Fitness and nutrition are two sides of the same coin — but for weight loss goals, focusing on nutrition is key. INTRODUCTION Like everything else in life, fitness is just a matter of math and discipline. Nutrition (“energy in”) and training (“energy out”) are key, but let’s focus on nutrition for now — since it's the more important factor for weight loss goals. ( Read more: What is a Calorie, Really? Understanding 'Energy In and Out' ) These evidence-based “rules” are heuris