Our recommendation for physically active adults is to consume 15% of your calories as protein, which equates to 93g a day for a 70Kg (154lb) male. That’s 1.3 grams per kilogram of body mass. Elite athletes will require closer to 2gm per kilogram.
This is slightly less than the 100g per day average in America. Overconsumption, particularly in inactive individuals, can encourage tumour growth and can lead to gout and gouty arthritis with uric acid crystals being deposited in the joints.
Inactive adults should limit their protein intake to the RDA for sedentary adults which is 56g/day. That’s 0.8 grams per kilogram of body mass.
Protein content of foods
Since fresh foods are not sold with detailed contents labels, it is useful to get a general idea of the composition of basic foods. An excellent source of this kind of data is ‘The Healthy Diet Calorie Counter’ by Kirsten Hartvig. Some representative examples of the proportion of protein to be found in a variety of foods are shown above. The protein content of many common foods is shown as g per portion, which can readily be converted to g per 100g for ease of comparison. Meat, fish, eggs and cheese are the most concentrated source of quality animal protein, with legumes (peas, beans and lentils), nuts and seeds being the plant protein-rich alternatives. Nuts and grain contain more than 10% of their calories as protein, as do fruit and vegetables, but all foods contain varying amounts of water which will lower the actual concentration of protein in the food. For example, to obtain 56g of protein from one food alone it would be necessary to consume either 14kg of apples, 7kg of lettuce, 240g of roast beef, 154g of Parmesan cheese, or 15 slices (600g) of wholemeal bread. Based on the daily protein requirement, the above table can be used to design a healthy, balanced range of protein sources. Eating enough protein each day for a healthy existence almost certainly provides enough calories because in food, protein is typically associated with carbohydrate and fat. Sugar-rich, low protein junk food diets may not supply adequate protein. We must also remember that the RDA guidelines have to be flexible to take account of the increased protein requirements of women who are pregnant or breastfeeding, and for those who undertake high intensity exercise.
Protein and dietary misconceptions
Critics of high fat/high protein/low carbohydrate diets often quote a flawed study in 1905 [ref] which proposed that 50% of dietary protein was converted to sugar. This was proposed to raise blood sugar and insulin levels as would a carbohydrate diet. Later research [ref] demonstrated that a high protein diet did not cause a significant rise in the blood glucose levels of normal or diabetic subjects. In addition, the perceived link between high protein diets and kidney stones is not supported, but protein restricted diets help patients with kidney disease [ref]. However, eating a high protein diet with adequate fat does not cause kidney problems because the dietary fat-soluble vitamins and saturated fatty acids associated with animal proteins or plant proteins are vital for properly functioning kidneys .
High or low protein diets
Insufficient dietary protein can be unhealthy and even fatal [ref]. Lack of protein can cause retardation of growth, loss of muscle, decreased immunity [ref], weakening of the heart [ref] and the respiratory system , mental retardation and death. These symptoms are seen in severely malnourished children as infantile marasmus and are largely irreversible. Too little protein is clearly a problem but can we get too much protein in our diet? Some research has suggested that eating a high protein diet may result in too much calcium being excreted in the urine. During protein digestion there is a release of acids into the blood. This acidity is largely neutralised by calcium. On average, 1mg of calcium may be lost in urine for every 1g rise in protein consumed in the diet [ref]. This loss of calcium may affect bone strength. Can we eat too much protein? Eating a high protein diet for a few weeks will not have much effect on bone strength. However, in the long term, a high protein diet without calcium supplementation may weaken bones, as was suggested by the results of the Nurses’ Health Study. In this study, women who ate more than 95g of protein a day were 20% more likely to have broken a wrist over a 12 year period compared to those who consumed an average amount of protein (less than 68g a day), but calcium intake and urinary loss were not measured [ref]. However, a recent scientific study has suggested that increasing dietary protein does not affect the amount of calcium excreted in the urine, but has been associated with higher circulating levels of a bone growth factor in the blood. This suggests that a high protein diet would enhance bone strength [ref], but only in the presence of adequate dietary calcium to compensate for the increased calcium loss associated with the breakdown of protein for energy. The role of protein in bone strength appears complex and is not fully understood, although it probably depends on age, health, exercise and the presence of other nutrients in the diet. What is apparent is that a balanced diet with ample fruit and vegetables and adequate protein seems important for bone mineral density [ref]. Recent research supports this, with particular reference to the consumption of dark green vegetables such as broccoli, kale, cabbage and spinach. In addition to valuable antioxidants, these vegetables are a rich source of vitamin K1 which plays a crucial role in bone formation and the prevention of osteoporosis and calcification (hardening) of the arteries. Surveys show that subjects with low blood levels of vitamins K1 or K2 are more prone to limb fractures and osteoarthritis.
