Is a Calorie a Calorie?

Yes. 250kcal is still 250kcal whether it originally existed as an avocado, a packet of sweets, stored glycogen or stored body fat. Calories are measured after a substance has been broken down and utilised to produce energy. By the time the final metabolic processes are taking place to actually generate energy from the byproducts of the breakdown of different foods, those byproducts are the same regardless of their original source.

However, this is a completely different question to "does eating 250kcal of avocado have the same effect on weight loss and health as eating 250kcal of sweet?" The answer to this question is no.

A better way to look at this, is that one of the only things 250kcal of avocado and 250kcal of sweets have in common is that by the time they are broken down and utilised by the body they each have the potential to provide 250kcal worth of energy. Apart from this they are almost entirely different.

Avocados: High in monounsaturated fats, low in sugar and provide vitamins C, K, E and B6 as well as magnesium and potassium. Avocados are tasty, but also fairly filling. You are unlikely to sit and eat through several avocados in one sitting amassing an intake of 500kcal+

Sweets: High in sugar, low in vitamins and minerals. Sweets are tasty and very non-filling, it is perfectly possible to eat through a 500-600kcal packet of sweets without noticing it.

While calorie intake relative to calorie expenditure will ultimately determine changes in bodyweight, sourcing your calories from nutrient dense, filling foods like avocados, lean meats, whole grains, fruits and veggies will not only provide your body with calories, but also the necessary vitamins and minerals, fibre, proteins and fats to keep you energised for your training sessions, recovery and muscle building. These foods are also typically filling enough to not lead to excessive over consumption and subsequent weight gain.

Diets high in sugary, non-filling, low nutrient foods on the other hand can lead to excessive calorie intake and weight gain due to overconsumption. Alternatively if calorie intake remains appropriate for weight maintenance or weight loss but the diet is still high in these foods they will likely be being eaten instead of nutrient dense, quality foods (in order to keep calorie intake appropriately low). 

This can lead to a diet that might be appropriate in calorie intake but low in protein, essential fats and other essential micronutrients. This is not a good picture for health, performance, energy levels, mood or muscle building!

As always, balance is useful! A diet that prioritises nutrient dense, filling foods can afford a small amount of "treats" within the plan. We know that the calories from sweets/chocolate/crisps are not any more "fattening" in and of themselves than the calories from avocados or fruits but the context still suggests we keep their intake moderate to enjoy a well balanced diet overall.