I have a qualification in advanced sports nutrition and this is perhaps the area i am most interested in ... so my two penneth for what its worth.
Determine the amount of calories required to put you into a calorie deficit to lose weight. Add calorie burning activities, if you don't want to restrict to diet/calories, only.
If the only goal is to lose weight, you can eat whatever, at whatever portion you want, as many times as you want as long as you are in a calorie deficit.
Although Calories do matter, they are not the whole story.
Why isnt a calorie a calorie?
1) differernt foods produce different metabolic and hormonal responces. 100Kcals of sugar and 100kcals of fat will not produce the same result metabolically or hormonally. And as we all know very well know, our hormones have a dramatic effect on our ability to loose body fat.
2) Gut Flora and enzyme production has a direct impact on our ability to actually extract energy from our food. We all know that one skinny guy who eats junk food all day and never seems to get fat. It is looking increasingly likely that this is due to the make up of their gut flora, the mocrobiome and the enzyme production that allows them to digest foods. This make up will directly impact how much you get from what you eat.
3) State of the body during consumption of food. If the body is in a fed state the amount of nutriant absorbtion is different from if the body is in a fasted, exhausted or stressed state. Again this relates to hormones and also production of digestive juices.
So basically, yes eating less calories CAN work, but is not the only relevant metric. If you eat less calories but they are all carbs the insulin responce will cause you some problems with fat loss. If you eat more calories than you need but they are mainly fats you may still loose body fat due to initially malabsorbtion and then due to ketosis.
On helping people loose body fat.
The key is to make it gradual, progressive and absolutely NOT give them a laundry list of things to cut out and do immediately.
There are simply tools that you can use as a 'lead domino' that will impact the rest of the day. A very well proven first step is simply 30gms of Protien within 30 minutes of waking. There are a number of reasons why this is so effective. But i would suggest making this first step for a couple of weeks, then move onto the next step, which may be to reduce carbs in the lunchtime meal ... another couple of weeks, next step ... etc. It is a marathon not a sprint
Perhaps most important however is that everyone really is different when it comes to nutrition. There is NOT a one size fits all. Some people are better on high fat diets, some on moderate to high carbs, some on higher calories, some on lower. There is evidence for pretty much every dietary plan you can think of so you have to monitor how you 'look, feel and perform' at around 30 days then make changes if needed.
It is also worth mentioning that studies suggest the make up of your gut flora actually impacts the sorts of food you want. If you have an abundance of sugar loving bacteria in your gut you will crave sugar ... yes they actually affect your mind! So it takes time to starve them and change the balance of bacteria in the gut.
As with martial arts trianing, things should be individualised to the person, and not one size fits all.
thanks.