Myth of milk as a good source of protein and calcium As part of a healthy diet, milk has long been thought of as an excellent source of both protein and minerals and is associated with dietary calcium and strong bones. Milk is undoubtedly a good source of protein. However, it is not a good source of calcium for bone strength. In a 12-year Harvard study of 78,000 women, it was found that those who drank milk three times a day actually broke more bones than women who rarely drank milk. Similarly, a study of elderly Australian men and women showed that higher dairy product consumption was associated with increased bone fracture risk. Furthermore, those with the highest dairy product consumption had approximately double the risk of hip fracture compared to those with the lowest consumption [ref]. This may be partly as a result of the high level of phosphate in 66 milk. Phosphate in cow’s milk may bind with calcium in the digestive tract and sharply reduces its absorption. A more controversial reason may be that the protein in milk, as with other high protein foods, leads to a build-up of acid in our blood. Calcium is leached from bones to neutralise this acid. So milk may actually be no good for our bones at all.
Which proteins are the healthiest to consume? Proteins can come from a variety of different sources. Some of the protein will contain all the amino acids needed to make new proteins. This kind of protein is called complete protein. Animal proteins are usually complete. Proteins that lack essential amino acids are termed incomplete proteins. These usually come from fruits, vegetables, grains, and nuts. The consumption of a combination of different plant proteins, termed complementation, can supply the complete range of amino acids needed by the body, so with planning, vegetarians can get all the protein they need for a healthy lifestyle from plant material.
Protein complementation for vegetarians
The three essential amino acids which are most commonly lacking in plant foods are lysine (Lys), tryptophan (Trp) and methionine (Met). The protein complementation table below lists common plant foods and their relative composition of the three essential amino acids most lacking in vegetarian diets. These three amino acids can be obtained by varying the type of plant food consumed. Thus, combining corn and beans can use the excess lysine in beans to offset the shortage of lysine in corn. This is the basis of the staple Central American diet, with rice and beans being important in many Asian diets. In fact, beans form the most important source of lysine and are a crucial component of vegan and vegetarian diets. In terms of healthy eating, we recommend a diet with a higher proportion of plant proteins than animal proteins. One reason for this is that cooking certain meats at high temperatures results in the formation of toxic substances that are known to have a negative effect on health. Examples of these substances are heterocyclic aromatic amines and polycyclic aromatic hydrocarbons, which are formed when meats such as beef, lamb, pork, fowl and fish are cooked at high temperatures [ref]. These high temperatures are most often achieved by roasting, grilling or barbecuing meat. These toxic substances, some of which may be cancer causing agents [ref], are formed when amino acids and creatine (found in muscles) react together at high temperature. The same will apply to the cooking of plant material under excessive heat, but this tends not to occur when cooking plant products as they are usually steamed or boiled.
In short-term studies, a diet that involves eating less carbohydrate and more protein and fat is more effective for losing weight or keeping weight steady than a high carbohydrate, low protein, low fat diet [ref]. Eating foods high in protein such as beef, chicken, fish or beans reduces hunger pangs because the food is released more slowly from the stomach to the intestine. This may help to reduce and delay hunger and can lead to eating fewer calories. Eating protein maintains the steady state blood glucose concentration at approximately 100mg/ml without the peaks and troughs associated with sugar consumption. Insulin secretion after a protein meal is extremely low when compared to the amount secreted after a carbohydrate-rich meal. Eating protein-rich, low carbohydrate meals avoids the rapid increases and drops in blood sugar that can trigger the feeling of hunger. This type of diet can also help diabetics to control blood sugar levels and insulin requirements. Highs and lows in blood sugar are common in people who eat quick energy foods such as refined carbohydrates, and as a result they experience hunger sooner after a meal. At present, very few studies have been performed concerning the long-term effects of a high-protein/fat diet on weight control, although the number of studies is increasing rapidly and we will be in a better position in the near future to evaluate the health aspects of such diets.
Protein and chronic disease
Dietary protein may play a significant role in protection from a number of chronic diseases such as obesity, heart disease, diabetes and cancer.
