The desire for rapid fat loss is everywhere—crash diets, extreme cleanses, and aggressive calorie cuts all promise to make the scale drop in record time. It is tempting: seeing a big number change overnight feels rewarding. However, the reality is that fast weight loss is often not the same as fast fat loss. The scale can be misleading, reflecting water, glycogen, and even muscle loss, not just fat.

Fast Fat Loss

There is reasonably clear evidence that losing weight quickly, leads to higher amounts of lean tissue loss, as a proportion of total fat lost:

• Weiss et al. (2007) compared moderate (1,200 kcal/day) and very low-calorie diets (800 kcal/day). Both groups lost weight, but the rapid weight loss group lost significantly more lean mass along with fat, especially after the first few weeks (Weiss et al., 2007).
• Hall (2008) used mathematical modeling to show that extremely rapid weight loss is initially possible due to water and glycogen loss, but true fat loss slows as the body adapts (Hall, 2008). Both water and glycogen fall into the category of lean mass.

Maintaining Lean Tissue

• Garthe et al. (2011) studied athletes and found that those losing weight at a slower rate (about 0.7% of body weight per week) lost less muscle and more fat compared to those losing faster (Garthe et al., 2011).
• Johannsen et al. (2012) in the CALERIE study, found that a modest calorie deficit of 10–12% below maintenance produced steady fat loss while preserving muscle, averaging about 0.5–1% of body weight per week (Johannsen et al., 2012).

Theoretical Maximum

All of that being said, the theoretical maximum rate of human fat oxidation appears to be somewhere around 31 kcal/kg of fat mass per day (Alpert, 2007). For a 90kg person with 30% body fat, this is about 2-3lbs of fat loss per week at the upper end. The trouble is, of course, this is only looking at fat loss in isolation. Meaning while humans might be capable of these rates, it does not tell you what level of restriction would be necessary to achieve this, and the effects that would have on lean tissue (specifically muscle mass).

A moderately active 90kg male with 30% body fat would likely need to eat about ~2500 calories per day to maintain weight. If we were to aim for the theoretical maximum of fat loss, this person we need to eat about 1000 calories per day, assuming no change in activity/exercise. If they are eating a high protein diet (which they should be to preserve lean mass) somewhere between 60-80% of their daily calories would be coming from protein. At face-value, it is probably easy to see why this is likely not going to work. Not only is that terribly fewer calories than this individual is accustomed to, but expecting them to keep that up the whole week and still live their normal lives is unreasonable. Beyond that, it is highly unlikely they would be able to maintain the quantity, or quality, of resistance training sessions needed to preserve muscle (given that resistance training is the primary factor driving muscle preservation/growth).

Practical Maximum

Considering the above information, setting targets for a rate of 0.5-1% of total body weight loss per week is probably a good idea. It is faster enough to still see very tangible progress in a short period of time, but not so fast that the wheels fall off the bus (see my previous blog post about the downsides of losing weight too quickly). You will be able to eat reasonably filling meals, get all necessary nutrients, and support proper exercise and activity throughout the week. It is much more conducive to long term success, and will look much more similar to what your long-term routine will actually look like. So don’t get caught in the crash dieting trap, it does not lead to better outcomes!

All the best,

Kevin Rogers, RDN, LD

Founder | Redesign Nutrition

References

• Alpert, M. A. (2007). The maximum rate of fat oxidation. European Journal of Applied Physiology, 99(4), 411–418. https://doi.org/10.1007/s00421-006-0356-z
• Weiss, E. P., et al. (2007). Weight loss, weight maintenance, and cardiovascular risk factors. Obesity, 15(4), 1003–1013. https://doi.org/10.1038/oby.2007.528
• Johannsen, D. L., et al. (2012). Effect of 6-month calorie restriction on biomarkers of longevity, metabolic adaptation, and oxidative stress in overweight individuals. American Journal of Clinical Nutrition, 96(2), 337–346. https://doi.org/10.3945/ajcn.112.035782
• Hall, K. D. (2008). What is the required energy deficit per unit weight loss? Cell Metabolism, 8(5), 371–373. https://doi.org/10.1016/j.cmet.2008.09.002
• Garthe, I., et al. (2011). Effect of two different weight-loss rates on body composition and strength and power-related performance in elite athletes. Medicine & Science in Sports & Exercise, 43(2), 318–326. https://doi.org/10.1249/MSS.0b013e318207f2e0