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Burning fat or keeping fit?

MagazineApril 2009 (Vol. 20 Issue 1)Burning fat or keeping fit?

We've all been imbued with the notion that regular exercise will keep us trim

We've all been imbued with the notion that regular exercise will keep us trim. WDDTY begins a special series on exercise: what does what for you, and which is best for maintaining health and losing weight.

Both the UK and US governments have stated that at least 30 minutes of moderately intense physical activity five times a week will maintain general health and reduce the risk of chronic disease. Other official reports argue that 30 minutes is not enough, and

still others, such as the British Medical Journal, claim that just walking for 30 to 60 minutes a week is beneficial.

Needless to say, these instructions leave many of us still wondering how much, doing what and for what?

Whether aerobic exercise is good for you depends on its frequency, duration and intensity. The benefits can range from general health improvement and disease prevention to cardiovascular fitness and weight loss.

But, if you want to lose weight or be 'heart-healthy' through aerobic exercise alone, it will require far more work than the officials tell you.

Weight control

Although most people believe that regular exercise helps weight loss, the truth is that basic regular aerobic exercise, as suggested by the govern-ment, may only prevent weight gain. In a Duke University trial involving 175 overweight volunteers, walking 11 miles a week prevented significant fat accumulation. Indeed, 30 minutes or more of a moderate-intensity activity on all or most days of the week can prevent weight gain.

However, losing weight requires working harder. In this study, a modest increase in exercise resulted in a sig-nificant decrease in body fat (J Appl Physiol, 2005; 99: 1613-8), but others find that you have to work up far more of a sweat to work off excess poundage.

In 2002, the US Institute of Medi-cine recommended that to control weight required moderate exercise for 60 minutes a day on most days-and the UK agrees. Then, the 2005 dietary guidelines from the US Department of Health and Human Services and Department of Agriculture told us that losing weight requires 60 to 90 minutes of moderate exercise daily.

Both food and exercise are usually examined in terms of calories. One calorie is the amount of heat needed to raise the temperature of one kilogram of water by one degree Celsius. One pound of fat equals about 3500 calories, which takes quite a while to burn off, especially as the average aerobic session burns just 152 calories (see box below). If this amount seems surprisingly low, it's probably because you're used to the digital readouts on exercise machines that may be using conversion factors that overestimate caloric expenditure (Phys Sportsmed, 2003; 31 3: e-pub).

So, exactly how hard do we have to work to lose those love handles?

What the evidence says

The body uses different fuel for different exercise intensities, produc-ing more carbon dioxide when burning carbs compared with fat. Burning fat is highest during lower-intensity exercise and, as the intensity increases, the body starts breaking down carbs (DReferences for Intakes for Energy, Carbohydrates, Fiber, Fat, Protein and Amino Acids [Macro-nutrients]. Institute of Medicine, 2002). This doesn't mean that low-intensity exercise is better in general, just that we can get away with it and still shed pounds.

A retrospective analysis of the US-based National Weight Control Registry found that long-term weight loss can be successfully maintained by an average energy expenditure of 2800 kcal/week (Am J Clin Nutr, 2003; 78: 669-70).

Another study compared two different exercise regimes: one burning 1000 kcal/week, the other, 2500 kcal/week. The subjects were all overweight and on calorie-controlled diets. It took 12 months, but the higher-calorie group eventually showed significantly greater weight loss (Am J Clin Nutr, 2003;78: 684-9).

One way to steadily lose weight through aerobic exercise is to engage in frequent bouts of low-to-moderate exercise such as continuous cycling, walking or swimming. Not only will you lose weight-and keep it off-but you'll also increase your levels of 'good' (HDL) cholesterol (Circulation, 1995; 91: 2596-604).

Scientists refer to calories burned during exercise as METS, or metabolic equivalents-the amount of heat produced by the body during exercise. Thus, exercise levels are measured in METS, with 1 MET used during sitting and resting and, say, 8 METS if you are jogging. According to the MET system, you need to do around an hour of 6 or 7 MET activity (such as hill-climbing or skiing) every day to lose weight. If you're new to exercise, expending this many calories will require a lengthy workout, so a more workable strategy may be to split it into several 20-minute sessions every day.

However, weight loss involves more variables than calories expended such as metabolic type, sugar-burning efficiency and exposure to allergies, which can cause bloating and weight gain. Unless you're prepared to spend 60 to 90 minutes a day on it, exercise may be better viewed as a way to increase your overall health-but not the easiest way to lose weight.

Kim Wallace and Tina Tan

Why exercise doesn't shift pounds?

