This is what is taught in medical school about the effect of low calorie dieting on metabolism.  This is why 'diets FAIL, why most dieters regain the weight they lose.  This is why weight managment is about long term, healthy, living, lifestyle, NOT going on a diet.

When you do not eat for a day, or any other prolonged period of time, your body produces the energy it needs to live and move by "eating" your muscles instead. Muscle weighs more than fat, so when you step on the scale, you will see that you have lost weight.
When you are not consuming addequate nutrition, minerals and vitamins, your body leaches these from your own lean tissue body mass, effectivelly reducing even your bode density. 'Osteoporosis, the thinning of the bones, is often called, 'the slimmers disease', for good reason. The human body needs calcium and minerals for the function of it's vital organs including the heart and if insuficient nutrition is consumed the body will draw nourishment from the bones to protect the vital organs. When calories are reduced the quality of those calories is more important than ever before.

Consider, that muscle takes the most energy for your body to maintain. A person with a lot of muscle mass burns more calories per day than a similarly built person who has less muscle, even if both are just sitting on the couch watching TV.

So, when someone stops fasting, he or she will have lost weight because they have lost muscle mass. However, his or her body will now burn fewer calories per day than it did before they started fasting. If that person were to start eating again as they did before the fast, he or she would now gain weight, whereas before the fast, they broke even and neither gained or lost.

It is quite possible that the same is true for when you eat an extremely low amount of calories per day. Your body needs energy to keep moving and living. It is not getting that energy from food, so it burns up some muscle, because muscle a) provides more nutrients than fat, and b) requires more energy to maintain, energy that your body no longer has.

The belief that your body goes into starvation mode, is a simplistic way of explaining the lowering effect on your metabolism of low calorie diets. Your body does not think "oh, no food, better lower my metabolism for a while". Your body is a machine really; it does not think much of anything at all. Eating too little, however, does lower your metabolism because your body is eating up its muscle mass. This is not a temporary change though, as the words "starvation mode" may lead you to believe. The lowering of your metabolic rate will be permanent until you exercise to build back the muscle you have lost.

 

Here are some pros and cons of exercising at different times of the day so you can decide what’s best for you.

Mornings are best because... Research carried out by Associate Professor Steve Boutcher, Director of the Fat Loss Laboratory at the University of NSW, shows that if you want to lose fat the best time to exercise is in the morning before breakfast.

Professor Boutcher explains, “The body has effectively run out of carbohydrates or sugars for fuel and it switches to burning fats instead. One way that it does this is by releasing a substance that ‘turns on’ the fat cells to release their fats.”

If you’re one of those people who can’t exercise on an empty stomach, Professor Boutcher advises, “If you can’t face working out before breakfast or you’re really hungry, have a small snack, then after exercising wait for at least 15 minutes before you eat.” 

Other studies support Professor Boutcher’s research findings on morning workouts and suggest that early exercisers are more likely to stick with a fitness regime than those who leave it until later in the day.

In addition, a recent study by Glasgow University found that morning exercise enhances mood by 30 percent more than evening exertion. Getting up and active early can help boost your physical energy and mental alertness for the day ahead.

On the flip side, your body temperature is at its lowest after waking so you’ll naturally have lower energy levels and blood flow in the morning. Therefore it’s very important to warm up well to prevent injury.

Lunchtimes are best because... If you know you can put the time aside to exercise regularly, lunchtime exercise is a good option. You’ll be awake and alert and your muscles will be warmed up so you might be less likely to injure yourself. Plus, you can grab an exercise partner – a friend or colleague – and this will help you commit to exercise regularly and make it a lot more fun.

Exercising in the middle of the day could help to relieve daily stresses and boost blood flow to your brain so you’re sharper for your afternoon duties. Additionally, you might find that exercising in the middle of the day makes you more mindful of what you’re eating for lunch and later on.

On the down side, time constraints might mean that you can’t get a full workout in.

Evenings are best because... If you want to gain muscle mass, exercising later might be your best option. “Exercising in the evening is a good way to gain strength and that’s why strength trainers like weightlifters train in the evenings,” says Professor Boutcher. “The muscles have warmed up by the afternoon or evening and certain hormones that are necessary for muscle-resistance work are optimal at this time.”

Evening exercise might also help you to regulate the amount of food you feel like eating for dinner, which is beneficial if you tend to eat big meals at night. It can also be a great stress reliever after a busy day at work or home.

On the downside, some research suggests that vigorous activity just before bedtime isn’t a good idea, so you’ll need to exercise one to three hours before you go to sleep. This is because exercise raises your body temperature and increases hormone activity, which could interfere with sleep. The only way to see if evening exercise works or doesn’t work for you is to try it and see how you feel.

Anytime is best because... Experts agree that the most important thing is not so much the time of day that you exercise, but that you actually do it – and do it on a regular basis! That means that anytime you can fit it into your day is ultimately the best time for you.

Summery:  With that, I'm about to pop on my gym clothes and head to Aquamoves.