One large prospective study performed in 1999 investigated the association between dietary protein and heart disease. In this study [ref], women who ate the most protein (approximately 110g per day) were 25% less likely to have had a heart attack and/or die of heart disease than the women who ate the least protein (about 68g per day) over a 14-year period. The source of the protein, animal or vegetable or whether it was part of a fat controlled diet did not seem to be important. These results suggested that eating a high protein diet does not harm the heart. Eating more protein while cutting back on easily digested carbohydrates may actually benefit the heart, by lowering insulin levels which controls fat production. More recently in 2003 a study has suggested that an increased intake of protein, particularly plant protein, may lower blood pressure and reduce the risk of CVD [ref]. Many reports have suggested that soy food products can help reduce the risk of heart disease . In 1995, an analysis of 38 controlled clinical trials indicated that eating approximately 50g of soy protein a day in place of animal protein played a role in reducing serum TGs by 10.5% [ref]. The reduction in serum TG is now thought to be far more important than the effect on serum cholesterol [ref]. Such reductions could potentially mean a reduction in the risk of developing heart disease in a high risk population. So, from this we can say that eating a high protein diet does not harm the heart. A note of caution should be introduced here with respect to diets rich in soy products causing acquired mineral deficiency.
Diabetes .Proteins found in cow’s milk may play a role in the development of Type 1 diabetes (insulin dependent diabetes or juvenile diabetes). This is one reason why cow’s milk is not universally recommended for infants [ref]. Other studies have disputed these findings and suggested that the protein in milk is not responsible for development of Type 1 diabetes. Later in life the amount of protein in the diet does not seem to adversely affect the development of Type 2 diabetes (adult onset), although research in this area is ongoing.
Cancer. There is no conclusive evidence that eating too little or too much protein will increase cancer risk. However, soy-based foods including soy protein have been suggested to play a role in the prevention of breast and prostate cancer [ref]. These claims made for soy products may be thanks to the high concentrations of isoflavones, which are forms of plant oestrogen or phytoestrogen. For this reason it has been suggested that soy products are protective against breast cancer. The phytoestrogens in soy protein foods may act to block the binding of oestrogens to receptors on the surface of tumour cells, thus blocking oestrogen dependent tumour growth. If this occurs in breast tissue, then eating soy may play a role in reducing the risk of breast cancer. However, up to now scientific studies have not provided clear evidence of this effect. Some studies have shown a benefit and others show no association between soy consumption and breast cancer [ref]. Some reports suggest that concentrated supplements of soy protein may actually stimulate the growth of breast cancer cells [ref]. It has also been suggested that soy may be involved in the development of pancreatic cancer and may play a role in the development of other forms of cancer [ref], the depression of thyroid function [ref], and interfere with mineral and vitamin uptake from food, potentially leading to stunted growth. Soybeans are high in a substance called phytic acid. This substance can block the intestinal uptake of essential minerals such as calcium, magnesium, copper, iron and especially zinc. Many scientists agree that diets high in phytates derived from soy have contributed to widespread mineral deficiencies in Third World countries, even in areas where minerals are not in short supply. If soy causes mineral deficiencies, the question that arises is how have the Japanese consumed soy for generations with no apparent damaging health effects. Traditionally, the Japanese eat a small amount of tofu or miso as part of a mineral rich fish broth, followed by a serving of meat or fish. It is important to know that miso and other traditional Japanese foods such as natto are soya bean products which have been fermented with Bacillus subtilis strains which lower the phytic acid and isoflavone content of these foods. It seems that the negative qualities of unfermented soy are more than compensated for by other nutritious components in the Japanese diet. However, if vegetarians consume tofu and bean curd as a total substitute for meat, they will be at risk of severe mineral deficiencies. Animal protein will invariably be associated with animal fat consumption. It is extremely difficult to separate the two to any great extent. A diet that is high in animal protein and low in carbohydrate will usually be a diet rich in fat, unless lean meat such as chicken and non-oily fish are selected. The consequences of a fat-rich diet are discussed in the next chapter.
Unhealthy types of meat
If possible, one should try to avoid eating chicken raised in battery conditions, as much for health reasons as for moral and ethical reasons. Battery chickens tend to be exposed to more chemicals and diseases than free range chickens. The consumption of excessive amounts of smoked meats or luncheon meats, sausage, hot dogs, salami, bologna and pastrami should be avoided. Many of these meats contain significant levels of nitrate or nitrite. Consuming these compounds in the diet has been shown to induce stomach and colorectal cancer [ref]. If possible, one should avoid eating too much of any meats that are processed, salted or preserved, unless consumed with adequate amounts of antioxidant-rich salads, and water to counteract the salt.
