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How many calories should I eat to lose weight fast? (5kg per month)

Daniel Brady | November 25, 2012 in Nutrition

In this article we will look at various weight loss studies to see what weight loss is achievable with various calorie intake levels. Let’s start with a 500 calorie daily deficit. A calorie deficit means that you eat X amount of calories less than your BMR plus your burned calories from exercise, which will lead to weight loss.

500 Calorie Deficit

90 obese subjects had a 500 calorie deficit for 48 weeks, about 11 months.[1] On average they lost 8.5kg +/- 7.8kg of fat, a range of 0.7kg to 16.3kg for the majority. There was a roughly 4:1 ratio of fat lost to muscle & water lost, e.g. 8kg fat and 2kg muscle lost. On average they lost 0.8kg fat per month and 0.2kg muscle & water per month, for a total of 1kg weight lost.

There are 7700 calories in a kilogram of fat, so theoretically at a 500 calorie deficit they should have lost 2kg per month, but in reality they lost 1kg.

Extreme: 1000+ Calorie Deficit

This study went to extremes, prescribing calorie intakes of only 1000 calories per day and below, and also exercise. This leads to a huge calorie deficit. The results were also extreme, with the PSMF (1000 calories of meat, fish and fowl per day and nothing else) group with exercise, losing 15kg in 12 weeks, a rate of 1.25kg per week, or over 5kg per month. The same diet without exercise lost 13kg in 12 weeks. The second group, BCDD (1000 calories of balanced food) with exercise, lost 14kg in the 12 weeks, nearly the same as the other group.

1000+ Calorie Deficit Diet Results
Exercise as an adjunct to weight loss and maintenance in moderately obese subjects.[2]

There were two additional groups consuming only 420 calories and 800 calories. The 420 calorie group lost 13kg in 8 weeks, a rate of 1.6kg per week, or 7kg per month. But such a small calorie intake would require doctor supervision, because it may be unsafe, so I will detail these groups no further.

Considering the large calorie deficit in these groups, they were all required to take daily multivitamins. Clearly the protein-only group which got great results without being as extreme as the 420 calorie group, is still going to have nutrient deficiency, so water (obviously) and vegetables may be added for a more nutritious diet.

Bounce Back

There is one last conclusion that is worth noting. The groups who exercised managed to keep the weight off, while the diet-only groups bounced back almost to where they started. Clearly exercise is beneficial. The exercising groups lost more weight in total too. While the groups who exercised increased their strength significantly over the 12 weeks, the groups who didn’t exercise lost a minor amount of strength, another clear benefit.

Exercive Versus No Exercise Diet Bounce Back


If you want to lose weight fast, you should eat only 1000 calories per day, but don’t forget to keep your nutrients up. If you want to lose weight even faster, and keep it off too, then make sure to exercise. This method can have you lose 5kg+ per month. That is if you have the self control to limit your calories so significantly.

If you want to eat more but still want to lose weight, then you should eat at a 500 calorie deficit and you’ll likely lose 1kg per month.

  1. Effect of Energy-Reduced Diets High in Dairy Products and Fiber on Weight Loss in Obese Adults. Warren G. Thompson, Nicole Rostad Holdman, Denise J. Janzow, Jeffrey M. Slezak, Kristin L. Morris, Michael B. Zemel []
  2. Exercise as an adjunct to weight loss and maintenance in moderately obese subjects. Konstantin N Pavlou, Suzanna Krev, and William P Steffee []

Daniel has a strong interest in evidence supported fitness training, preparation and supplementation. His goal is to provide clear information that simply works. He's currently 16 kg of muscle above his starting weight (7kg in the past 12 months), and targeting a continued growth rate of 5kg+ per year.
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