ll eventually start getting digestion problems. If you don’t eat any vitamin C, you’ll eventually die of scurvy. If you scarf down too much saturated fat (emphasis on saturated) then you’ll eventually hit heart problems. But the key here is “eventually”. Scurvy takes months to develop and heart disease takes decades. You can’t stave off heart disease by not eating chips for a month. But nor will you be hospitalised for malnutrition if you eat KFC every day. The problem in the Western world is usually too many calories and almost never too few nutrients.

Nor, crucially, can you lose weight by avoiding certain types of foods, or by over-indulging in others – except in so far as such alterations to your diet cause you to ingest fewer calories as a byproduct. And, so that raises a couple of other issues. The first is a variation of the Hawthorne Effect in which people who are on a diet – any diet – tend to lose weight at least at first simply because they are more aware of what they are eating. Just keeping a food diary, writing down everything you eat, can help many people to lose weight, because it helps to prevent mindless snacking.

Other faddish diet, like the famous Atkins eat-all-the-cream-and-red-meat-you-like-but-stay-off-the-pasta diet add to this principle by giving you an additional appetite suppressant. If you go to a steakhouse and order steak and chips with béarnaise sauce (and I hope that you would), here’s how the calories break down. A big rump steak might weigh 400g, which will supply about 500 calories. The béarnaise adds about another 120 calories, depending on how much they dollop on. The fries are the hardest to estimate, but assuming a largeish portion of shoestring fries, they will probably run you around another 400-600. So clearly, if you have the steak without the fries (or indeed the fries without the steak), you will roughly halve your total number of calories. But if you have scoffed down your own plate of steak and chips and your dining companion has left half their fries behind, you might very well pick at them until they’re all gone. It’s rather less likely that if they leave half their steak behind that you’ll want to start in on that. The protein-rich steak fills you up more than the starchy fries do, so the Atkins diet gives you a similar feeling of fullness for fewer calories.

Cutting down fat makes sense if you want to diet, again because it generally results in cutting down on calories. Fats are the most calorie-rich foods, so eating less of them is generally good. But beware of low-fat foods which compensate for the lack of delicious fat by loading you up with sugar instead. Fat contains about 9 calories per gram. Sugar, although better, still contains around 4 calories per gram. (Since you asked, alcohol contains around 7 calories per gram, but nobody drinks pure alcohol.) Once again, it’s vital to distinguish between health and weight-management. A diet coke contains almost nothing of nutritional value, but it clearly also contains no toxins (why would a capitalist company choose to poison its customers?) so it makes an ideal drink for a dieter. A delicious glass of healthy orange juice contains lots of health-giving vitamin C and lovely fibre, but will also set you back around 90 calories. Which is more important right now? Extra vitamins or fewer calories? As long as you’re clear about what you’re eating and drinking and why, there’s no problem. But if you add extra “healthy” nuts and fruit to your diet, you won’t lose weight. You’re just adding an extra source of calories.

If you really need convincing that it’s how many calories you eat and not what you eat that matters with weight loss, then consider the case of nutritionist Mark Haub who set out to test this very issue by putting himself on all-Twinkie diet. A Twinkie (a sort of sponge cake with a creamy filling, beloved of American 7-11s) is hardly a healthy food, being loaded with sugar and fat and little else of nutritional value, but because the calories are printed on the packet, you can know exactly how many you are ingesting and so regulate your weight. Mark ate one Twinkie every three hours (plus a protein shake and a multivitamin once a day and a few celery stalks of an evening) thus limiting his calorific intake to 1800 calories per day. He lost 27lb in two months.

So – finally – here’s my plan.

Step one – count calories

Using myfitnesspal.com and its companion iPhone app, I’ve selected a calorie goal of 1480 calories per day. I drink little other than black coffee (no sugar) and diet coke, both of which are negligible in terms of their calorie content. I eat a lot of M&S ready-meals (which I reckon have improved dramatically in the last five years) because, like the Twinkies, they have the calories printed on the box. Yesterday I had a toasted muffin for breakfast (226 calories including the butter), a Pret brie baguette for lunch (396 calories) and an M&S Gastropub Cottage Pie with a whole pack of Classic Layered Vegetables (665 calories total) for supper and I went to bed feeling quite satiated. With the iPhone app, I can snap the barcodes and add the meals to my food diary instantly.

Step two – cardiovascular exercise

Exercising more means you use up more calories. You also might stimulate your metabolic rate a bit (but only a bit – see above). It also helps me to feel like I’m doing something, getting somewhere. But running burns about 500 calories per hour. If you run for 15 minutes and then eat a crème egg, you’ve done more harm than good. I hate gyms, they depress me, but running at least feels not entirely pointless. Following, of all people, Charlie Brooker’s recommendation, I’m using an iPhone app called Get Running. You run three times a week and the programme ramps up each week. You can listen to music or an audio book and the app chips in every so often with fresh instructions – “run for three minutes”, “cool down by walking for a minute-and-a-half” and so on. At the start of the process, you only run for a minute or so at a time. By the end of week 9, you’re running for thirty continuous minutes. I’m on week 5. I’m also working my way through the 100 push ups programme.

Step three – record everything

What motivates me is seeing progress. I weigh myself every morning just before jumping in the shower and record the results in a simple spreadsheet. Weight fluctuates considerably – a change of up to 2lb in 24 hours in not unknown – so I run a five-day moving average to smooth out the noise in the data. On 5 January I weighed 160lb. Probably as heavy as I’ve ever been and just nudging into overweight on the BMI chart. It’s possible I was heavier earlier this year, before a horrible throat infection which turned me off pretty much all food for about a week. Today, my bathroom scales have packed up, but yesterday morning I weighed 149.6lb. Last time round, I got down from 156lb to 147lb but gave up in mid-February. This time, my target is 140lb by mid-March when I turn 40.

It would probably be better – certainly more sustainable – to just get out of the habit of munching through an entire block of cheddar in an evening, but this will at least be a start.