There are many compelling reasons to exercise, so don’t read this title as an excuse to not to work out as hard and as often as you can – exercise is super important! Physical activity helps prevent many diseases, it boosts mental function, it makes you feel and look better, it can improve sleep quality, it can actually prolong your life.

But exercise on its own – without reducing caloric intake – results in less than expected weight loss.

Which is counterintuitive, right? To lose weight we need to achieve a negative caloric balance, and to put caloric balance in the red we could either increase energy spending, or reduce the energy we consume. Junk and fast food manufacturers promote the notion that the imbalance could be addressed through exercise; they’re also sponsors of exercise programs, from school and amateur leagues all the way to World Cups and the Olympic Games, driving this subtle but powerful message.

It really would have been nice if rather than reduce caloric intake we could just exercise a little more.

Alas, obesity isn’t an exercise deficit problem, Americans today aren’t that much less active than they were 30 years ago, and exercise alone isn’t a weight loss plan, studies have shown. Exercise calories are partly compensated for, so the deficit they’re supposedly creating is small.

Why’s that?

Several options: Upping exercise might lead to energy conservation during the rest of the day (more tired time on the couch after an intense spin class?) to compensatory reduction in metabolic rate (the body doing its internal work more efficiently to keep the status quo), or simply to a bigger appetite and slightly bigger portions post workouts.

Calories burnt – but not forgotten

A new study in the American Journal of Clinical Nutrition assigned about 200 women and men randomly to 3 treatments that lasted 6 months: no exercise (control), a little exercise of 8kcal per kg weight per day (about 700 calories a week), or more intensive exercise of 20kcal per kg weight per day (1700 calories a week). The participants were not instructed to change their diet. The volunteers' energy expenditure and body composition were followed tightly, and energy (i.e. food) intake was measured with the best available method – doubly labeled water.

Exercise sessions were under supervision and monitoring, and compliance was good, so energy spend did go up in the exercise groups. Weight loss, however, did not concur with the energy-out balance sheet expectations – it was about 40 percent less than expected. In fact, almost half of the people in the moderate exercise group lost no weight and even gained some, as did 24 percent in the high exercise group.

This carefully done study did not find that energy was conserved by reducing activity after exercise. It also didn’t observe any slowing in the metabolism of the participants.

The main reason the participants didn’t lose much weight: food. People ate more when they exercised.

The control group continued to eat pretty much the same amount, while the moderate exercisers upped caloric intake by about 90 calories a day, and the higher dose exercise group increased caloric intake by 124 calories daily. People who exercised reported greater hunger and increased cravings for sweets and indulgent foods.

Exercise is still the best advice

This study joins many others that demonstrate that exercise isn’t – by itself – a weight loss strategy; it’s unique in its careful study of the mechanism through which diet underperforms for weight loss.

No doubt, people should exercise for fitness’ sake.

But if the goal is also weight loss, the authors conclude that physical activity needs to be combined with dietary change.

Exercise in some ways under-performs, and in others over performs. Physical activity is keystone habit, and sometimes a change in exercise habits, such as joining a team, a cycling group, a yoga studio or a physically demanding hobby changes other things in a cascading fashion. Habits tend to cluster, and exercise clusters with really good things: People who exercise will often also start eating better, watch their weight, consider the nutrition label, avoid smoking and get enough sleep.

Physical activity can also help avoid weight regain, and stabilize weight.

But the calories burnt on your app or treadmill, forget these, they’re merely a guess; they're definitely not something you should add to a post-exercise snack.

Dr. Ayala