To determine your average daily caloric expenditure, first determine your unique resting metabolic rate (RMR)-the amount of calories you burn at rest. Use the following formulas:

- Men: 66.473 1 [13.751 3 (body weight in kg)] 1

[5.0033 3 (height in cm)] 2 [6.755 3 (age in years)]

- Women: 655.0955 1 [9.463 3 (body weight in kg)] 1

[1.8496 3 (height in cm)] 2 [4.6756 3 (age in years)]

To find out how many calories you burn per minute, divide your RMR by the number of minutes in a day (1440). If your RMR is 1100, divide 1100 by 1440, which means you burn 0.76 calories per minute at rest and at each metabolic equivalent (MET) level.

Let's say you do an aerobics exercise class for 50 minutes, which generally has a MET level of 6. You would then calculate 50 (minutes) 3 6 (METS) 3 0.76 (calories per minute) and-voil`a!-you've burned 228 calories doing that class.

But remember: you're always substituting one activity for another-at the very least, your resting MET or, more likely, another activity such as sitting at your desk working or fixing dinner. If you then subtract 2 (fixing dinner) from 6 (aerobics), you get a MET level of 4.

So, to determine the true number of calories burned per minute, you must plug 4 METS into your calculations, not 6. In this case, you find that you only burned 152 calories (instead of 228). A standard calorie counter will only show what you've burned in total, not what you've burned more than normal.

Rimonabant: suicidal slimming

A panel of US federal health ad-visors has unan-imously rejected Sanofi Aventis' weight-loss drug rimonabant-hailed as a magic bullet against obesity-after it was revealed that it increases the risk of suicidal thoughts and psychiatric side-effects.

Already sold in the UK as Acomplia, Sanofi intended to market the drug in the US as Zimulti. It's the first of a new class of agents that block cannaboid receptor-1 (CB1), found in the brain and in fat. Controlling these receptors supposedly dampens the urges that can lead to excess food intake.

A review of a series of clinical trials found that 20 mg of rimonabant plus a low-calorie diet effectively reduced body weight by about 5 per cent more than diet alone over one year. But this dose was also associated with a load of side-effects, including mood disorders.

Although the trials excluded those with a history of psychiatric illness or attempted suicide, a meta-analysis of all the data revealed that those taking rimonabant were twice as likely to have suicidal thoughts and behaviour compared with a placebo. In fact, two patients actually committed suicide while taking the drug.

Others reported delusional symp-toms, psychotic behaviour and aggress-ion with rimonabant, including a man who tried to strangle his daughter and another who beat his wife. Within one month (May to June 2007), there were 30 reports of adverse psychiatric events with the drug made to the FDA's Division of Metabolism and Endocrin-ology Products.

Other side-effects with the drug include dizziness, tremors, loss of sense of pain and touch, amnesia, seizures, and disturbances in thinking and perception-not surprising given that the drug interferes with parts of the brain that affect memory, motor function and reward behaviours.

The FDA reviewers commented that these neurological adverse events may well also have contributed to the disproportionately large number of patients sustaining injuries (such as contusions, concussions, falls, road traffic accidents and whiplash) during the trials in the rimonabant 20-mg group (6.9 per cent) compared with the placebo group (3.8 per cent).

Yet another cause for worry are four cases of confirmed or suspected multiple sclerosis (MS) among those taking rimonabant, reported either during or shortly after the trials, compared with just one with placebo. Of the four, only one had a history of MS-and her symptoms got worse while taking the drug. Another woman, who developed MS several months after starting the treatment, had a near-complete recovery within weeks of stopping the drug. Nevertheless, the precise relationship between the drug and MS is unclear at this time.

Sanofi Aventis had hoped to persuade the FDA that rimonabant's benefits-specifically, its ability to produce significant weight loss-"clearly outweigh the defined risks that are manageable in clinical practice". But the Committee's decision not to recommend the drug means it's not likely that Zimulti will get US approval.

Still, Sanofi has succeeded in other parts of the world. Alarmingly, the drug is already approved in over 30 countries across Europe and the Americas, and in India. In the two countries with the highest sales-the UK and Germany-more than 64,000 patients took the drug between June and November 2006.

As long-term studies of rimonabant have yet to be completed, the true impact of the drug on these patients' future health remains unknown. What we do know is that patients who stop taking rimonabant soon regain any weight they have lost-which means that it has to be taken for life for it to work. Considering the raft of side-effects that comes with the drug, it's hardly the miracle the obesity epidemic has been looking for.

Joanna Evans


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