1) A decrease in Resting Metabolic Rate (RMR)
As we age, we burn fewer calories. One reason for this is the decline in resting metabolic rate (RMR), which is the number of calories we burn at rest. This has been shown to fall linearly with age. The fewer calories we burn, the fewer calories we must eat to maintain our weight, and so adherence to a deficit becomes more challenging.
2) Lifestyle changes
Typically our lifestyles at 50+ are not the same as they were when we were 25. Our activity levels are far lower, and so our energy expenditures are also far lower. 
So how do we overcome these hurdles?
One way we can help prevent the RMR from declining is by maintaining muscle mass. One of the reasons our RMR falls as we age, is because skeletal muscle mass tends to decline as we get older . There are other factors which contribute to the decrease in RMR (eg, a decrease in sodium-potassium pump activity) so some slowdown may be inevitable. However, by weight-training and preserving as much skeletal muscle mass as we can, we can mitigate as much as possible the decline in RMR and stop body fat increasing.
As for lifestyle, we simply need to keep as active as possible. More sitting around and less activity will lead to fat accumulation. Staying active will keep energy expenditure high, and thus make fat-loss easier. While we may lose speed and agility, walking, swimming, cycling and other low-impact activities are all good ways to burn calories. Better to move slowly than not at all.
To sum up, those who maintain the same activity level and who continue to weight train into older age, see just as good results. Take the example of Albert (results below), 66 years old, who in the space of 3 months lost over 6kg (a stone). Not only that, but the vast majority of it (5.1kg) was body fat; less than a kilo was lean mass. In our experience, people often lose 1kg of lean mass for every 3kg of fat, so not only has he done better than most, but he has done so at 66 years of age (our clients' median age is 37). Age is just a number, don’t let it define your results.
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