Eating and Exercising to Lose Weight

Dr Shannon McCarthy Endocrinologist MBBS, FRACP

I’m exercising and eating healthily but I’m still not losing weight. What’s going on?

Our bodies are very, very resistant to losing weight - your body still thinks you are a caveman and the food supply could dry up at any time, so you need those fat stores to get you through the hard times.

However, our current environment has a constant supply of high calorie foods that have been scientifically engineered to taste really, really good. Our brains will quickly become accustomed to this constant influx of calories and feel-good food if we let them. So if you reach for a chocolate bar or a glass of wine every time you are upset, your brain learns to crave these things because they make you feel better.

Your body can also tell when food is scarce and it turns down your metabolism to try to prevent weight loss, and turns up your hunger hormones. When participants in programs like The Biggest Loser are studied scientifically, their metabolic rate goes down when they restrict their caloric intake, their hunger hormones go up, and these changes can last for FIVE YEARS after they have lost their weight.

The other problem is that most people don’t realise how much exercise they need to do, and how far they need to restrict their calories, to lose a significant amount of weight.

Let’s give an example.

Lisa is age 25, 165 cm tall, and weighs 100 kgs.

She would like to lose 1 kg per week.

1kg of fat has 7,700 calories of energy in it.

Therefore she has to be in a negative calorie balance of 1,100 calories per day.

To maintain a weight of 100 kgs she must be eating at least 2000 calories per day, and that is with absolutely no movement (ie if she were lying in a hospital bed). If she is up and walking around, but not really exercising, she is probably eating around 2500 calories per day.

Therefore to lose a kilogram per week her calorie allowance is 2500 -1100 = 1400 calories per day. Given that when she starts to calorie restrict, her body will turn her resting metabolic rate down, her allowance is probably more like 1000 calories per day.

In summary, if she is eating 2500 calories per day now, she has to reduce her caloric intake by MORE THAN HALF to lose this kilo a week, if she doesn’t add in any exercise.

So how could she do this?

Current calories


1 bowl of muesli with dried fruit and nuts, half a cup of full-fat milk, small glass of orange juice

375 calories and 52 grams of carbohydrate

On the way to work

1 large latte 225 calories 17 grams of carbohydrate

Morning tea

2 scotch finger biscuits and 1 large latte 400 calories and 40 grams of carbohydrate


Multigrain sandwich with butter, cheese, ham and tomato 567 calories and 40 grams of carbohydrate

Afternoon tea

1 large latte and 1 banana. 325 calories and 39 grams of carbohydrate


Lasagne, garlic bread, salad (lettuce, tomato, carrot, olives, olive oil, balsamic) and 2 glasses white wine. 1100 calories and 100 grams of carbohydrate


Two small scoops Peters Light and Creamy Ice Cream 71 calories and 13 grams of carbohydrate

Late night snack

Small tub of peaches 100 calories

22 drinks over the day: water, black tea no sugar or milk, diet cordial

Total = 3163 calories and 323 grams of carbohydrates

So this doesn’t sound terrible right? Aside from the biscuits it’s not particularly “unhealthy” - it’s just WAY more energy than she needs.

The Alternative - If her allowance was 1000 cal per day


Optifast bar, black coffee. 214 calories and 18 grams of carbohydrate


Optifast bar, black coffee. 214 calories and 18 grams of carbohydrate

Afternoon snack

Optifast bar, black tea or water. 214 calories and 18 grams of carbohydrate


Steamed chicken breast, mixed low-carbohydrate vegetables. 300 calories and 4 grams of carbohydrate

No snacks, no alcohol, no lattes, no calories in any drinks.

Total = 942 calories and 58 grams of carbohydrates

If Lisa adds some exercise, this will both keep her metabolism up, build some muscle (which burns calories even at rest), and increases her calorie budget.

But how much exercise?

Let's say Lisa was willing to give up her juice, her lattes, the garlic bread, the ice cream and the wine Total calories - 1800. But she is aiming for 1000 calories per day. Lisa needs 800 calories of exercise every day!

So how much exercise is 800 calories?

Riding on a stationary bike at 20 km/hr for 2 hours

Swimming laps in the pool fast for 65 minutes

Lifting heavy weights for 2 hours

Walking at a speed of 6 km/hr for 3.5 hours

“But I’m on my feet all day at work!”

Walking at a speed of 2.5 km/hr for 4 hours a day (ie half the day walking slowly and half the day just standing) burns only an extra 280 calories


So Lisa needs to halve her calories and exercise moderately hard for two hours a day to lose 1 kg per week.

And keep in mind that if she does this for six days of the week, then on Sunday doesn’t exercise and eats her old 3163 calories, she cancels out almost 3 of the past 6 days work, and she will lose less than half a kilo that week.

In summary

Losing weight takes a LOT of CONSISTENT effort. The more exercise you can do, the better, and harder that exercise, the better.

Also, many people find in the first 4 weeks of trying to lose weight by increasing their exercise, they don’t lose ANY weight.

This is thought to be due to fluid retention due to the stress of the exercise and damage to the muscles. Don’t despair; you’re doing the right thing and the scales will eventually reflect all of your hard work.