The consumer should choose organic beef, free-range chicken and poultry. Game birds and animals are low in saturated fat and contaminating chemicals and hormones. The method of preparation of meat is important. As far as possible, we would recommend baking or poaching meat and avoiding excessive broiling/grilling and frying. Although it is extremely tempting, especially in the summer, one should avoid consuming too much barbecued meat. These meats contain carcinogens formed from the oxidation of fat and protein [ref]. If you must barbecue meat, make sure you eat plenty of fresh salad vegetables and fruit to counteract the potential damaging effects of the burned meat. It is worth mentioning that the compounds responsible for lung cancer found in cigarette smoke are very similar to those produced on the surface of a barbecued joint (polycyclic aromatic hydrocarbons (PAHs), such as benzopyrene). Seafood is an excellent source of nutrition but must be fresh. Oily fish is a good source of omega-3 fatty acids and micronutrients. A healthy diet should contain at least one meal of fish per week. It is advisable to avoid eating too much smoked, salted, fried or breaded fish. As for shellfish, shrimp, clams, scallops and lobster, be careful of their origin. These fish are scavengers or filter feeders and can often retain relatively high levels of contaminants and pollutants in their bodies. Some fish such as tuna contain relatively high levels of mercury and should not be eaten more than once a week. This having been said, the benefits of consuming seafood far outweigh the risks. The heavy metal story may have been overplayed, and it should be remembered that many more people become ill through the microbial spoilage of seafood and fresh foods in general. Bacteria present as faecal contaminants in some prepared fresh foods, such as E. coli, Clostridia, Salmonella and Campylobacter, have caused serious food poisoning outbreaks often with fatal consequences. Some of these bacteria can survive and even grow slowly in chilled food products, which are becoming increasingly popular as ‘healthy’ food options. It has been calculated that tens of thousands of people in the UK and hundreds of thousands in the US become ill through bacterial food poisoning. This represents only those who were ill enough to notify a doctor. You can eat two eggs three to four times a week. Make sure that they are soft boiled, poached, lightly scrambled, or fried, and try to avoid hard-boiled eggs. Contrary to what you may have previously understood, eggs do not raise serum cholesterol or blood pressure. Some vegetarians tend to get their protein from processed food sources such as vegetarian sausages, burgers and other textured protein products made from Quorn and soy protein. These foods invariably contain partially hydrogenated fat and should be avoided. Vegetarians should limit their intake of soy protein for reasons indicated earlier in this chapter. Egg and cheese products contain high quality protein and fat and can be supplemented with protein from legumes. True vegans have more of a problem obtaining a diet balanced in protein and need to eat a wide variety of plant products. Some vegetarians supplement their diets with oily fish; this is an excellent idea as the fish provide protein and essential omega-3 polyunsaturated fatty acids.
Commercial ice cream and dessert toppings such as cream substitutes should be avoided, and one should try to reduce milk consumption. A study of elderly Australian men and women showed that higher dairy product consumption was associated with increased fracture risk. Further, those with the highest dairy product consumption had approximately double the risk of hip fracture compared to those with the lowest consumption [ref].
Cows milk contains a high level of soluble phosphate which can react with calcium salts under the alkaline conditions of the intestine to form insoluble calcium phosphate salts which cannot be absorbed. One should also avoid consuming powdered milk and sweetened yogurts .
We are the only species that drinks the milk of another animal. Moreover, milk is produced to maximise the growth and development of the immature animals and contains growth factors that may play a role in the development of human cancers. Many health-conscious people accept skimmed milk as a healthy low fat alternative to full cream milk. It should be noted that reduced fat milk may contain added powdered milk to ‘improve’ palatability. The production of milk powder causes oxidation of cholesterol and recent research has indicated that it is oxidised cholesterol that is a risk factor in heart disease . This is the same product that Kritchevsky fed to rabbits to induce atherosclerosis and to support his theory about the role of cholesterol in heart disease.
Very high protein diets can be lethal: Early Arctic explorers were well aware of the dangers of consuming too much protein in the absence of fat and carbohydrate. Several on lean meat diets suffered protein poisoning and some died [ref]. This has particular reference to modern dieting where protein is perceived as being healthy and fat as unhealthy. The latest advice is to limit protein intake to one gram per kilo gram of body weight per day.