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Activité physique et le contrôle du poids

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Strength Training for Fitness

Physical Activity and Weight Control

Introduction

In this unit we will examine the links between physical activity and weight management as well as considering the importance of adequate pre- and post-exercise nutrition and hydration.
In examining the role of physical activity in weight management, we will compare physical activity and dieting as weight loss methods and discuss the relative benefits of physical activity, such as an improved blood lipid profile and increased lean tissue.

What we eat before and after exercise can have a significant effect on sport and exercise performance, therefore the unit will also consider pre and post exercise nutrition and hydration for optimal performance.

Activity, Diet and Weight Control

Dieting alone is not a successful strategy for weight control.

There is a clear relationship between physical activity and measures of body composition such as waist to hip ratio, waist circumference and body fat. Yet modern research shows that in 2004 only one-third of men and one-quarter of women were taking thirty minutes of exercise at least five times a week.

Low levels of physical activity are associated with poor diet and obesity, which may in turn be a barrier to being active. According to statistics the main reasons given by adults for not participating in active sports during the last year were: their health wasn’t good enough (50 per cent), followed by difficulty in finding time (18 per cent) and not being interested (15 per cent). In this unit we will consider why this inactivity is of concern in relation to healthy weight and why physical activity is a key component for weight loss and weight maintenance.

Benefits of Physical Activity

Physical activity influences appetite and leads to improved overall fitness levels. In turn, higher fitness levels mean you gain advantages that benefit your weight control, because of the increased use of body fat as an energy source, without losing lean muscle mass.

One of the major physiological advantages of exercise is that levels of fat in the blood are reduced. We will consider how exercise can influence cholesterol in the blood, and therefore reduce the risk of coronary heart disease (CHD).

Activity and Appetite

Physical activity and food intake are the two key components of energy balance. Effects on intake are influenced by the duration, intensity and frequency of exercise.

Appetite is a complex phenomenon and is influenced by several factors. In the brain, within a region called the hypothalamus, is the control centre for food intake, the appestat. Many psychological factors influence the desire to eat. Physiological factors, such as blood sugar levels and hormones, also influence the appestat. It is argued that regular exercise helps the appestat to adjust calorie intake to energy expenditure.

For exercise to be of benefit in weight loss then high-fat foods must be avoided. Exercise does not provide us with permission to eat high-fat foods; there is a trade-off between the calorie loss from the physical activity and the calorie intake from the foods consumed.

Post-Meal Lipidemia

Lipidemia, the presence of fat in the blood is associated with atherosclerosis, a disease of the arteries caused by the deposition of fatty material on the inner walls.

Studies have shown that exercise either just before or after a meal is effective in reducing lipidemia, by increasing fat utilization. Research states that exercise before eating may inhibit the appetite and increase fat metabolism, since the metabolic rate remains high immediately after exercise, and that a post-meal walk or other physical activity is also able to reduce lipidemia.

Because metabolism remains high after exercise, this may be the best time to eat a meal if weight loss is the goal.

Eating for Optimum Performance

A low-fat diet with plentiful, low GI (glycaemic index) carbohydrates and a suitable amount of protein and plenty of vitamins and minerals is most suitable for athletes.

During exercise, fatigue may be caused by depletion of muscle glycogen stores and low blood sugar levels. To prepare for exercise, high-carbohydrate meals and snacks are needed to maximize stores.

The pre-exercise meal should be taken around two hours before exercise and should consist of low to moderate GI carbohydrates with small amounts of protein and some vitamins and minerals, for example, a chicken salad sandwich made with wholemeal bread. Both immediately before and during exercise which lasts for more than one hour, carbohydrate foods with a high GI value will delay the time before muscle glycogen stores become depleted.

Eating for Optimum Performance (Continued)

The important aspect of post-exercise recovery is the replacement of carbohydrate.

Depending on the intensity of exercise, 7 to 12 grams per kilogram of body weight per day of carbohydrate are required.

Evidence suggests that higher GI snack foods may also be more appropriate immediately after exercise since they promote glycogen storage. Glycogen storage occurs faster in the first two hours after exercise but does not begin until after 1 gram per kilogram of body weight has been consumed (Burke, 2007).

Therefore it is very important to refuel quickly, using a suitable snack, when there is limited time between training sessions. One example of a high GI snack providing 75 grams of carbohydrate would be a plain bagel (90 grams) with two heaped teaspoons of jam (Bean, 2006) and this snack would begin the refuelling process. This should be followed by a carbohydrate-based main meal.

Role of high GI foods for athletes

Foods with a high GI value have a useful role in the diet of athletes since they can be helpful in the speedy replacement of muscle glycogen stores. Table 1 shows examples of some high, moderate and low GI foods.

Hi GI

Moderate GI

Low GI

High GI Foods

Parsnips

Honey

Baked Potato

French Baguette

Energy Drinks

Moderate GI Foods

Bagel

Wholemeal Bread

Cereal

White Rice

Raisins

Low GI Foods

Sweet Potato

Bananas

Oats

Beans

Milk

Hydration

It is essential for health that good levels of hydration are maintained.

Water is the most important aspect of our diet because we can’t manage without daily supplies, and fluid losses must be replaced quickly to avoid long-term damage, for example, to the brain and vital organs.

The body only has a small reserve of water, and dehydration of tissues results in weakness, headaches, tiredness and loss of concentration, followed by collapse and eventually death.

During physical activity dehydration results from sweating. It is important not to rely on your thirst as an indicator of dehydration, because by then it is already too late as the effects of dehydration are already impacting on performance. Even a loss of water representing 1 to 2 per cent of your body weight can result in a lack of concentration and loss of performance.

END of UNIT

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