It's one of the most commonly stated fitness proverbs of all time that whatever happens on your plate determines 90% of the results you get.

Whether you're on the skinny side looking to pack on some muscle or on the chubby side looking to shed some pounds, you should know that no amount of work in the gym will ultimately make up for bad diet habits outside of it.

Where does too much become too little though? How do you know how much to eat? Nutritionist and Gym Manager, Hollie Miles, takes a look here:

How Much Should You Eat?

There's a "scientific" approach to working out how many calories you should eat, and it's a confusing and messy business getting your head around it. Think that sounds fatalistic? Let's illustrate.

Before you can work out how many calories you need to eat to either gain or lose weight, you need to work out your maintenance calorie intake (that is, how many calories you need to eat to maintain your current weight).

There are 6 biological factors that determine your maintenance calorie requirements:

• Basal Metabolic Rate (BMR)[1] -- Essentially how many calories you expend on essential physical systems (just staying alive). This is correlated with your lean body mass and accounts for typically 60-75% of all calories you eat.
• Thermic Effect of Food (TEF)[2] -- Number of calories burned digesting your food. Typically, it's 10% of the calories you eat.
• Non-Exercise Physical Activity (NEPA) -- Calories burned through intentional but casual movement, like walking to the fridge. Combined with NEAT, this usually accounts for 200-400 calories though it can vary wildly.
• Non-Exercise Activity Thermogenesis (NEAT)[3] -- Calories burned through casual, absent-minded movement during the day, like fidgeting. Combined with NEPA, this usually accounts for 200-400 calories though it again can vary wildly.
• Activity Levels -- How active you are, in terms of deliberate physical exertion or exercise. This can vary wildly from 0 calories burned to thousands.
• Adaptive Component -- How your metabolic rate changes when you overeat or under-eat. Eat less, burn fewer calories, eat more, burn more calories.

As you can see, accurately calculating your maintenance calories can be an extremely intricate scientific procedure. There just isn't a single equation which will work for everyone. It's no accident that there are dozens of formulas out there for trying to figure all of this stuff out, like the Katch-McArdle formula, the Mifflin-St Jeor formula,  and the WHO/FAO/UNU formula- the list goes on.  

Never fear though as there is an easier way to get a decent handle on roughly where your maintenance calories lie:

• Follow your daily routine as normal while tracking all of the calories you eat- Do this day-by-day, for around 2 Add the total number of calories from each day together and divide this figure by the total number of days to work out your average daily calorie intake.
• If you're losing weight when you don't want to be- Increase your intake by a small number each week -- around 200-500 calories -- until you notice a change in the right direction then stick at that number until your results taper off. Re-calculate when needed.
• If you're gaining weight when you don't want to be- Reduce your intake by a small number each week -- around 200-500 calories -- until you notice a change, then stick at that number until your results taper off. Re-calculate when needed.

When it comes to figuring out much you should be eating to meet your goals, the key is constant self-analysis, tweaking, and experimentation. There's only one way to do that -- by tracking your calories meticulously, day-by-day, and recording your weight every week.

It's not all a matter of calories, either. You need to ensure that you're getting enough of the different macronutrients (protein, carbohydrates, and fats) and micronutrients (vitamins and minerals) regardless of whether you're aiming to lose or gain weight.

As you would expect, there's some debate on what the best macronutrient ratios are. A decent rule of thumb is to try and get roughly an equal amount of your daily calories from proteins, carbohydrates, and fats. You can tweak these ratios experimentally by yourself as you go along. As for micronutrients, a varied diet is the best way of getting those in order, with a good multivitamin supplement coming in second place.

Does Meal Frequency Matter?

One prominent bit of fitness dogma which has recently fallen out of fashion is the notion that you need to eat a small meal roughly every 3 hours throughout the day to keep your body in an "anabolic" state.

This idea seems to rest on a few faulty premises, such as the idea that your body can only digest a small amount of protein in one go, so you need to keep feeding it small doses throughout the day to add up to your daily total.

It turns out, however, that digestion is actually a very gradual process. It takes around 5 hours for your stomach to empty itself after you've filled it -- and it can take around 30-40 hours before an entire digestion cycle is completed.[4]

Obviously, this means that if you eat one enormous meal in the morning, with all of your day's protein allowance packed into it, your body will digest the food just as surely as if you had eaten 8 small meals that day.

Meal frequency likely doesn't make a major difference to your fitness goals. Focus on hitting your overall calorie and macronutrient targets and space your meals however you find most convenient.

"Making use of fitness/nutrition apps such as 'my fitness pal' are great tools to use to help you track your macros and your daily calorie intake," Hollie says. "In order to achieve your weight loss/weight gain goals, it is important to log your exercise activity and nutrition so you can track your progression and also see how your body best responds."

Hollie Miles, GM and Nutritionist.

[1] http://www.ncbi.nlm.nih.gov/pubmed/16277816

[2] https://nutritionandmetabolism.biomedcentral.com/articles/10.1186/1743-7075-1-5

[3] http://www.ncbi.nlm.nih.gov/pubmed/11101470

[4] http://arbl.cvmbs.colostate.edu/hbooks/pathphys/digestion/basics/transit.html